Amaurobioidea: Rummaging through a Wastebasket

A representative of the strikingly-coloured Nicodamidae from Australia. Photo by Nick Monaghan. While such spiders were previously identified as Nicodamus bicolor, there are no less than 23 species in seven genera that have previously been included under that name.

One term that you may come across in discussions of phylogeny is the concept of a "wastebasket" taxon. As the name suggests, a wastebasket taxon is one into which authors tend to throw everything that they can't really deal with. Often, a wastebasket will include the members of a group that are relatively unspecialised, often primitive, and united less by their shared characters than their lack of distinct features to connect them to one or another of the specialised subgroups that the author may recognise within the parent group. Phalangodidae among short-legged harvestmen, Sylviidae among passerine birds and Perciformes among spiny-finned fishes are all examples of taxa that have become wastebaskets in the past. Some wastebasket taxa are explicitly established as such, like the 'Deuteromycota' that included asexual fungi before techniques were developed that made it significantly easier to relate asexual and sexual fungal taxa. More often, though, a taxon originally based on a certain combination of features will develop into a wastebasket over time as phylogenetic studies show that the original basis characters for that taxon represent plesiomorphies (ancestral characters). This week's highlight taxon, the spider superfamily Amaurobioidea, perhaps belongs to the latter group.


Tegenaria gigantea (Agelenidae). Photo from Wikipedia. Agelenids build funnel-shaped webs and are apparently often called some variant of "funnel spiders" in North America, but such names are likely to cause confusion here in Australia with a certain notorious mygalomorphs. Some species of Tegenaria such as the hobo spider are also known for being toxic, but nowhere near as toxic as the Australian funnel-web.

In an earlier post, I included a quick overview of basal spider phylogeny, going as far down as the clade Araneoclada that unites those spiders that have only a single pair of book lungs (ancestrally, at least - many families of Araneoclada have lost the book lungs entirely, or evolved tracheae in their place). Members of the Araneoclada are further divided between the Haplogynae and the Entelegynae, originally based on the presence (Entelegynae) or absence (Haplogynae) in females of paired copulatory ducts opening on a sclerotised plate called the epigyne. While the absence of such ducts in the Haplogynae is obviously a primitive character and no longer regarded as uniting them, the group has funnily enough been supported as monophyletic based on a number of other characters (except for a small number of 'haplogyne' taxa that are phylogenetically entelegynes) (Coddington & Levi, 1991). However, the Amaurobioidea belong to the Entelegynae, which is by far the larger of the two clades. Within the Entelegynae, the primary division was long based on whether or not a species possessed a cribellum, a plate-like structure among the spinnerets that bears hundreds of tiny silk-producing spigots. As these spigots exude silk simultaneously, the spider uses a specialised arrangement of bristles on the fourth pair of legs to weave them together to form a woolly thread (see here for a more detailed description). Because this woolly thread is composed of multiple tangled strands, it can effectively entangle prey such as small insects that get caught among the strands. Unfortunately, as knowledge of entelegyne spiders improved it became clear that possession of a cribellum did not define a phylogenetically coherent group. A number of cases were identified of pairs of taxa clearly related by other characters in which one taxon possessed a cribellum and the other did not. The eventual conclusion was that the cribellum was an ancestral character for the Entelegynae (as also supported by its presence in one haplogyne family, the Filistatidae) that had been lost on numerous occassions.


Ctenus floweri (Ctenidae), from Singapore. Photo by David Court. Ctenids are active hunters.

In general, the Amaurobioidea included cribellate spiders with unbranched abdominal median tracheae, as opposed to Dictynoidea with branched abdominal median tracheae (Coddington & Levi, 1991). Families that have been assigned to Amaurobioidea include (among others) Amaurobiidae, Agelenidae, Ctenidae, Amphinectidae and Nicodamidae, but relatively little unites these families. Most of them are generally ground-dwellers (which may explain the common name of one of the best-known members, the hobo spider Tegenaria agrestis). Many members build small sheet-webs, but others are active hunters. Both the characters referred to above have since been shown to represent plesiomorphies of larger clades, with the alternative conditions arising multiple times. The phylogenetic analysis of entelegyne spiders by Griswold et al. (1999) found the 'Amaurobioidea' to fall within a clade that was sister to the clade including the orb-weavers, but the same clade included the Dictynoidea and Lycosoidea (wolf spiders and such) nested within 'amaurobioids'. Indeed, not even the type family of Amaurobiidae was monophyletic, with some members closer to the lycosoids while others were closer to the agelenoids. The Amaurobioidea, it seems, was a bust.

Coming up - science and art, whether taxonomy is science, why family names are so awful, micro-spiders, and Parapseudoleptomesochrella almoravidensis.

REFERENCES

Coddington, J. A., & H. W. Levi. 1991. Systematics and evolution of spiders (Araneae). Annual Review of Ecology and Systematics 22: 565-592.

Griswold, C. E., J. A. Coddington, N. I. Platnick & R. R. Forster. 1999. Towards a phylogeny of entelegyne spiders (Araneae, Araneomorphae, Entelegynae). Journal of Arachnology 27: 53-63.

In Which I Reveal Just How Much of a Freak I Am

It's all there in the subtitle to this site. In the last few days I've decided to set myself a task that will probably be ridiculously time-confusing, gut-wrenchingly futile and will doubtless cause me to become even older before my time than I already am. But it's something that hasn't been done since 1923, and I think the time is ripe for it to be done again. I'm thinking of compiling an index for all described taxa of long-legged harvestmen. With a few thousand species involved, this is no small task.

But the thing is, and this is the freakish part, I actually really like nomenclature. Nomenclature is the specific part of the taxonomic process where the researcher sifts through the assortment of available names and works out which is the correct name to use for the organism sitting before them. It is important to distinguish the identification of the correct nomenclature from the identification of the organism itself - the nature of the specimen won't somehow magically change if the name attached to it does. Nomenclature is simply the system of labels that researchers have agreed to use in order to allow communication. As such, many people seem to regard the identification of the appropriate label as a somewhat arduous and uninspiring task, but personally I find it can be quite a lot of fun. As frustrating as past confusions can be, there is also something appealing in the challenge of sorting them out.

As a group, harvestmen have their share of nomenclatural challenges. I've just linked to my post on the mess that is Gagrella in which I just scratched the surface. There are no less than five taxa laying claim to the name Gagrella bispinosa as a result of its repeated use as a subspecific name. The oldest harvestman genus, Phalangium, was originally used by Linnaeus for pretty much any arachnid that wasn't a spider or a scorpion, leading to a fair number of homonyms spread between a number of orders. These are the sort of things I'd like to delve into for the next few years. Sure it's a big call, but if you can't be a little hubristic as a grad student, when can you be?

Araneidae - With Web and With Scent

The St. Andrew's Cross (Argiope keyserlingi). Photo by Louise Docker.

