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#38: Arachnids

Harvestman (aka daddy longlegs)

Mite

Spider

 

Litinsects #38: Arachnids

Today we are taking a little excursion outside the world of insects into a neighboring class of arthropods, the ARACHNIDS. Most people think spiders when they hear arachnids but the group includes so much more than that!
  • In addition to spiders 🕷, the arachnids consist of mites and ticks, scorpions 🦂, pseudoscorpions, tailless whip scorpions, and harvestmen (aka daddy longlegs which are NOT actually spiders!). The closest relatives of arachnids are horseshoe crabs
  • Like insects (and all other arthropods), arachnids have jointed appendages and a hard exoskeleton and grow by molting
  • Arachnids have neither wings nor antennae. They possess only simple and no compound eyes
  • Unlike insects, which have 3 body segments (head, thorax, and abdomen) arachnids generally have two body sections, the PROSOMA, aka head, and the OPISTHOSOMA, aka abdomen (but the two may be fused into one in some groups)
  • Arachnids are easily recognized by their 8 legs but they also possess two additional pairs of appendages towards their heads. CHELICERAE are basically jaws that can be used as pincers or modified into fangs to inject venom. PEDIPALPS are used either for feeding or locomotion and can be highly specialized in various groups. (In scorpions for example, the pedipalps are modified into huge pincers, making it seem like they actually have 10 legs).
  • This group has an almost worldwide distribution. The majority of species are terrestrial and predatory
  • Among the smallest arachnids are mites, some as tiny as 0.1mm! (More on my mite friends to come later!) On the other end of the spectrum sits the goliath bird eater, a spider that can measure up to 30cm
If there is one thing I want you all to take away from this post it is this: spiders are NOT insects and they are not the only arachnids!
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