Adventures in Spider Misidentification: Spitting Scytodes

Spitting Spider Scytodes cf thoracica (Latreille, 1802)

Finished sweeping yesterday's snow off the walkways, had my coffee, and now it's time to get back to work. Sunday should be, at least in part, a day of rest, even if a deadline is fast approaching. Chapters won't write themselves, of course, but this week looks like working at a computer would be preferable to being outside. Average weather in Edmonton is an oxymoron: Pacific Lows and Arctic Highs push back and forth showering us with extremes. This week looks to be a depressing and Vitamin D-free seesaw of snow, sleet, ice and sunless days. So how about a quick dissemble to a sunnier clime with an ancient photo of a Spitting Spider?

Looks like a week for the snow shovel and ice scrapper

I think this is the cosmopolitan Scytodes thoracica (Latreille, 1802) and, although she inhabited our closet in Brisbane, one might find a similar slowly stalking spitter in many closets in North America. The Spitting Spider spits silk from its chelicerae (fangs) to tie its victims down before eating them. At first this seems strange: silk should come from spinnerets at the other end of the body. But, like most things arthropods do, it is really the jointed limbs that are the key. The fangs are just the first pair of limbs modified for feeding: spinnerets are derived from couple of pairs of more posterior limbs. Arthropods are composed of a series of what were originally very similar segments, each with a pair of limbs and each with the genetic potential to produce silk (silk is, basically, an excretory product). Scytodes are unusual spiders only in that they have turned on the silk producing abilities of the segment bearing their fangs.

More than somewhat alien-looking and silk issues from either end

But in a sense, this is reverting to what most silky arachnids do. Pseudoscorpions and mites, for example, also produce produce silk from their mouthparts and some mites use silk to trap prey (Spinibdella, a genus of snout mites being one such). Pseudoscorpions use their silk to make nests in which to hide away, as do many mites.

Silken cells under bark and some exposed pseudoscorpions

Of course, the gardener's least favourite mites, spider mites, also spin silk, but from their second pair of limbs, the pedipalps. Unfortunately, spider mites feed on plants and not on each other, but use silk much like many arachnids - to make nests and even to disperse on the wind - a method analogous to ballooning in spiderlings.

Two-spotter Spider Mites resting, moulting, and laying eggs in their web 

Come to think of it, we tend to assume what is most common is normal. Spiders are our common experience of silk spinning in arachnids and arthropods in general, but they are the only ones I can think of that produce silk from limb remnants near the middle of their body (over time, the spinnerets have become less limb-like and moved rearward - see the 'living fossil' Mesothelae). Tarantulas were once rumoured to produce silk from their feet, but this hypothesis seems to have come from too much Spiderman and not enough experimental rigour. Even caterpillars produce silk from their mouthparts and the only insect that I know of that has silk producing legs (Embioptera) uses its front feet.

Sticky feet yes, but not silky

Well, enough bludging for a Sunday morning - but not all in vain. I discovered a lapse I'd make yesterday about where silk in sea spiders (Pycnogonida) comes from - and learned that spiders are stranger than I thought.