The orb-weavers are undoubtedly the best-known of all spiders. Ask anyone to imagine a spider and they will probably picture an orb-weaver (they may also have transcribed the words "some pig" in the web). This is something of an unfair characterisation - of the more than 100 recognised families of spiders, less than ten are orb-weavers. Still, it is one of the orb-weaving genera that holds the name of "spider", Araneus, which, as the only generic name used in Clerck's (1757) Aranei Svecici, the only taxonomic work recognised by the ICZN that predates the 1758 tenth edition of Linnaeus' Systema Naturae, is officially the oldest generic name in zoological nomenclature*. That's right - spiders came before humans. Nyeh nyeh nyeh.

*Admittedly Clerck did use the name Araneus for all spiders, not just species included in the modern Araneus.

The Araneidae are the largest family of orb-weaving spiders, with a little less than 3000 described species. They are actually a lot more numerous than you might realise - many species build their webs only at night, taking them down in the morning before hiding during the day and rebuilding the web every evening. The family is decidedly diverse in appearance - from the gaudy colours and spines of the Christmas spiders to the idiosyncratic figures of the tailed spiders to one group whose common name describes their appearance perfectly - the bird-dropping spiders.



The classic orb-web is made by first floating a line of sticky silk horizontally across a space between two anhoring points (such as a pair of branches), then running a second looser non-sticky strand along the initial strand. The spider then drops herself* from the centre-point of the second strand, trailing a third strand behind her, so that the second and third strands form a Y-shape. The vertex of the Y will be the centre of the web. The spider next constructs an outer frame, as shown above in a diagram by Ed Nieuwenhuys (the page linked to has diagrams of each of the stages in orb-web construction), then runs a series of spokes from the centre of the web to the outside. She then runs a broad spiral of non-sticky thread from the centre of the web until she reaches the outer edge. After that, it travels back to the centre laying a much tighter spiral of sticky thread, removing the non-sticky scaffold as she goes. As the sticky thread is stretched, the sticky coating breaks into a series of globules of coiled thread, which is how the web is able to be so elastic and stand up to the thrashings of captured prey. The spider herself is able to move about without being trapped by means of secretions produced by glands near the mouth with which she coats her legs. Forster & Forster (1999) refer to an experiment where the tips of a spider's legs were dipped in solvent before the spider was returned to its web. The spider initially showed great difficulty in moving due to the removal of its protective coating, though it was able to renew the covering and regain mobility. After the web has been completed, the spider will take up residence at the central hub, legs resting on the radiating spokes in order to feel for any vibrations. Araneid eyesight is almost non-existent, and orb-weavers are incapable of hunting without a web. They are perhaps the closest thing to a terrestrial filter-feeder, filtering the air for small animals.

*All spiders are referred to as female unless proven otherwise, like ships and birds of prey. It's another one of those things that make the English language so damn confusing.


An unidentified member of the genus Gasteracantha. These spiders come in a dazzling array of colours and ornamentations, and unlike many other araneids are often visible during the day, earning them such names as "jewel spider" or "Christmas spider". Colour patterns can vary significantly even between members of the same species. Photo from here.

Many araneids may vary the basic orb-web design further. Ladder-web spiders, for instance, have a long narrow web instead of the usual circle. The function of these is not really understood, though it has been suggested as a specialisation for catching moths - moths have a covering of loose scales on their wings which would normally allow them to shake off a web and escape, but it is suggested that the elongate shape of a ladder-web means that as the moth shakes off its scales, it falls onto a lower part of the web until eventually it is no longer able to escape. Many orb-weavers construct a stabilimentum, a zig-zag ladder-shaped structure that extends upwards from the central hub. Again, despite being such a distinctive structure, the function of the stabilimentum remains largely unknown, though subject to intense debate - suggested roles include strengthening the web (the original source of the name), disguising the position of the spider from predators or making the spider look bigger, attracting prey or even making the web more visible for larger animals and so reducing the risk of them walking or flying through it. One large and striking araneid found here in Australia, the St. Andrew's cross (Argiope keyserlingi), shown at the top of this post, gets its name because it builds four stabilimenta radiating from the central hub, while the spider itself sits with the front two and rear two pairs of legs held alongside each other, so the spider itself forms the eponymous cross shape.

Perhaps the most remarkable thing about araneids, however, is that despite the total dependence of most species on their webs for survival, some species no longer make them. The aforementioned bird-dropping spider (Celaenia) is so-called because its lumpy brown-and-white-splotched abdomen really does look like a lump of bird poo, offering excellent camouflage from discerning predators. Instead of constructing a full web, Celaenia simply hang from a leaf or a thread with their legs outstreched. There they catch and feed on moths (excellent pictures of hunting Celaenia can be seen at Esperance Blog). It used to be a mystery how this seemingly limited and haphazard means of capture could possibly feed the spider (after all, how many moths could reasonably be expected to pass by one point over the course of a night) until it was observed that a surprising proportion of the moths being caught (that is, all of them) were males, and that, far from passing by the spider accidentally, male moths will actually approach the spider and remain close by it until caught. It seems that the spider actually emits pheromones that mimic those of a female moth, luring their prey in with the false promise of sexual gratification (like a Trojan virus attached to a spam e-mail). The bolas spiders of the tribe Mastophoreae have refined this process further - as well as producing attractive pheromones, they also dangle a single sticky thread below themselves. When a moth approaches close enough, the spider spins the sticky thread around in the air until it sticks to the moth and they are able to draw it in. How bird-dropping and bolas spiders make their living until they become large enough to handle moths seems a little confused - Brunet (1996) claims that Celaenia construct standard orb-webs until they reach maturity, while bolas spiders produce different pheromones for attracting different-sized moths when at different ages. Forster & Forster (1999) and Yeargan (1994), in contrast, both claim that Celaenia spiderlings produce pheromones to attract psychodid midges. Interestingly, while bird-dropping and bolas spiders are both members of the subfamily Araneinae, it is debatable whether they are each other's closest relatives within the subfamily (Yeargan, 1994), so it is possible that their amazing pheromone-capture techniques could have arisen separately of each other!

REFERENCES

Brunet, B. 1996. Spiderwatch: A Guide to Australian Spiders. Reed New Holland: Sydney.

Forster, R. R., L. M. Forster. 1999. Spiders of New Zealand and their Worldwide Kin. University of Otago Press: Dunedin (New Zealand), and Otago Museum: Dunedin.

Yeargan, K. V. 1994. Biology of bolas spiders. Annual Review of Entomology 39: 81-99.

Gnah! Gagrella! Headdesk!

'Gagrella' splendens - is this the face of Evil? Photo by sswroom.

Please permit me to vent some frustration. I've written before about the ghastly legacy left to many areas of harvestman taxonomy by the work in the first half of the 1900s of Carl-Friedrich Roewer, henceforth referred to as the Antichrist of Arachnology, through his use of artificial classification systems and slipshod taxonomy. In the past, I personally have managed to remain relatively unscathed by the dark influence of Roewer, who did not deal much with the Australian opilionidan fauna. In the last few days, this has sadly changed dramatically. I have found myself wandering into the toxic wasteland that is Gagrella.

