Friday arachnid blogging (outsourced version)

Today I'll let somebody else do the FABing for a change. Dinesh's Spiderblog links to a page about the tetragnathid spider Pachygnatha zappa Bosmans & Bosselaers 1994. (The linked page, it appears, is part of a site devoted to organisms named for the late musician Frank Zappa.)

The names of Robert Bosmans and Jan Bosselaers are familiar to anybody in Europe (and probably far beyond) with an interest in arachnology. If you were wondering why they settled on Zappa as a namesake for this spider, they explain that it's not just that they happened to be listening to Gregory Peccary when they collected their specimen. No, they urge us; find yourself a female, flip her over and have a look at her belly. There you will see two dark markings that, together, look remarkably like Frank's moustache and soul-patch. (Are the spiders at all common, I wonder? The markings are clearly a miraculous sign, like Our Lady of the Grilled Cheese; but if anybody can scoop up a handful of these creatures in his garden, there's little chance of fetching a decent price on eBay.)

The page is well worth a visit. Not the same thing as reading the original description, of course. You'll find that in Zoologica Scripta 23(4), though this number is from long enough ago that it is not archived online, so you will not have the chance to pay Blackwell's lots of money for it and will have to go to the library instead (assuming you are one of those few benighted souls who do not maintain a full bound set of the Scripta at home). Still, the page does tell you a bit about the spider and has a couple of wonderfully clear drawings by Jan Bosselaers.

The second of these shows the underside of the female's belly. Go on, you'll see: B&B are right about that moustache.

Of even more interest is the first drawing, which shows the male of the species. You can tell he's a male by his pedipalps. As a faithful reader of FAB, you already know that these are (among other things) his organs of generation and normally look, to the naked eye, like tiny boxing gloves at the palps' ends. What you might not have known is that male spiders, like some men, are capable of 'getting wood'. When the spider is in the mood, the tips of his palps swell and distend. You don't really see it when the boxing gloves are in storage mode, but they comprise a number of sclerotised plates as well as erectile tissue. When expanded, the plates form a sort of complicated key that fits into the 'lock' of the female's epigynum. If you have never before had a close-up look at a randy spider, here is your chance.

And just abaft the palps you will notice the jaws (it would be hard to miss them). Technically, they're called chelicerae. These are the first in a series of paired appendages that develop in the larval spider. In the original chelicerates (the group to which the spiders belong), they were pincers, rather like a hand with a thumb and one finger. If you are ever lucky enough to find a horsehoe crab (not a crab at all but rather a large and 'primitive' marine chelicerate that turns up annually on beaches in, I believe, three parts of the world, including the northeastern seaboard of the United States), flip it over. (They might look fearsome, but they won't hurt you.) The first pair of appendages are chelicerae pretty much in the original form. If you live in the desert, maybe you've seen a solpugid (sun spider, camel spider). These are not spiders but their relatives, and they too have something like the 'original model' chelicerae, as you may see in this picture from Phillip Glogoza's solpugid website. Unlike spiders, they have no venom, but their jaws are large and very strong, so handle with care; they can give you quite a nasty nip if they feel threatened.

In spiders, by contrast, the chelicerae have been highly modified from the ancestral form. The chelicera is no longer a box-like 'hand' with two opposed pincers sticking out. One of the pincers has been lost altogether, and the other is bent back (often fitting into a groove in the cheliceral base, or what's left of the 'hand'). The derived pincer is now a fang; a hypodermic syringe, really, for injecting prey with venom. In some spiders, the fang is also used to 'chew' the prey by mashing it up against the base of the chelicera, which may even have spiny 'teeth' for better mashing.

P. zappa and its pachygnathid relatives are, emphatically, spiders of this sort. Their jaws are ridiculously enlarged. They tend not to be as long as those of some of their fellow Tetragnathidae, but they are much stouter. And hence their generic name, 'Pachygnatha', from the Greek for 'thick-jawed'.