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.