The Gagrellinae are a sizable subfamily found in tropical and subtropical Asia and the Americas. The centre of diversity for the subfamily is in Asia, whence about 700 species have been described. Many of these species are exceedingly colourful, strikingly ornamented and just downright remarkable. The problem is that most of the Gagrellinae, including Gagrella itself, have not really been revised since Roewer's massive investigation of the group, culminating in a monograph of the Asian Gagrellinae published in four parts over 1954 and 1955. As with other groups of harvestmen, Roewer used an artificial classification system based on characters such as the numbers of nodules in the legs or spines on the carapace - characters which have sometimes since been shown to not even be consistent between members of the same species, let alone the same genus (there are specimens of other taxa that, if one was to use Roewer's identification system, the left side of the animal would key out as one genus while the right side would key out as another). It is therefore quite likely that, were a full revision to be conducted, many species would have to be placed in different genera from their current position. With, as I said, about 700 species involved, this would be a mammoth task.

And yet there is a second layer of ghastliness, to make the problem even more difficult. Of the species assigned to Gagrellinae, a little less than 200 have been assigned to the largest genus, Gagrella. The type species of this genus is one Gagrella signata. However, when Roewer worked on the genus, he moved G. signata out of Gagrella and into another genus, Crassicippus, leaving the remaining species as Gagrella. Because the genus name is required to always stay with the type species, this actually meant that what Roewer called Crassicippus should have been called Gagrella, while what he called Gagrella should have been something else! Unfortunately, almost all authors following Roewer used his inaccurate sense of Gagrella, meaning that of those nearly 200 species, none of them actually belong to Gagrella unless they happen by coincidence to have been placed in the wrong genus and are actually closer to G. signata.


Unidentified South American Gagrellinae, congregating suspiciously. Photo by Bruno Buzatto.

Under strict application of the rules of nomenclature, what is now 'Gagrella' should actually be called Hexomma, that being the oldest genus name synonymised with Gagrella sensu Roewer in the past (Crawford, 1992). Unfortunately, the name Hexomma has been used as a valid genus in all of about three publications since it was first published back in 1876. Also, there are serious doubts about whether the type species, Hexomma vulcanicum, is actually identifiable - Roewer (1954), who may or may not have seen the type specimens*, claimed it to be based on unidentifiable juveniles, and the type specimen(s) seems to have since gone AWOL (Crawford, 1992).

*Roewer (1954) claims to have examined type specimens that he had borrowed from Vienna, but then eight lines later lists "Holotypus (Thorell) (Mus. Stockholm, Genua?)". As the Vienna specimens were correctly attributed to Doleschall rather than the later Thorell (albeit from the wrong publication), the Vienna record seems more likely to be accurate. Thorell later redescribed Hexomma vulcanicum as Gagrella vulcanica, and the Stockholm/Genoa specimen listing probably refers to the specimen used by Thorell for the redescription (and so not actually a holotype).

Why does all this matter to me? I am currently trying to write a description of a new species of Gagrellinae. Seeing as I am not in a position to conduct the full revision of the Asian Gagrellinae, my best option for now would be to follow the Roewer classification, despite its faults. Unfortunately, the species keys out to 'Gagrella'... What am I (or other workers on Asian Gagrellinae) to do about this huge ugly pile of taxonomic blancmange? Among the options:

1. Move everything currently in Gagrella to Hexomma: Probably not that good an idea, considering that Hexomma is not a well-defined genus and many (if not most) of the species will end up having to be moved out anyway.


2. Conserve the definition of Gagrella as used by Roewer: One option might be ask the ICZN to conserve Roewer's usage of Gagrella. Under normal conditions, this would be ideal, but in this case much the same issues apply as with the first option - what's the point with conserving Gagrella in its current condition if most of the species are probably going to end up having to move anyway?


3. Run screaming in horror from the entire concept of Asian Gagrellinae and end up crouched into foetal position and whimpering in the darkest recesses of the wet collection: At the moment, this option is looking increasingly attractive...

REFERENCES

Crawford, R. L. 1992. Catalogue of the genera and type species of the harvestman superfamily Phalangioidea (Arachnida). Burke Museum Contributions in Anthropology and Natural History 8: 1-60.

Roewer, C. F. 1954. Indoaustralische Gagrellinae (Opiliones, Arachnidae). (Weitere Weberknechte XVIII). 1. Teil. Senckenbergiana Biologica 35 (3-4): 181-236.

Spiders Losing their Lungs

Hypochilus petrunkevitchi - photo by Marshal Hedin from Wikipedia.

The current Taxon of the Week arguably has a pretty poor claim on the title, because it is no longer recognised as a valid taxonomic grouping. As I have explained before, modern spiders can be divided into three suborders or infraorders or what-have-you. The Mesothelae or Liphistiomorphae (segmented spiders) are a small group distinguishable from all other spiders by their obviously segmented abdomens. The Mygalomorphae (vertical-fanged spiders) have fangs that move straight up and down, and include the trapdoor and funnel-web spiders and American tarantulas. The largest group of spiders by far is the Araneomorphae (cross-fanged spiders), with fangs angled towards each other, including orb-weavers, cobweb spiders, jumping spiders, wolf spiders, and pretty much any other spider family you're likely to be familiar with. However, some older references may list a fourth group, the Hypochilomorphae, and it's with the latter that we're dealing today.


The Tasmanian cave-dwelling austrochilid Hickmania troglodytes. The four yellow spots visible on the underside correspond to the positions of the book lungs. Photo by Niall Doran from here.

The 'hypochilomorphs' include three small families, the Hypochilidae, Austrochilidae and Gradungulidae, that are now regarded as basal members of the Araneomorphae. Like other araneomorphs, they possess fangs that are angled towards each other rather than parallel. Where they differ from other araneomorphs is in the number of book lungs they possess. Book lungs are the ancestral respiratory structure for all arachnids, and evolved from the gills of their aquatic ancestors as they adapted to life on land. They are little more than gills recessed into the underside of the animal and covered over to prevent moisture loss, and the name "book lung" refers to their appearance in cross-section like leaves of a book. The ancestral number of book lungs in arachnids is four, though many arachnids (particularly the smaller forms, and including some spiders) have independently replaced the book lungs with tracheae, or lack any specialised respiratory structures entirely. Most araneomorphs with book lungs have lost the posterior pair and only have two book lungs. Hypochilomorphs retain the posterior pair, demonstrating their basal position to other araneomorphs and causing them to all too often be damned with the execrable title of "living fossil". However, because this is an ancestral feature rather than a derived one, it does not indicate that hypochilomorphs form a group exclusive of other araneomorphs, and other features make it clear that Austrochiloidea (Grandungulidae and Austrochilidae) are more closely related to the other araneomorphs than they are to Hypochilidae (Griswold et al., 1999). The remaining araneomorphs have usually been presented as a single clade (the Araneoclada), though at least one species of Filistatidae, Kukulcania hibernalis, possesses posterior book lungs as a juvenile, suggesting that family lost the posterior book lungs independently of other araneomorphs, and Lopardo et al. (2004) suggested that Filistatidae may be outside the Austrochiloidea + Araneoclada clade.

