July 15, 2006

Why Spiders Aren't Insects: I

This is the first in a series of discussions on the evolutionary significance of the spider and its relatives.

Perhaps nothing will spark a lengthy dissertation from an entomologist more quickly than calling a spider a "bug." And lengthy can be well, hours.

Truly, spiders do seem rather buggish; they're creepy, have loads of legs and the thick outer structure (an exoskeleton) that other bugs possess. In short, if it looks like it, feels like it, tastes like it (?!), well, it must be...

That rule doesn't apply here. When you look more closely at a spider, one thing becomes immediately clear: it only has two segments, the most important of which is called the prosoma. The prosoma in spiders is the smaller segment and bears both the spider's head and all of the spider's walking legs, while the larger part, the abdomen, bears another part spiders are famous for, silk-secreting spinnerets (we'll discuss silk production in a later post).

The major distinction between spiders and insects is in the mouth. While insects have evolved leaf and flesh shredding mandibles from small appendages on the head evolved from a common ancestor of both spiders and insects, spiders have more primitive feeding parts called chelicerae tipped with well-known and well-feared fangs with which spiders subdue and tear prey into digestible pieces. Chelicerae can be used like knives or scissors depending on the species of spider.

For this reason more than any other, spiders are placed as a class under the subphylum Chelicerata (Greek: claw) and insects are placed into the subphylum Mandibulata*, (Greek: jaw).

Chelicerata incorporates not only the arachnids (spiders, scorpions and mites), but also the extant horseshoe crabs, the extinct eurypterids (perhaps the largest arthropods ever to live, reaching lengths of over six feet) and the relatively obscure pycnogonids or "sea spiders."

So in essence, spiders are more closely related to horseshoe crabs than insects. Not only do they have a prosoma and chelicerae, but they also respire in much the same way, from a oxygen exchanging structure that closely resembles a book.

Tomorrow we'll discuss the ancestry of spiders; once upon a time, they may have left the sea millions of years ago just to scare the curls out of you in your basement.

*With the inclusion of extinct arthropods into this subphylum, taxonomists dispute whether or not more than one subphylum is required to accurately classify these organisms.


  1. Although I tend to give them a wide berth, they fascinate me, so I'm looking forward to part II!


  2. Hey, that's great, BV! Spiders are fascinating creatures; most people don't know how deep the hole goes, so to speak, so I'll do my best to elaborate.

  3. The word 'insect' can indeed be used to refer to any terrestrial arthropod. The word you want is 'Insecta'. You can find this information in any good dictionary. Quit spreading this falsehood.

    "Main Entry:1insect
    Inflected Form:-s
    Etymology:Latin insectum, from neuter of insectus, past participle of insecare to cut into, from in- 2in- + secare to cut; translation of Greek entomon * more at SAW, ENTOMOLOGY

    1 a : any of numerous small invertebrate animals that are more or less obviously segmented and that include members of the class Insecta and others (as spiders, mites, ticks, centipedes, sowbugs) having superficial resemblance to members of Insecta not used technically b [New Latin Insecta] : a member of the class Insecta (as an ant, bee, fly)
    2 now chiefly substandard : any of various small animals (as an earthworm, coral polyp, turtle)
    3 : a small, trivial, or contemptible person"

  4. sweetscholar11:03 PM

    Why spiders SHOULD be considered "insects". Or at least a sub-category of insects. Exo-skeleton. Multiple legs. Creepy crawly. And if it quacks like a duck, people. So for some stuffy scientists, biologists, zoologists, paleantolgists, etc, to come up with artificial nit-picky labelings and distinctions, to make spiders a totally different species, like "arachnids", is silly. Why can't "aranchids" simply be considered a TYPE of "insect"??? (As well as scorpions, or dust mites, or whatever else.) The average person (and even insect and animal lover) doesn't generally in practical purposes separate spiders from the category of "bugs" or "insects". Why?? Cuz even though there ARE SOME basic differences here and there (like 8 legs instead of 6, two body segments instead of three, and claw jaw munching distinctions, whatever blah blah, those are just hair-splitting differences in the overall picture of it. Cuz again, just who came up with these uptight definitions of these things in the first place. When you see a spider in your bathtub (and I like spiders, by the way), you automatically think "bug" or "insect" or "creepy crawly" not "aracnid". So let's lose the silliness already on this subject. (By the way, "evolve" is presumed as fact, without sound solid basis for it, though 80% of scientists dogmatically insist differently. No real evidence. Gaps are still gappish. Just a side point and another topic.)