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.