The Hypochilidae are large spiders found in Asia and North America. They construct a unique web for snaring prey, often referred to as a "lampshade web" in reference to its shape, though if the description in Forster & Forster (1999) holds for all hypochilids, then the photo on the Wikipedia page for this family is quite possibly upside down. According to Forster & Forster, Hypochilus builds its web on the underside of an overhanging rock, with a tightly woven upper section flaring out around the lower circular edge. Hypochilids are mostly montane species.



The Austrochiloidea are restricted to Southern Hemisphere continents - the Austrochilidae are found in southern South America and Tasmania, while the Gradungulidae are found in eastern Australia and the South Island of New Zealand (Forster & Forster, 1999). The Austrochilidae build large horizontal webs, but many Grandungulidae have abandoned web-building and become active hunters. The Gradungulidae are characterised by the significant increase in size of one of the claws on the legs, which is used to great effect in seizing prey. In one of the Australian web-building species, the cave-dwelling Progradungula carraiensis, a long, sparse web is built between the ground and an overhang, up to and exceeding a metre in height. The spider itself sits head downwards at the base of the web, low enough that the front legs are near the ground. Any suitable prey that walks by the spider is grabbed with the front legs and bitten. The prey may be eaten where it is caught, or carried up to the top part of the web that also serves as a retreat for the spider. One of the New Zealand species, Pianoa isolata, has abandoned the web but hangs down among strands of dense moss, catching its prey in a similar manner to Progradungula. A New Zealand cave-dwelling species, Spelungula cavernicola, shown above in a photo by Paddy Ryan, is an active hunter but often feeds on its prey suspended in mid-air from a silk dragline. The round egg-sacs are also hung from draglines, probably as protection from potential predators.

REFERENCES

Forster, R. R., & L. M. Forster. 1999. Spiders of New Zealand and their Worldwide Kin. University of Otago Press: Dunedin (New Zealand), and Otago Museum: Dunedin.

Griswold, C. E., J. A. Coddington, N. I. Platnick & R. R. Forster. 1999. Towards a phylogeny of entelegyne spiders (Araneae, Araneomorphae, Entelegynae). Journal of Arachnology 27: 53-63.

Lopardo, L., M. J. Ramírez, C. Grismado & L. A. Compagnucci. 2004. Web building behavior and the phylogeny of austrochiline spiders. Journal of Arachnology 32: 42-54.

Your Little Friends that are With You Always

Demodex folliculorum, male above and female below, from Desch and Nutting (1972).

At all times, you are surrounded by life. Micro-organisms swarm on your skin, swim in your gut, and set up shop in your organs. Indeed, at any one time, there are considerably more microbial cells around your person than there even are of your own cells. The micro-organisms are not living on you - you are living among the micro-organisms.

They're not all bacteria, either. I just thought that I'd briefly introduce you to one of the more distinctive micro-organisms that you are almost certainly carrying about with you - the follicle mite. I also challenge you to keep from scratching yourself while reading this.


Demodex brevis, male on left and female on right, from Desch and Nutting (1972)..

Demodex, the follicle mite, is a specialised inhabitant of the follicles and pores of mammalian skin. A significant number of species appear to have been described from different hosts, ranging from humans to dogs to cattle to marsupial mice to honest-to-goodness mice. The most distinctive feature of Demodex is its elongate body shape, which allows it to live head-first inside the follicles of its host, feeding on cells within the follicle. Humans are actually inhabited by two species, Demodex folliculorum and D. brevis (Desch & Nutting, 1972). The more elongate D. folliculorum lives in the follicles themselves, and feeds on the epithelial lining. The rarer D. brevis is shorter, and is found within the sebaceous gland on which it feeds. Demodex brevis is found on less people, and also at lower numbers - multiple D. folliculorum may be found in a single follicle (as shown below in an image from here), but usually only one D. brevis.




The question of whether Demodex causes any harm to its human host is a difficult one. The sheer universality of Demodex within the human population implies that its presence is usually of no concern to the host. However, another species, D. canis, is widely connected with mange in dogs, and Demodex has been connected with skin disorders (demodicidosis) in a number of humans. The difficult question is whether Demodex is a direct causative agent or not. As one might reasonably expect Demodex to be present anyway, it's mere presence at an infection site does not automatically indicate its responsibility for the infection. It seems likely that Demodex may be a facultative agitator of problems arising from other ultimate causes, such as a suppressed immune system (Jansen et al., 2001) or an already-damaged follicle (Pena & Andrade Filho, 2000). It is noteworthy in this light that prevalence of demodicidosis varies seasonally (it is most common in spring), but the prevalence of Demodex itself does not (Desch & Nutting, 1972). Demodex folliculorum may cause damage when more than six individuals are present in the same follicle (Desch & Nutting, 1972).

REFERENCES

Desch, C., & W. B. Nutting. 1972. Demodex folliculorum (Simon) and D. brevis Akbulatova of man: redescription and reevaluation. Journal of Parasitology 58 (1): 169-177.

Jansen, T., U. Kastner, A. Kreuter & P. Altmeyer. 2001. Rosacea-like demodicidosis associated with acquired immunodeficiency syndrome. British Journal of Dermatology 144 (1): 139–142.

Pena, G. P., & J. de S. Andrade Filho. 2000. Is Demodex really non-pathogenic? Revista do Instituto de Medicina Tropical de São Paulo 42 (3): 171-173.

Tarantulas sans Tarantella

It's interesting how different people perceive levels of risk. Someone once asked how I could be completely unafraid of spiders, but be extremely nervous around cars (I am - a friend of mine once banned me from riding in the passenger seat when she was driving, because the sight of my knuckles turning white as I gripped onto the handlebar would make her nervous). I asked him in return how I could possibly be otherwise - hardly anyone is ever seriously hurt by a spider, but cars kill large numbers of people on a regular basis. The point of that little anecdote, in case you were wondering, is to introduce a family of spiders that have provided stock horror film fodder for years, but are widely known to be fairly harmless - the Theraphosidae.



Theraphosidae are a family of large spiders found mostly in ex-Gondwanan landmasses - South America, Africa, India and Australia, as well as in south-east Asia. These are the spiders best known as bird-eating spiders or tarantulas, though the name "tarantula" originally applied to a member of a quite different family of spiders, the European wolf spider Lycosa tarantula. The photo at the top of the post (from here) shows an Australian species of Selenocosmia. The photo just above this paragraph (from Tarantulas from Uruguay*) of Theraphosa leblondi gives a good idea of the size some theraphosids reach. Theraphosidae include the largest living spiders - indeed, since the Carboniferous Megarachne was reidentified as an eurypterid, modern Theraphosidae include the largest spiders known to have existed ever.

*I rather enjoyed the Tarantulas from Uruguay page, but if you're at work you might want to be forewarned that the page does play music at you.

