Friday arachnid blogging: Spot the gnaphosid

To warm you up on these cold wintry days, here is a spider from the sunny Mediterranean. She is Poecilochroa, probably P. variana.
 

Members of this genus can be found in most of Europe, bar the northernmost bits and the British Isles. The two commonest species are P. variana and P. conspicua. If you're a real pro, you'll tell them apart by peering at their genitals through a microscope. The females of most spiders have something called an epigynum. This is a sclerotised plate covering the female genital opening. It corresponds to the complicated tips of the male's pedipalps, and together they work like lock-and-key. P. variana's epigynum is flat and very lightly sclerotised, not much more than a slit; by contrast, P. conspicua's is heavy and arrow-shaped. If you're at all thrilled by the thought of spider genitalia, you can see drawings of Poecilochroa epigyna and palpal tips at the Spinnen Mitteleuropas website (the upper reaches of this site exist in an English version, but when you get down to species level, some pages, including this one, haven't yet been translated from the original German).

I'm not a real pro, though, and I don't have a decent microscope. (My dream model, from Zeiss, costs over €20,000 with all bells and whistles, and even a decent used binocular dissecting scope from the old East German Zeiss would likely set me back nearly a thousand.) What I do have is a 'microscope' that looks like a fat fountain pen. Its magnifying powers are adequate for looking at most bits of all but very small spiders, but it's extremely hard to use. So I spared my spider the embarassment of a genital examination, and instead decided she is P. variana because she has that white spot at the end of her opisthosoma (abdomen); P. conspicua doesn't.

That's a quick and dirty method of determination, of course, and it could well be wrong. Though P. variana and P. conspicua are common and widespread in Europe, Fauna Ibérica records four other species of Poecilochroa in Spain. I don't know which of these live in the Balears (the spider above is from Formentera), but P. albomaculata, at least, is recorded in Corsica as well as Spain so it's reasonable to think you can find them on an island in between. And the white spot on the prosoma (i.e., the front bit, sometimes called the 'cephalothorax'; the white spot covers the 'head' bit, distinguishing it from the 'thorax' bit) does look like the sort of thing that might have prompted the epithet 'albomaculata'. But I have access to neither pictures nor diagnostic drawings of P. albomaculata (or, for that matter, the other Poecilochroa spp. recorded in Spain), so I am going to have to go with variana as a tentative best guess. (Though I'd welcome correction from any Iberian arachnologists who might happen upon this post).

But maybe you're not so interested in spiders that you care which species of Poecilochroa this is. Even so, when a spider crosses your path, you've probably found yourself wondering what kind it is. Focusing on the slightly bigger picture, the spider above is a gnaphosid. The Gnaphosidae are a family of medium-sized to fairly biggish spiders. They are hunters, not web-spinners. They tend to be solidly built, with heavy armour on their prosoma and legs; and, though our Poecilochroa is attractively patterned, many gnaphosids are drab. (An earlier edition of FAB showed a typical representative of the family.) Drab or not, though, there's an easy way to tell whether what you are looking at is a gnaphosid. And you won't even need a microscope (though with smaller examples you might want a magnifying glass). And that way is to look at their bums.

A spider's silk comes out of little nozzle-like things called spinnerets. Originally, these were appendages much like legs. Just as evolution has modified appendages up front into the spider's 'jaws' (and its pedipalps, whose derivation is plain to see), it has modified appendages just behind the legs into spinnerets. (In most spiders, these appendages have migrated bumwards, but in a few 'primitive' sorts you'll still find them at the bottom of the belly just behind the legs.) And the number, position and/or shape of the spinnerets can be an important clue for determining spiders at taxa above species.

A gnaphosid's spinnerets are large and cylindrical. Here, let's have a look:

And here's an even closer look. I must apologise for the fuzziness of the image, but this is a tiny piece cropped from a much larger photo, and we are running up hard against the limits of 72 pixels/inch resolution. Squint a bit, if that helps. (And try to avoid thinking that you could squeeze silk from them as you'd squeeze milk from a cow's similar-looking teats. Your hands are much too big; and if they weren't, well -- gnaphosids are less docile than cows.) Now, what's so important about those cylindrical spinnerets? Well, there is another large family of similarly-sized, similarly-shaped, ground-dwelling, mostly drab hunting spiders called the Clubionidae. The quickest way to tell gnaphosids from clubionids is to look at the spinnerets. Whilst the gnaphosid's are cylindrical, the clubionid's are long tapering cones. There are other visual clues that differentiate the two families -- e.g., the eye pattern and the look of the chelicerae ('fangs') -- but none is quicker, easier or as sure as the shape of the spinnerets.