Not really a spider

Arachnidae, Opiliones
Harvestman

Most people seeing the small creature above (body length about 5mm) would say immediately that it’s a spider. Most of the rest would count the legs – yes, eight not six – and then say it’s a spider. Most of the rest of the rest would think it’s a spider and say “Eeek!” even though it’s so small. And then there’s the tiny minority who would say, “That’s interesting. I think that’s a Harvestman.” These last few would be right, too.

Mike Downes brought this one to me after saying just that, and patiently held its home rock while I took my photos. This happened on the Wildlife Queensland walk along the Dalrymple Track, up near Broadwater, last Sunday. There will be is more about that walk here and on the WQ blog in due course – but what are Harvestmen, and why aren’t they spiders?

Spiders are the best known group of a larger category of invertebrates called Arachnids, all of which have eight legs. The other arachnids include scorpions, ticks, mites and several quite obscure and unfamiliar groups; Wikipedia sets out the characteristics and relationships on this page, but the important thing for now is that Harvestmen are as distinct from spiders (evolutionarily) as scorpions, however much they look like spiders. Technically, Scorpiones (Scorpions), Opiliones (Harvestmen), Acari (Mites and Ticks) and Araneae (Spiders) are Orders within the Class Arachnida.

As Wikipedia says, “The most obvious difference between harvestmen and spiders is that in harvestmen the connection between the cephalothorax and abdomen is broad, so that the body appears to be a single oval structure.” Their legs are also longer than those of most spiders, though not usually as long as our Daddy Long-legs, but other differences are not reliably visible to the naked eye. Speaking of eyes, Harvestmen have only two whereas most spiders have eight.

arachnid
Harvestman – frontal view. The two pale dots on the abdomen are parasitic mites.

Further reading

  • CSIRO’s invertebrate site is usually good for information about Australian creatures but its page on Opiliones doesn’t have much to say except, “Harvestmen are most commonly found throughout the damper regions of Australia although some species have adapted to life in the more arid regions of the country. Most live in moist leaf litter but can also be found living under rocks and logs or under the bark of trees.”
  • Arachne.org has one page on Opiliones, with some good photos, amongst its “Other Arachnids”.
  • The only other resource I can recommend, Wikipedia’s Opiliones page which I quoted earlier, is not about Australian Harvestmen in particular but has plenty of good information about the whole order.

[Updated 8.11.15 to add links to associated reports.]

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