Theraphosidae belong to the group of spiders known as mygalomorphs. Spiders can be divided into three major groups - liphistiomorphs, mygalomorphs and araneomorphs. Liphistiomorphs are a small group found in eastern Asia that represent the sister group of all other spiders, and can be distinguished from other spiders by their retaining an obviously segmented abdomen. The other two groups of spiders can most easily be distinguished by their chelicerae (fangs). Mygalomorphs retain the more primitive condition of having the fangs directed straight up and down, and so are only able to stab down with them. Araneomorphs, by far the larger and more diverse of the three groups, have the fangs directed towards each other and are able to pinch prey or attackers between the chelicerae (the Wikipedia page for Araneomorphae has a good pair of photos showing the difference). Mygalomorphs are mostly relatively large spiders (there are a few exceptions). They also tend to be far less sexually dimorphic than many araneomorphs, with relatively little difference between males and females.

While the bites of Theraphosidae are apparently not particularly notable as far as humans are concerned, of more concern for people handling tarantulas is the presence on the abdomen of many South American species of urticating hairs - specialised hairs with minute barbs that can break off and irritate the skin of any threatening predators. Members of the subfamily Theraphosinae can even propel the hairs directly at a threat by rubbing the legs against the abdomen. Members of two genera of theraphosids have also been recorded to incorporate shed urticating hairs into the silk of egg-sacs, which was demonstrated to increase the defense offered by the egg-sac against insect egg predators (Marshall & Uetz, 1990).

The South American Avicularia metallica (image from here).

Many species of Theraphosidae are popular as pets, and females may live for up to thirty years in captivity (males, in contrast, do not survive long after mating). Unfortunately, while pet individuals of the more popular species such as the red-kneed tarantula (Brachypelma smithi) are generally captive-bred, a substantial market (in many places, such as Australia, a largely illegal market) exists in wild-caught specimens, especially of rare and unusual species. Many theraphosid species have very limited ranges, and are severely threatened by collection for the pet trade, and I have been informed that at least some Australian species have actually become extinct due to over-collection. This is especially tragic as a large proportion of the Australian theraphosid population remains undescribed, necessitating a race against time to recognise their diversity before the opportunity to protect it is lost forever.

REFERENCES

Marshall, S. D., & G. W. Uetz. 1990. Incorporation of urticating hairs into silk: a novel defense mechanism in two Neotropical tarantulas (Araneae, Theraphosidae). Journal of Arachnology 18: 143-149.

The One about Sexual Cannibalism

Sometimes, the history of people's misconceptions about organisms are nearly as interesting as the organisms themselves - not so much because of what it says about the organism itself, but what it says about us as people. One common pattern is the "pendulum swing" in conceptions about certain organisms. An originally negative over-simplification about a given organism ("wolves are savage human-killers that should be exterminated before they exterminate us"/"whales and dolphins are just fish and can be hunted as such") is replaced by a reactionary viewpoint which is more positive, but arguably just as erroneous an oversimplification ("wolves are completely harmless to humans, and would never attack somebody"/"whales and dolphins are super-intelligent, and, like, just filled with spiritual wisdom"). It is only as our attitudes mature, and the question becomes less politically charged, that opinions settle towards the generally more accurate but usually more complex middle ground.

Sexual cannibalism is one behaviour that has fallen victim to the human tendency to mythologise. Many people are aware of the idea that female spiders, mantids and other such carnivorous arthropods have a habit of eating the male during mating. Any feminist readers I have out there might be interested to consider how the popularity of this concept reflects our own attitudes on the relationship between the sexes (the female eating the male seems scandalous because, of course, we live in a society that tells us it should be the other way around). However, if you open a textbook you will probably be told that this story is generally not true. Bug Girl has recently corrected Isabella Rossellini on just this point*. For the most part, Bug Girl is right - sexual cannibalism is a fairly rare occurrence that usually only happens when things go wrong (for instance, if the male makes his move before properly placating the female). However, there is at least one species in which sexual cannibalism is an integral part of the mating process.

*Actually, speaking of societal attitudes, perhaps the most remarkable thing about the video Bug Girl has posted - other, of course, than the fact that Isabella Rossellini is making videos about arthropods in the first place - is the obvious cultural differences when it comes to talking about sex. The American interviewer with her giggling prudishness does not compare well to Rossellini's far more relaxed attitude.

That species is Latrodectus hasselti, the redback spider. The image at the top of the page (from here) shows two redbacks. The larger individual is a mature female, while the smaller white individual is a male. The genus Latrodectus has a wide distribution around the world, including the black widow (L. mactans) of North America (which does not regularly engage in sexual cannibalism), with most species having a well-deserved reputation for toxicity. The redback had done particularly well out of human civilisation - it is unclear where exactly in Australia it originated*, but it has since been spread throughout the continent, as well as establishing populations in other countries such as New Zealand and Japan. Latrodectus hasselti specialises in constructing its webs between hot, dry, facing surfaces, and humans are very good at building hot, dry, facing surfaces.

*Some people have suggested that the redback is not a native to Australia because of the absence of early records of this species, and its close association in most areas with humans. Conflicting with this is the absence of redbacks in any other part of the world, apart from areas where it has obviously been imported in Australia. It is far more likely that the redback had a much more restricted distribution in Australia prior to European settlement (probably somewhere west of the Great Dividing Range) and has since been spread to the remainder of the country.



In other Latrodectus species, the small male first climbs onto the female's web and approaches her cautiously with regular stops to vibrate the web. The female will usually chase him away a few times, but eventually she calms down and the male is able to climb onto the underside of the female as she hangs upside-down on the web, as shown above in Latrodectus hesperus (image from here). The female remains largely immobile while the male mates with her, and he is able to make his escape quite easily. In the redback, the male approaches and mounts the female as in other Latrodectus but once he has inserted his pedipalp into her he performs a back-flip that brings his abdomen alongside her mouthparts. The female, seemingly unable to resist the temptation, bites into the male and begins chewing. After a while, the male pulls away from the female and rips his badly damaged abdomen out of her grasp. Nevertheless, after a period of grooming the male returns to the female, and inserts and repositions himself as before. This time, the female does not allow him to escape - the male does not survive the second mating.

Why does the male submit himself to this fatal attack? There are a few possibilities that have been suggested. The nutrients the female receives from eating the male may aid in the development of the eggs he has fertilised. Also, the female remains mating with the male for longer while feeding on him than she would have otherwise. Not only does this allow the male more time to fertilise her eggs, it also denies other males the chance to mate with her in this time. A thesis abstract available here indicates that some males try to escape the role of victim, attempting to sneak in and mate with the female without offering themselves. However, females react even more aggressively to such cheaters, who were much more likely to be cannibalised before mating was successfully completed.

REFERENCES

Forster, R., & L. Forster. 1999. Spiders of New Zealand and their Worldwide Kin. University of Otago Press: Dunedin, in association with Otago Museum.

Taxon of the Week: Cynortula, Cynortula

Vonones ornatus, one of the few species of Cosmetidae found in the southern United States. Photo by Lynnette Schimming from Bug Guide.

