Friday arachnid blogging: not a spider

Well now we've all seen plenty of these creatures. Sometimes they are busily legging it through the undergrowth. Other times crowds of them are gathered on the shady side of a house, doing nothing much for hours on end.

One thing they aren't, though, is spiders. They are only spider relatives, and not terribly close ones either. They're opiliones, better known as harvestmen or daddy-long-legs; not to be confused with the spider Pholcus, which is also called daddy-long-legs, and certainly not to be confused with the crane fly, an insect that also shares this common name.

There's another, even cooler looking opilione beneath the fold; it's a very big picture, so if you have a slow connection you can make yourself a sandwich while it downloads.

Opiliones do have a rather spider-like habitus, or overall 'look'. Many (but by no means all) have the exaggeratedly long, thin legs that are the source of one of its common names. A few spiders have legs like that, but most don't. The really obvious difference between the two groups is that spiders have two main body sections, the cephalothorax and the abdomen, whilst opiliones appear to have only one. 'Appear', because they actually do have the same body division as spiders. Unlike in spiders, though, their two body sections are joined by a broad, fused connection rather than a dainty petiole, so they look like one big blob. If you look carefully at this one, though, you can see the division:

For all their spider-like appearance, though, opiliones seem to be as unrelated to spiders as one can be within the Arachnida. On the Tree of Life website, David Maddison divides the Arachnida into two basic groups. One contains, among other things, the spiders. Maddison places opiliones basal to the second group, whose most famous members are the scorpions. According to Maddison's cladogramme, opiliones (and scorpions etc.) are even less closely related to spiders than are the Acari (ticks and mites), a group universally accepted as Arachnida but (unlike opiliones) traditionally excluded from the discipline of arachnology. (Acari are simply too large a group, and morphologically/behaviourally too distinct from other arachnids; they get a whole discipline of their own, acarology.)

The other animals in the opiliones' subgroup are fierce indeed. Everybody knows about scorpions and their stings. Then there are the solfugids (wind-spiders or camel-spiders), which lack the venom of spiders or scorpions but have huge jaws that can easily slice into a human finger. Then there are the pseudoscorpions, which are unterrifying (to us) only because they are so tiny. Were we the size of an ant, we would find pseudoscorpions (which inject venom with their fingers and shoot silk out their jaws) significantly less cute.

Opiliones, though, are the pacifists of the Arachnida. 'Pacifist' in relative terms, of course, for the Arachnida are on the whole quite voracious predators. Some opiliones specialise in (for example) devouring snails. Most, though, will take what they can get. That could be something they kill, but it could also be something they scavenge, or even odd bits of decaying plant material. And two things make them outliers among all the Arachnida. They can include in their diet a bit of solid food (most Arachnida can only suck up the goop they have made by spewing digestive fluids onto or into their prey). And male opiliones have something that other arachnid males can only envy: a penis.

You'll find another opilione here No you won't; the 'other opilione' is the one just above there. Thanks to Aidan for the catch. The T6I proofreader who let that one get by is being flayed alive in our dungeons even now.