Friday arachnid blogging in absentia; or, Early Monday morning is losing its appeal

As you read this I will be finishing packing up. We are taking the brood off for two weeks' holiday in the Illes Baleares. Specifically, we are headed to Formentera, a small island just south of Ibiza but virtually devoid of the discos etc. that make Ibiza so nice a place to get off a plane and onto a ferry.

So there'll be no posting, and therefore no Friday arachnids, till after the middle of April. Now I know there are millions of you who turn faithfully to this site every Friday for your spider fix. To keep you from jonesing too badly, I've stuck not one but two spiders below the fold; and a couple of pictures of each. Both are spiders I collected on Formentera in 2001; both are a bit unusual.

We'll start with Loxosceles rufescens. This is the Mediterranean cousin to the infamous American brown recluse, L. reclusa. Here he is while he still walked among the living:

You'll note that he has but six eyes, arranged in three pairs or dyads, like past Friday Arachnid S. thoracica. In fact, these two spiders are from closely related families. You'll also note from the swollen tips of his pedipalps that he is a sexually mature male. The simple structure of these 'manual genitals' marks L. rufescens as a member of the Haplogynae, a relatively small and 'primitive' group within the araneomorph spiders (i.e., the main branch; more on this below).

Here's a close-up of his prosoma. You can see the 'violin-shaped' marking that gives his American cousin the nickname 'fiddleback spider'.

If you look closely, you can also see the fine-pointed ends of his emboli, the turkey baster-like thingies at the ends of his palps. These he fills with sperm, later to be squirted into his mate.

Judging by this specimen, fully grown at 5 mm in body length, I'd say that L. rufescens is a bit smaller than L. reclusa (of which there is a picture here). Presumably they have similar venom. It's nasty stuff altogether. The bite itself, so I've read, is not painful, indeed most victims don't feel it. And most victims never know they've been bitten because the venom has no effect. But in a significant minority an ugly, slow-healing necrotic wound develops at the site of the bite (a much smaller minority will have a dangerous and potentially lethal systemic reaction.) An interesting and little-known fact about L. reclusa, recently reported by Jamel Sandidge in Nature, is that this spider, unlike almost all others, is a scavenger: it prefers dead prey to living. I could not begin to say whether L. rufescens has the same habits, but I will keep a cold eye on any dead flies I may note on the windowsill.

Now we'll turn to another spider that is a bit of an outlier. This is Nemesia sp. Again, we'll start with a picture of her still alive, in a little glass phial:

Nemesia is a mygalomorph spider. That is, she is part of a discrete group considered more 'primitive' (closer to the ancestral state) than the araneomorphs, or 'true' spiders. Mygalomorphs as a general rule are bigger and longer-lived than araneomorphs. They include those huge hairy dinner-plate sized things we all call 'tarantulas', though this is a misnomer - the real tarantula is an araneomorph, a Mediterranean wolf spider that, though quite large, isn't nearly as big as the mygalomorph 'tarantulas'. But the mygalomorphs also include quite tiny creatures, including a minuscule (and endangered) American sort that lives in the moss growing on the sides of trees. As for our Nemesia, she is 17 mm long, ignoring her legs.

Mygalomorphs differ from 'true' spiders in a number of ways, some of which we'll look at. The most obvious difference is the orientation of the chelicerae, the 'jaws' bearing fangs at their ends. Mygalomorphs are orthognath; that is, their chelicerae move in parallel, striking downwards. Araneomorphs are labidognath - the chelicerae face each other and the fangs close in a pinching movement. If you want to imitate an araneomorph, you need to put your thumb and index finger in front of your mouth and snap them open and shut. To imitate a mygalomorph, put your index and middle fingers in front of your mouth and move them up and down.

Nemesia is a 'trap-door' spider. She lives in a burrow with a hatch on top, and pops out to nab passing prey. That's about as sophisticated as trapping technology gets among the mygalomorphs. Most wander about and eat whatever they come across and can take down. Some, like Nemesia, build trap-door burrows. A few - the purse-web spiders - extend the burrow outwards with a silken tube, and bite their prey through it. The Australian funnel-web spider, Atrax robustus, builds primitive webs a bit like those of the araneomorph Agelenidae. (Atrax, BTW, is a very dangerous spider, with venom that can be deadly for humans and other primates but not, oddly, for other mammals.) But no mygalomorphs build the marvelous webs most people associate with spiders.

Let's take a closer look at our Nemesia, who is now quite docile, because dead:

Here you can clearly see how her chelicerae jut straight forward. Thery're very big, compared to those of most araneomorphs, but her venom glands are small, being contained within the chelicerae (in araneomorphs, they extend back into and occupy a good bit of the body). Note also the eyes (she has eight of them), crowded together on a tiny hump. Finally, note her spinnerets, two of which extend visibly beyond her tail end.

If we flip her over, here's what we see:

Spiders don't have lungs as we know them, i.e., inflatable bags that are pumped full of air. They have 'book lungs', a sort of internal cavern with a large surface area across which oxygen diffuses. The air gets in through a slit on the belly. Virtualy all araneomorphs have one pair of book lungs. Their ancestors bore a second pair but this has become reduced to a so-called trachaeal slit (most spiders have one, a few have two) that leads to small tubes bringing an extra bit of air to the spider's interior. The mygalomorphs still have both ancestral pairs of book lungs. In the picture above, you can very clearly see the posterior pair (dark slits just in front of the midpoint of the belly). The anterior pair are a bit harder to see, as their slits run right along the epigastric furrow, a sort of trench running from side to side along the front part of the belly, beneath which the genitalia lie.

And, if you're looking at the epigastric furrow, there's something you won't see: an epigynum. This is a sclerotised plate covering the female genital openings. It's only present in the entelygynae, i.e., the 'higher' araneomorph spiders. The absense of an epigynum makes it very hard to idenitify Nemesia specifically. To do so with any precision, you need to catch a sexually mature male, and those are hard to come by. With any luck, I'll find one in the next two weeks.