The systematics of South American harvestmen have long been one of the taxonomic world's God-awful messes, with the painstaking work of Pinto-da-Rocha, Kury and associates only recently managing to go some way towards drawing it from the mire. The blame for this morass can be placed almost entirely with a single person - Carl-Friedrich Roewer, who described about a third of the world's total of harvestmen species, some 2,260 taxa. He was able to attain this prodigious output by employing a highly artificial mode of classification. Individual specimens were assigned to species and species assigned to higher taxa on the basis of quite superficial characters such as the number of sub-segments in the legs or the number of spines on the abdomen. Character systems such as genitalia that are now regarded as highly significant were not considered*. Many of the features used by Roewer have since turned out to be variable within individuals of a single species, and sometimes within a single individual - in the case of number of tarsal segments, more than one author (such as Hickman, 1939) has described specimens that have differing numbers of segments on the left side from the right, which would require that each side belong to a different genus, if not subfamily!

*To be fair, Roewer could probably be forgiven for his neglect of genitalic characters. While genitalia had been used by some authors in taxonomy by the early 1900s, the practice was not yet widespread and its importance not widely recognised.

It has to be said that the names he gave his excessive outpourings of taxa were not exactly inspired either. Many of them were derived by sticking some suffix or prefix onto a pre-existing name. For instance, from the original name Cynorta, he gave us Cynortula, Cynortoides, Eucynorta, Cynortella, Cynortellana, Cynortellina, Eucynortula, and I'll stop now before my head explodes. Trust me, there's a lot more. To quote Kury (2003), "The dreadful, uninspired and sometimes cumbersome names created by Roewer and Mello-Leitão and followers, and which are deformations of place names, people's names and (the worst!) pre-existing generic names, are best left alone."

The above-listed genera belong to the family Cosmetidae, one of the largest harvestman families in the Neotropics. While still officially divided into two subfamilies, these are divided solely by whether the claws on legs III and IV are smooth or pectinate and this distinction is not expected to stand up to proper phylogenetic analysis when someone should one ever be conducted (Kury & Pinto-da-Rocha, 2007). While the work of Kury and associates has vastly improved matters with the Gonyleptidae, the other major Neotropical family of short-legged harvestmen*, the Cosmetidae remain almost untouched by modern researchers**.

*Harvestmen fall into three groups, the mite-like, long-legged and short-legged harvestmen. I covered mite-like harvestmen once before. Long-legged harvestmen are the daddy-long-legs type harvestmen. Short-legged harvestmen are generally more heavily armoured, and while they do tend to have shorter legs than long-legged harvestmen, they probably have what would be fairly long legs for any other group of animal. The first episode of Life in the Undergrowth included footage of egg-guarding behaviour in a short-legged harvestman.

**Fortunately, I have reasons, such as the publication of Kury et al. (2007), to hope this may change over the coming years.



The genus Cynortula Roewer, 1912, as it currently stands, contains 32 species from throughout tropical Central and South America, from Mexico and the Bahamas to Bolivia and Brazil (the illustration above, from Goodnight & Goodnight, 1947, shows Cynortula granulata from Trinidad). Lord only knows what will happen to this genus in the future, however. Roewer (1923) seems to have supplied the last description of the genus, and described it as "Schlanke Tiere mit langen, dünnen Beine. 1. und 3. Area mit je 1 mittlerer Tuberkel-Paar; 2., 4. und 5. Area und 1.-3. frei Tergit unbewehrt. 2. Chelicere-Glied auch beim ♂ klein und normal gebaut oder seltener beim ♂ viel dicker als beim ♀ unten oben das 1. Chelicere-Glied weit überragend. Beine: die basal Glied des 3. und 4. Bein auch beim ♂ von gleichem Habitus und gleicher Stärke wie die des 1. und. 2. Bein; Endabschnitt des 2. Tarsus 3-gliedrig; 1. Tarsus 6-gliedrig; 2.-4. Tarsus jeweils mehr als 6-gliedrig, variabel. Sekundäre Geschlechtsmerkmale des ♂ bisweilen am 4. Bein."* This roughly translates (if I translate it correctly through the gibberish of BabelFish) as "Slim animals with long, thin legs. 1st and 3rd areas always with 1 central pair of tubercles; 2nd, 4th and 5th areas and 1st-3rd free tergites unarmed. 2nd cheliceral segment of ♂ small and normally built or more rarely with ♂ much larger than ♀. Legs: basal segments of 3rd and 4th legs the same as 1st and 2nd legs; Final section of 2nd tarsus 3-segemented; 1st tarsus 6-segmented; 2nd-4th tarsus in each case more than 6-segmented, variable. Secondary sexual characteristics sometimes present in 4th leg of ♂." For those not familiar with variation in harvestmen, that's not a very impressive list of distinguishing features. In fact, in Roewer's key to the Cosmetidae, only one character is used to key Cynortula out from similar genera - whether the dorsal ornamentation is a tubercle (Cynortula) or a spine (other genera). Not convincing.

*If there are any German speakers reading this, I apologise profusely for the errors that I have no doubt are all through that. As a result of its publication not too long after the Great War, with materials in short supply in Germany, Roewer (1923) was condensed as much as possible for publication and hence is entirely composed in a series of arcane abbreviations. Any grammatical errors are therefore probably the result of my attempts to restore the description to a readable form.

REFERENCES

Goodnight, C. J., & M. L. Goodnight. 1947. Studies of the phalangid fauna of Trinidad. American Museum Novitates 1351: 1-13.

Hickman, V. V. 1939. Opiliones and Araneae. British, Australian and New Zealand Antarctic Research Expedition Reports Series B 4: 157-188.

Kury, A. B. 2003. Annotated catalogue of the Laniatores of the New World (Arachida, Opiliones). Revista Ibérica de Aracnología, special monographic volume 1: 1-337.

Kury, A. B., & R. Pinto-da-Rocha. 2007. Cosmetidae. In Harvestmen: The Biology of Opiliones (R. Pinto-da-Rocha, G. Machado & G. Giribet, eds.) pp. 182-185. Harvard University Press: Cambridge (Massachusetts).

Kury, A. B., O. Villarreal-Manzanilla & C. Sampaio. 2007. Redescription of the type species of Cynorta (Arachnida, Opiliones, Cosmetidae). Journal of Arachnology 35 (2): 325-333.

Roewer, C. F. 1923. Die Weberknechte der Erde: Systematisches Bearbeitung der bisher bekannten Opiliones. Gustav Fischer: Jena.

Taxon of the Week: Eye of the Spider

I'm afraid that today's "Taxon of the Week" must needs be a short one. I'm tutoring a lab course on invertebrate surveying this week, so I don't have the time to write an extensive post. I can only give you a whirlwind introduction to the lynx spiders of the family Oxyopidae (image above from Wikimedia).



Oxyopidae are one of the families of hunting spiders - that is, rather than building a web to catche prey in, they actively hunt for small insects. The name "lynx spider" is apparently supposed to refer to their sharp eyesight, though there seems to be some doubt as to just how sharp their eyesight is compared to, for instance, the jumping spiders of the Salticidae. Still, like other families of hunting spiders, Oxyopidae have all of their eyes directed more or less forward, and a sharp downwards bend to the front of the prosoma means that the four central eyes are looking straight ahead, as shown spectacularly well in the picture above from Ed Nieuwenhuys. In both the pictures above, if you look closely you may also make out the long spiny hairs sticking out at right angles from the legs that also seem to be characteristic of this family.



