Archive | November 21, 2012

A biblical arachnid

The male Atewa dinospider (Ricinoides atewa) from Ghana has a huge second pair of legs, which are probably used in male-to-male combat. [Canon 1D MkII, Canon MP-E 65mm macro, Canon MT-24EX twin light]

In 1837 a small fossilized beetle was discovered in Carboniferous deposits of England, and its description was promptly published in “The Bridgewater treatises on the power, wisdom and goodness of God as manifested in the creation.” Unfortunately, two things were wrong with this publication. As it turned out, the creature was not a beetle, but a member of a previously unknown group of arachnids. Only a year later, French entomologist F.E. Guérin-Méneville described a living species of the same group, now known as Ricinueli, and thus turned the animal into a “Lazarus taxon”. This biblical name is applied to groups that were first described as fossils (i.e., all members are presumed dead), only to be subsequently discovered as extant organisms. Coelacanth is another example of such an animal, as is the recently found new order of insects, Mantophasmatodea.

And, speaking of biblical references, one species of Ricinulei, Pseudocellus krejcae, should be considered both a “Lazarus taxon” and a “Jesus Christ species” (two guys who apparently met at some point) as it is known only from a single specimen found walking on the surface of the water deep in a cave in Belize.

Immature dinospiders, like this Cryptocellus sp. from Costa Rica, are usually brightly colored. [Canon 1Ds MkII, Canon MP-E 65mm macro, Canon MT-24EX twin light]

Ricinulei are often referred to as hooded tick-spiders, but I prefer a name invented by a friend of mine, the dinospiders. Since the initial discovery of living dinospiders in West Africa 175 years ago we have learned quite a bit about these cryptic animals. Further paleontological evidence has confirmed that they indeed date back at least 319 million years, and their closest living relatives are probably anactinotrichinid mites. Their distribution also points to their origin before the breakup of the Gondwanaland, and these days they are known only from tropical and subtropical regions of the Americas, and a relatively narrow area in West Africa. Only 68 species are recognized, making the Ricinulei the smallest order of arachnids.

At first sight the appearance of dinospiders may be underwhelming – most species are under a centimeter long, dull brown – until you notice one peculiar characteristic. These animals, a closer inspection reveals, don’t have a head. In the place where a spider or a daddy longlegs has eyes and mouthparts, dinospiders have nothing. The front of the body, where you would expect to find a mouth, ends in a vertical plate. (When I found my first dinospider I was convinced that it had been damaged, and was missing a critical part of the body.) Of course, they have to feed somehow. The plate that covers the front of the body (cucullus), can be lifted to reveal a pair of small, pincer-like chelicerae and the mouth opening. They have no distinct eyes, however, although this does not stop them from being extremely sensitive to light.

Dinospiders are surprisingly long-lived and can take several years to mature. [Canon 7D, Canon MP-E 65mm, 3 speedlights Canon 580EXII]

Most of the species are found in the soil and leaf litter of tropical forests, but a few are also found in caves. Little is known about their diet, but some species feed on ant larvae, termites, and dead invertebrates; cave species apparently also feed on bat guano. Their reproduction, in typical arachnid fashion, involves a complicated sperm-transfer mechanism. The male loads one of his legs up with sperm, and then inserts it into the female genital opening. The female usually lays only one egg, which she then carries until a tiny, six-legged baby hatches. Only after a few months does the larva turn into a nymph, and grows an additional pair of legs, making it an honest, eight-legged arachnid.

I had always been fascinated by these animals, and thus was ecstatic to discover a new species of dinospiders in the ancient and seriously threatened Atewa forest in Ghana, which I named, predictably, Ricinoides atewa. To make it even more exciting, this turned out to be the largest dinospider species in the world, growing to the whopping 11 mm long! How many taxonomists can claim that they discovered the largest species in an order?

Neotropical dinospiders, like this Cryptocellus sp., are smaller and distinctly “fuzzier” than their African counterparts. [Canon 1D MkII, Canon MP-E 65mm macro, Canon MT-24EX twin light]