Animal Form and Function 5: Chelicerates

The chelicerates are likely the most primitive of the extant arthropod groups, and they are the simplest anatomically.  Chelicerates are one of the first groups of animals known to have made the move from water to land, but their dominion over terrestrial ecosystems has not lasted.  Nowadays, the spaces that once belonged to this ancient lineage mostly belong to crustaceans in water and insects (so, different crustaceans) on land.  Still, chelicerates remain a major ecological force, thanks to the multitudinous mites and ticks and the prevalence of spiders and scorpions as insect predators.

And, they are magnificently weird.

Modern chelicerates comprise three lineages.  Most people have heard of Arachnida, and a few recognize the Merostomata, or horseshoe crabs.  The third lineage of chelicerates, the sea spiders or pycnogonids, tends to be written out of most surveys of the arthropods.  Even professional, technical texts often ignore this group, which is truly a shame.

Sea spiders are not spiders, but their own group within the chelicerates, much like horseshoe crabs. Alternately, they are the last remnants of an even more ancient lineage that predates the current chelicerate/mandibulate split.  Small species can be found in tropical reefs, but the largest ones (30+ cm in leg-span) live in deep antarctic waters.  This video shows some of their common anatomical features well, including their very large chelicerate mouthparts, their very short abdomens, and the extra sets of legs that their males use to carry balls of eggs.

(Horseshoe crabs are addressed in the students’ course materials, so I won’t feature them here.)

Within the theme of chelicerate diversity, the arachnids are a surprisingly diverse group.  Far from just containing spiders and scorpions, the Arachnida have at least 12 living orders, including harvestmen, pseudoscorpions, and mites.

One of the largest and most distinctive non-spider, non-scorpion arachnids is the vinegaroon, Mastigoproctus giganteus.

Vinegaroons are part of an order, the Thelyphonida, that is actually closer to spiders than it is to scorpions.  Their first set of walking legs is elongated and used as sensory appendages, effectively taking over the function of antennae, which chelicerates lack.  Thelyphonid claws are unique in that, in addition to the spike at the far end that turns a single line of segments into a claw, there is a spike on the next segment back.  So, a vinegaroon’s claws are actually two-part meta-claws.  The image below shows the claw proper, this meta-claw assembly, and the spiked gnathobase near where the claw joins the body, which the vinegaroon uses to help chew its food.

Mastigoproctus giganteus in threat display showing spiked pedipalp gnathobase, claw, and segment in between with secondary claw.
Mastigoproctus giganteus in threat display showing spiked pedipalp gnathobase, claw, and segment in between with secondary claw. Photo by R. Mitchell, University of Florida

Vinegaroons get their name because, instead of the scorpion’s stinger, they have a long tube through which they can squirt vinegar or formic acid when frightened.  Captive vinegaroons can thereby gas themselves if they are kept in too-small enclosures.  Vinegaroons are otherwise non-venomous and totally harmless.

Another unexpected subset of Arachnida is the group known as tailless whipscorpions or whip spiders, the Amblypygi.

Despite their fearsome appearance, whip spiders don’t even have the vinegaroon’s stench defense, and hunt their insect prey purely with the force of their spiked pedipalps.  Whip spiders are also on the short list of large arachnids that are social animals, maintaining dominance hierarchies and living in colonies.

And then there are the wind scorpions / sun spiders / camel spiders, the Solifugae.

Solifugae means “fleeing the sun,” and these non-venomous arachnids get part of their fearsome reputation from their habit of seeking shade in people’s shadows in their desert homes.  Like the Amblypygi, they rely on their long pedipalps to hold food, but they have huge chelicerate jaws to tear it apart, as their pedipalps are not particularly effective claws.  Camel spiders are widely rumored to be extremely venomous, extremely aggressive, or both, and local legends in their homelands claim that they are very dangerous to livestock.  None of these things are true, though they can deliver a mean pinch.

While we’re dispelling misconceptions, all of these organisms are called “daddy longlegs” somewhere:

That’s a non-spider arachnid, a spider, a cranefly, and a plant, for those of us keeping score.

The first animal is not actually even particularly closely related to spiders, and is part of a lineage of omnivorous arachnids whose prey includes fungi, earthworms, miscellaneous decayed plant material, and small insects.  These animals, also called “harvestmen,” can be distinguished from spiders by not having a distinct waist, as well as by their lack of venom and web-spinning ability.

The second animal is a cellar spider, a common cobweb-spinning species that prefers dark, relatively damp haunts such as cellars and basements.  It preferentially preys on other spiders, including dangerous species like the black widow.  The third animal is a crane fly, a nectar-feeding insect.  Its larvae are agricultural pests in Europe.  And the fourth thing is an Australian plant that seems to be part of the way toward itself becoming a predator of insects.

One thing all of these creatures have in common is that they are all totally harmless.  Only the cellar spider even has venom, let alone the “most deadly venom in the world” commonly attributed to it.  As far as anyone can tell, the urban legend of its lethality may stem from its habit of preying on spiders that are dangerous to humans, leading some to suppose that this must mean it is somehow more venomous than its prey.  The observation that the spider itself is benign added a detail to the urban legend: it is supposedly deadly venomous but unable to penetrate human skin, so its nuclear arsenal is no longer dangerous to us.  In reality, the pholcid spiders are quite capable of getting partway through human skin, but their bite is milder than a bee sting on the rare occasions that they can be cajoled into biting instead of hiding.  That has not stopped this mythical danger from propagating into the public perception of the other two daddy longlegs animals, that are even less dangerous than that.  With any luck, people will soon imagine that Stylidium flowers have weak fangs as well.

Something else that merits clearing up is that spiders do not routinely eat birds.  This kind of scene does happen…

…but very few spiders make webs strong enough, or are themselves strong enough, to subdue even small birds.  Birds usually have the opposite relationship with spiders, plucking them out of their webs with ease or crashing through them by accident at substantial cost to the spider.  Only the largest spiders, such as the over-dramatically named Goliath birdeater tarantula, can attack a sleeping or otherwise grounded bird with a reasonable chance of getting a meal, and most don’t try.  Many web-spinning spiders mark their webs with flower-like patterns to make them more visible to birds so that they do not get destroyed by accident, and also to confuse insects.  The golden silk orbweavers, Nephila sp, are not normally in the business of eating birds, but make strong webs that cross the space between entire trees and so are more often in the position to catch one, as this particularly impressive specimen from Asia demonstrates.  Also, they deposit reflective chemicals in their silk to give it a golden sheen, which gives them their name.

Another spider makes a far stranger web than that.

The diving bell spider, Argyroneta aquatica, captures a bubble of air with specialized hairs on its body and brings that air to a bubble that it maintains in its web further underwater.  It waits inside this bubble and exits to attack nearby animals, such as scuds, or to revisit the surface to replenish its oxygen.  Water has an extremely high capacity for carbon dioxide, so the animal’s respiratory output diffuses on its own out of its bubble.

And then there’s a spider that saw all of that and thought, “I can do better.”

The ogre-faced spiders of the family Deinopidae (“they of the terrible eyes”) hang from trees while holding a square of sticky webbing between their four front legs.  When an insect wanders by, the spider lunges at it and envelops it in silk.  The deinopids have turned the mostly passive, aerial-filter-feeding shape of the spider web into an active implement of attack.

Deinopis has weaponized the spider web.

It is a travesty that these animals are not household names.  So, I’m working on it.

Animal Form and Function 5: Chelicerates