While they may not build webs for catching prey, female lynx spiders do use silk to protect their eggs, which they stand guard over until the eggs are well-developed.

Cyphophthalmids Wait for the Mountain to Come to Them

Carl Zimmer beat me to it. I was planning to announce the recent pettalid work after the paper arrived in the mail last week, but it seems I've been scooped. But because good work always deserves a second look, I'll write on it anyway. Besides, I was at least able to pinch the photo from Carl's site.

Pettalidae are a family of Cyphophthalmi, what are called the mite-like harvestmen. Cyphophthalmids are a fairly small group as far as is known, with probably less than fifty described species, but the number of species has been rapidly increasing in recent years. Though they are divided into about five families, cyphophthalmids are a fairly conservative bunch in appearance - the photo above is of Pettalus cf. cimiciformis*, but it is fairly typical of the group as a whole. They are quite distinct from other harvestmen (in fact, it is generally agreed that they are the sister-group to all others), and rather than having the spindly build of more familiar members of the order, cyphophthalmids are minute, stocky armoured tanks. If you look closely at the picture above, you may see a light spot on either side of the body that looks a bit like an eye, but it is in fact an ozophore - a raised mound bearing the opening of a stink gland. Except for members of the family Stylocellidae, cyphophthalmids have been described in the past as eyeless, but SEM studies of Pettalidae have revealed minute (often lens-less) eyes hidden on the side of the ozophore (Boyer & Giribet, 2007).

*For those who aren't already in the know, the 'cf.' in the name stands for the Latin confer (compare). In this case, it indicates that the animal in question is very similar to Pettalus cimiciformis, but is not definitely a member of that species.

The really interesting thing about cyphophthalmids (beyond their own intrinsic charm, of course) is their distribution patterns. Each of the various families has a definite, disjunct distribution (Boyer et al., 2007). The family Stylocellidae are restricted to south-east Asia. The Sironidae are found in what once was Laurasia - Eurasia and North America. The Neogoveidae are found in Florida, tropical South America and tropical West Africa - the tropical parts of what once was Gondwana. Two genera placed in their own families, Ogovea and Troglosiro, are found in West Africa and New Caledonia, respectively. And Pettalidae has a classic Gondwanan distribution, found in southern South America, southern Africa (including Madagascar), Sri Lanka, Australia and New Zealand (see Carl Zimmer's post for a map).

I think I should say something here about "Gondwanan" distributions. Science has a tendency to go through fads like any other aspect of human culture. For many years, most organisms showing what we would now call a "Gondwanan" distribution were interpreted as relicts of a former world-wide distribution. As acceptance of "continental drift" and recognition of the previous existence of Gondwana increased, more and more researchers considered its potential significance for modern biogeography. Needless to say, the significance was especially apparent to workers in the southern continents, doubtless not without some aspect of asserting the importance of the all-too-often neglected Southern Hemisphere biota relative to the Northern Hemisphere. Gondwanan origins became the next big thing for everything from birds (Cracraft, 2001) to beeches (Linder & Crisp, 1995) to butterflies (de Jong, 2003). In the last few years, the pendulum has begun to sway the other way, probably towards a more reasonable median.

The idea of a Gondwanan distribution for a given group of harvestmen particularly merits a critical look. The fossil record of harvestmen is pretty abysmal relative to the age of the group, but what record there is speaks of a remarkable degree of morphological conservatism. The Carboniferous long-legged harvestman Brigantibunum is almost indistinguishable from modern taxa (Dunlop & Anderson, 2005). The cyphophthalmid Siro platypedibus Dunlop & Giribet, 2003, from Bitterfeld amber (probably Oligocene or Miocene in age) is so similar to modern species that it is included in a modern genus.

In order to test whether the distribution of Pettalidae is an actual Gondwanan distribution as opposed to a relictual one, Boyer et al. (2007) tested the phylogeny of the family with just about every morphological and molecular method imaginable. They demonstrated that most of the cyphophthalmid families were monophyletic, with distribution matching phylogeny (the exception was the Laurasian Sironidae, which came out paraphyletic to the northern Gondwanan Neogoveidae and Stylocellidae).

To add another level of interest to the whole deal, most of the genera within Pettalidae each have separate geographic distributions (Boyer & Giribet, 2007). Chileogovea in South America, Purcellia and Parapurcellia in southern Africa, Pettalus in Sri Lanka, Karripurcellia in Western Australia, Austropurcellia in eastern Australia. The exception is New Zealand. New Zealand has a remarkable diversity of Pettalidae, with more described species than everywhere else combined, in three genera. But let's look a little closer. In the South Island of New Zealand, the genus Rakaia is concentrated in the east, while the genus Aoraki is found in the west from Mount Cook* north. New Zealand actually lies on the boundary between the Indo-Australian and Pacific plates, and if you were to map the distributions of the genera, you would see that Rakaia is mostly found on the Pacific plate, while Aoraki dominates on the Indo-Australian!**

*The Maori name for which just happens to be Aoraki. Not a coincidence - the genus was named after the mountain.

**I know, I said three genera. The third genus is a single species, Neopurcellia salmoni, in the southwest of the South Island.

REFERENCES

Boyer, S. L., R. M. Clouse, L. R. Benavides, P. Sharma, P. J. Schwendinger, I. Karunarathna & G. Giribet. 2007. Biogeography of the world: a case study from cyphophthalmid Opiliones, a globally distributed group of arachnids. Journal of Biogeography, in press.

Boyer, S. L., & G. Giribet. 2007. A new model Gondwanan taxon: systematics and biogeography of the harvestman family Pettalidae (Arachnida, Opiliones, Cyphophthalmi), with a taxonomic revision of genera from Australia and New Zealand. Cladistics 23: 337-361.

Cracraft, J. 2001. Avian evolution, Gondwana biogeography and the Cretaceous-Tertiary mass extinction event. Proceedings of the Royal Society of London Series B – Biological Sciences 268: 459-469.

Dunlop, J. A., & L. I. Anderson. 2005. A fossil harvestman (Arachnida, Opiliones) from the Mississippian of East Kirkton, Scotland. Journal of Arachnology 33: 482-489.

Dunlop, J. A., & G. Giribet. 2003. The first fossil cyphophthalmid (Arachnida, Opiliones) from Bitterfeld amber, Germany. Journal of Arachnology 31: 371-378.

Jong, R. de. 2003. Are there butterflies with Gondwanan ancestry in the Australian region? Invertebrate Systematics 17: 143-156.

Linder, H. P., & M. D. Crisp. 1995. Nothofagus and Pacific biogeography. Cladistics 11: 5-32.

What is a Daddy-Longlegs?

