Uaraneida and the origin of silk in spiders

I do quite a bit of writing on spiders. Many entomologists deal with spiders, mites and other arachnids in their everyday working life. They’re important, ubiquitous predators. To this entomology undergrad, they’re essentially honorary insects.

Everyone knows spiders spin webs. They can be organized and beautiful, such as this orb weaver web

Orb-web weaver (Some Araneus species, probably) hanging out in it's web. Source: Wikipedia commons

They can be complex and multi-layered, like this Latrodectus geometricus web

Latrodectus geometricus in it's web. Source: Tolweb's Latrodectus geometricus page

They can even be as simple as a non-sticky sheet of silk with a hide like this Agelenopsis web.

Agelenopsis web. The hide where the spider lives is on the left by the paneling. Source: Wikipedia commons

Some spiders, like tarantulas, don’t even spin webs. They merely live in burrows lined with silk and use silk as tripwires to catch prey which stumbles by. This Poecilotheria formosa is an example.

Poecilotheria formosa mature female. Source: Personal, private collection.

Even spiders which don’t make their own retreat, such as the Saltacids use silk to cover their eggsacs and for guidelines to re-trace their steps.

Silk is an integral part of the lives of many spiders. They all spin silk from glands in their abdomen (opisthosoma) and this is their defining characteristic. This stuff has many uses. For many, such as those orb weavers above, this is their main means of finding food. Others use silk mostly as burrow lining. Many actually use silk to close off their burrows when moulting, perhaps to protect them from predators. Some spiders such as Liphistius sp. use silk to line their burrows, detect prey and to camoflauge their burrows by building a trapdoor over them.

Here’s a better picture of a similar Liphistius species, Liphistius malayanus

Liphistius malayanus. Photo courtesy of Tom Patterson, reprinted with permission.

That doesn’t look like a normal spider, does it now?

There are three sub-orders of spiders. The Araneomorphs, the Mygalamorphs and the Mesothelae. The Araneomorphs are the new kids on the block…evolutionarily speaking, they’re the youngest lineage. In this post, they’re represented by the first three pictures. The the next oldest is the Mygalamorphs which is represented by the Poecilotheria formosa. The third lineage, and the oldest, is the Mesothelae.

The Mesothelae in this post is represented by the Liphistius above. These spiders are very different from the Mygalamorphs and Araneomorphs (Opisthothelae) by two very important characteristics. First, they retain the primitive abdominal segmentation we see in spider ancestors (we’ll get to this soon, don’t worry) and they also are lacking something most folks associate with spiders-they are venomless. They’re not exactly harmless, though. Those chelicerae are strong enough to not only subdue their prey, but one of those spiders can bite you hard enough as to need stitches to close the wound.

Photo of Liphistius malayanus abdominal plates showing prominent macrosetae. Photograph reprinted with permission of 'Erigo' from arachnoboards.com.

The structures which produce in spiders are modified setae. These are the hollow hair-like structures which look like hair and make tarantulas and other spiders appear fuzzy. They’re normally concentrated on limbs, more specifically the finger like projections on their abdomen where the silk comes out that we call spinerettes. Some produce silk from their tarsal pads (the pads at the end of their legs) to help grip whatever surface they’re sticking to.

So…what makes Attercopus fimbriunguis a fossil so unusual?

It has silk spigots, but not spinerettes. The fossil has silk spigots along the outside edges of the abdomen, but they’re not centralized. Whereas silk in modern spiders comes out in lines, this guy spun sheets. It didn’t spin webs, but there were many other uses for the silk this guy spun…some of which I listed above. The silk it spun would have been useful for web-lining, eggsac covering, guidelines lines to help navigate or even possibly tripwires to help detect prey outside of the burrow.

Fig. 1. Attercopus fimbriunguis, Devonian of New York (localities: G, Gilboa; SM, South Mountain), macerated from matrix with HF and slide-mounted. (A) First-described “spinneret,” G 334.1b.34; darkness of cuticle reflects number of layers, so this fragment is folded over twice. (B) Palpal femur, SM 1.11.12; arrow indicates patch of distinctive spinules. (C) Piece of cuticle from corner of opisthosomal ventral plate showing setae, spigots, and possible silk strand, SM 1.11.4. (D) Close-up of E showing possible silk strand emerging from spigot shaft, SM 1.11.4. (E) Flagellar structure with 12 segments (including possible distalmost) from original Gilboa locality; segments show distal collars and setae, G 334.1a.4. (F) Close-up of cheliceral fang showing a number of holes (arrowed), the most distal of which had been interpreted as a venom-gland opening, G 329.22.9. (Scale bars: 0.5 mm, except F, 0.25 mm.)

As if that wasn’t enough, there’s another feature found on Attercopus which is completely foreign to most spiders, and that’s the flagella pictured in detail in plate E above. No other spider has this, and that puts this critter in an order, Uranaeida, all it’s own. There’s another specimen similar enough to Attercopus to be in this group, and it’s a fossil which was originally mistaken for a Mesothelae spider, Permarachne…and this is shown below.

Fig. 3. Paleozoic Araneae and Uraraneida. (A–C) Permarachne novokshonovi, Permian of Russia, PIN 4909/12. (A) Holotype part in rock matrix. (B) Explanatory drawing of A. (C) Close-up of flagellum showing whorls of setae. ch, chelicera; cx, coxa; fe, femur; mt, metatarsus; pa, patella; pl, ventral plate; st, sternum; ta, tarsus; ti, tibia. (D) Palaeothele montceauensis, Carboniferous of France, In 62050a, X-ray CT scan showing appendages buried in the rock matrix; note, anal tubercle (arrowed) is not a flagellum. (Scale bars: B, 1 mm; C and D, 0.1 mm.)

It has all the features of Mesothele spiders, and that flagellum was originally misinterpreted as a spinerette because of this. However, the discovery of Attercopus gave the researchers a reason to go back and re-analyze Permarachne. Specifically, they looked at how the flagella was attatched and how it was segmented. Turns out it was attatched to the critter in a different way than a spinerette (the middle as opposed to the sides for spinerettes) is and it was more highly segmented than a spinerette should be. It also didn’t have silk spigots…which means the evidence points to a flagellum.

Another fossil, Palaeothele montceaunsis, was analyzed to determine if there was a flagella present. There was a slight tubricle found, but no flagella so this specimen retains it’s status as Mesotheliae.

So we know now that the spiders most likely evolved from Uranaeida because it has features which are unique to spiders, as well as features present in other groups. It’s actually worth pointing out that there’s another extant group which has a long, multisegmented flagella and that’s the Uropygids.

Mastigoproctus giganteus from my private collection

These guys are neat. In this order, the flagellum is actually used to spray concentrated acid at whatever pisses them off. I have actually gotten mild acid burns from cleaning these cages.

At this point, it’s still uncertian whether the flagella is a convergent feature or if the Uaraneida evolved from the Uropygids. It’s possible that Uranaeida are an offshoot of the Uropygids as Aranae are an offshoot of the Uranaeida and it’s also possible that the flagellum is derived from a far more distant distant ancestor shared by the Uropygids and the Uraneida. However, these fossils do help us shed light on the origins of these enigmatic and beautiful creatures.

P. A. Selden, W. A. Shear, M. D. Sutton (2008). From the Cover: Fossil evidence for the origin of spider spinnerets, and a proposed arachnid order Proceedings of the National Academy of Sciences, 105 (52), 20781-20785 DOI: 10.1073/pnas.0809174106