Why Spiders Aren't Insects I: An Intro

I started this series of posts almost a year ago, incorporating some basics about taxonomy, evolution, and a little genetics while exploring my fascination with the Chelicerates. I'll be reposting the series, which is included in the Basic Concepts list, this week and next.

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.

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.

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Very interesting! This is something I've always wondered, but never remembered to look up when blankly staring at Google wondering how I'm going to waste the next hour or two of my life..!

I've never considered "bugs" to be synonymous with "insects"; I've always considered spiders to be bugs, although I know they're not insects. Even Merriam-Webster agrees:

bug: 1 a: an insect or other creeping or crawling invertebrate (as a spider or centipede)

Is this different in the world of entomologists?

I still think of "bug" as a generic term for any member of, archnids or insects. Some usages such as "I'm home sick. I caught a bug." seems to imply it also includes bacteria, and single celled critters.

Great stuff Jeremy. I've always been intrigued by the chelicerates myself, especially whip spiders and other critters that don't get as much attention. The biggest question I have at the moment about spiders, however, is how exactly do they know how to make the webs that they do? How is the behavior inherited, and how can it evolve (especially in terms of spiders who use their webs in unconventional ways, like lassos or lures in some species). I'll probably write something up on it if I can find anything, but for I can't honestly say that I have any clue.

I have a couple of papers layin' around here somewhere about the evolution of spinnerets in arachnids and I did want to get to it in this series eventually, but since I'm easily distracted by loud noises and shiny objects, I never did.

As far as the word "bug" goes, it's like any colloquialism; it's just not specific enough for a zoological discussion of these organisms. I'll talk about this a bit more tomorrow, after I repost the next in the series.

Actually, most of the antenna-heads I know (myself included) don't get offended by people who call non-insect arthropods "bugs". Strictly speaking, a true bug is an insect in the order Hemiptera (sometimes divided into two orders, Hemiptera and Homoptera, although current information seems to support lumping them together.) In that strict sense, referring to other insects like ants and grasshoppers as "bugs" isn't accurate either. So, if you see the common name "bug" used in the title of an entomological research paper, it's going to mean a hemipteran. But many of us use the word "bug" in casual conversation the same way everyone else does.

I often tease microbiologists, though, for using the word "bug" for bacteria. Then again, they tease me right back. :-)

It was always a bad idea to take current living-language words and try to give them precise scientific definitions -- as in 'bug', 'fly', 'worm', 'beetle'. Anything that irritates me is a 'bug', to include my little brother. For an entymologist to say something is not a 'true bug' is really a bit of nonsense, practically speaking. Entymologists have flies that don't fly, bugs that don't bug, worms that don't worm, and beetles that don't beetle.

Engineers have made similar mistakes. One is redefining 'steam' as invisible water vapor. The word 'steam' means the visible condensed water vapor seen over a lake, or seen forming over a hot horse on cold wet day. Watt and Newcomen called their machines 'steam engines' because they thought steam was the working fluid; later it turned out invisible water vapor was their working fluid.

By Rusty Knob (not verified) on 11 Jul 2007 #permalink

So what entomologically accurate term should I use for the category of land-dwelling exoskeletal beings that creep me out long before I pause to count the legs and/or body segments? (That includes spiders, potato bugs, scorpions, and centipedes.) I always thought "bug" was sufficiently general to cover all of them...

I don't think I've ever known anyone who uses the term 'bug' in its strict sence to mean Heteroptera. The zoologically accurate term for arthropods is, well, arthropods. Though of course, that also includes crustaceans, which most laypeople (and not a few biologists) tend to think of as another group due to their aquaticness (hmmm... I don't think that's a valid word...)

The picture Jeremy's posted includes two other orders of arachnids than spiders. See if you can tell which they are...

On the subject of common names: There have been efforts to standardize common names of American insects, and this task is currently headed by a standing committee of the Entomological Society of America. (The American Ornithologists' Union has done the same with the common names of American birds.)

One convention that I learned to follow in my first entomology class: If a common name includes a term like "bug", "fly", "bee", or the like, it's written as a separate word if the insect is a true bug (or fly or bee), but as a compound word if the insect is not a member of that group. For example:

Bed bug
Fruit fly
Bumble bee

All of the above are just what we're claiming they are, but in other cases:

Pillbug (actually a non-insect arthropod)
Firefly (actually a beetle)

Incidentally, the above convention is frequently violated in the literature. For instance, the "velvet ants" are actually mutillid wasps, so the name should be written "velvetant", but I've never seen that one in print.

Good point Julie; I nearly forgot that there's a whole order of "true bugs" (Hemiptera), many of which contain "bug" in their common names. Granted, outside of "bed bug" or "stink bug" most people probably aren't terribly familiar with them anyway.