"Daddy-longlegs" is one of the worst animal names there is. The name is widely used and generally recognised, but causes endless confusion because it is applied to no less than three very different animals. In a brief attempt at disambiguation, these are the animals involved:

First are harvestmen of the order Opiliones (picture from UMMZ). Harvestmen are often confused with spiders, but the body is not divided into a cephalothorax and abdomen, the opisthosoma (the posterior part of the body corresponding to the abdomen) is externally segmented, the chelicerae (mouthparts) are pincers rather than fangs, and harvestmen do not produce silk. The name "daddy-longlegs" as applied to harvestmen usually refers to the group known as "long-legged harvestmen" (Palpatores). There is some uncertainty about whether Palpatores are a monophyletic group, but that's a subject for another time.

Second are spiders of the family Pholcidae (picture is from Iziko Museums of Cape Town - the object the spider is holding is the egg-sac, which is carried by the female until the eggs hatch). Pholcids are true spiders, and so have a divided body, an unsegmented abdomen, fangs, and produce silk. Here in Australia and New Zealand, the 'daddy-longlegs' that are almost ubiquitously found in houses (particularly bathrooms) are pholcids, most often the introduced Pholcus phalangioides. Offhand, there is a common belief that daddy-longlegs (either pholcids or Opiliones) are "the most poisonous spiders in existence, but their fangs are too small to pierce human skin". I have come across this story many times, and have even been assured of it by people who really should know better. This story is absolute bunkum. The University of California, Riverside site has more info.

Finally, the third group accused of being 'daddy-longlegs' are crane flies of the family Tipulidae (picture from Wikipedia). Crane flies look a bit like giant mosquitoes, but they are not blood-suckers. They are a large family - the adults are nectarivores or do not feed, while the larvae, commonly called leatherjackets, feed on vegetation. Crane flies are easily distinguished from the other 'daddy-longlegs' - the wings are a bit of a give-away.

Another word on arachnid phylogeny

Shultz (2007) has just published a new paper on arachnid phylogenetics, based on morphology. As you can see if you scan my profile blurb, I'm currently working on arachnids - specifically on harvestmen (Opiliones), so I'm always happy to see work on them. Shultz (2007) sits in couterpoint to the most recent other publication on arachnid high-level phylogeny, Giribet et al. (2002), which used combined molecular and morphological data.

I do feel the need to make a few comments on character coding. One of the issues with high-level morphological phylogenetics is that it becomes increasingly difficult to code characters without interpretative bias. One example of this that I can spot in Shultz (2007) is his coding for the tracheal system (character 126). In the past, presence or absence of a tracheal system has generally been treated as a single character. Shultz argues on the basis of differences in the layout of the tracheal system that it has probably evolved independently a number of times (an idea that does gain some support from the definitely independent evolution of a tracheal system in some spiders), and codes the different tracheal systems as different character states. While Shultz is likely to be correct that the different tracheal systems have evolved independently, his coding of the systems separately a priori excludes the possibility of their homology. Also, in his comments on presence of a penis (character 160), a synapomorphy of Opiliones, Shultz notes that 'A clearly homologous structure is present in Cyphophthalmi (Opiliones) and apparently functions in depositing a spermatophore in the female’s genital chamber', then follows with 'The ‘penis’ in Oribatida is really a spermatopositor; it functions in construction of a spermatophore'. Is it really justifiable without prior phylogenetic expectations to code the cyphophthalmid structure as a 'penis', but not the oribatid structure?

Shultz uses a few exemplars from each of the living arachnid orders, as well as a fossil exemplars of a number of them (he includes more fossil taxa than Giribet et al.) plus the fossil Eurypterida, Trigonotarbida and Plesiosiro. An analysis is run without fossil taxa, then one with. At first glance, the inclusion or exclusion of fossil taxa has a significant impact on topology. Without fossils, and using Xiphosura (horseshoe crabs) as an outgroup, the recovered topology is (Palpigradi ((Ricinulei (Anactinotrichida, Actinotrichida)) ((Araneae (Amblypygi, Uropygi)) ((Scorpiones, Opiliones) (Pseudoscorpiones, Solifugae))))). However, through in a few fossil taxa and you get ((Scorpiones, Opiliones) (Palpigradi, (Actinotrichida (Ricinulei, Anactinotrichida), ((Pseudoscorpiones, Solifugae) (Araneae (Amblypygi, Uropygi)))))*.

*I wanted to use actual trees for this section, if only in ASCII format, because that would be one hell of a lot easier to read, but it looks like they won't show up properly in the final page. if anyone knows of a way I can put in trees on this site, I'd be ever so grateful to hear it).

Actually these two topologies are nowhere near as different as they appear - the support values for most supraordinal clades are ghastly. If we collapse all nodes in the final tree with less than 50% support, we get (Scorpiones, Opiliones, Palpigradi, (Ricinulei, Anactinotrichida, Actinotrichida), Solifugae, Pseudoscorpiones, (Trigonotarbida, Araneae (Plesiosiro (Amblypygi, Uropygi)))). To somewhat mitigate the drawbacks of this low support, however, Shultz does test his results against other past theories.

Shultz runs his neontological data set through a number of analyses constrained to recover particular clades. Clades suggested in the past that appear in trees only one step longer than Shultz's most parsimonious tree are Ricinulei + Anactinotrichida, Megoperculata (Palpigradi + Araneae + Amblypygi + Uropygi) and Rostrosomata (all arachnids except Scorpiones and Opiliones). The last one, notably, is what is recovered when palaeontological data are included. Scorpions sister to all other arachnids is only two steps longer, as is Micrura (arachnids except Scoropiones, Opiliones, Pseudoscorpiones and Solifugae). It is a little disappointing that these comparisons are run on the neontological data set alone rather than the complete data set, considering that the fossil taxa are not without influence on the result. Two comparisons are made using the full data set, testing a scorpion + eurypterid clade and a trigonotarbid + ricinuleid clade (the latter possibility was found by Giribet et al., 2002). Both possibilities are noticeably longer than the preferred tree.

A notable absence from Shultz (2007) is the Pycnogonida. Pycnogonids or 'sea spiders' are patently bizarre marine animals of very obscure relationships. Traditionallly they have been regarded as basal chelicerates owing to their possession of chelate pre-oral appendages, and many authors still support this view. Other authors regard pycnogonids as the sister group to all other living arthropods. When pycnogonids were included in the analysis of Giribet et al. (2002), they appeared in a completely unexpected position as sister to Palpigradi, within Arachnida. My impression on reading Giribet et al. is that the authors themselves are extremely sceptical of this result, and seem more inclined to attribute it to the high level of autapomorphy in pycnogonids. While it would have been interesting to see Shultz test the position of pycnogonids, it is possible that said degree of autapomorphy may have simply blown Shultz's analysis out of the water.

REFERENCES

Giribet, G., G. D. Edgecombe, W. C. Wheeler & C. Babbitt. 2002. Phylogeny and systematic position of Opiliones: a combined analysis of chelicerate relationships using morphological and molecular data. Cladistics 18: 5-70.

Shultz, J. W. 2007. A phylogenetic analysis of the arachnid orders based on morphological characters. Zoological Journal of the Linnean Society 150 (2): 221-265.