|MadSci Network: Zoology|
The answer to your question hinges, as you correctly surmise, on whether the "thousand-legger" that you're seeing is a millipede or a centipede. Because you describe these creatures as crawling out of sink and bathtub drains, I'm going to guess that they're centipedes. Centipedes are more likely than millipedes to be crawling out of drains because they don't have a waxy exoskeleton that would protect them from desiccation -- they are probably in the drains in the first place because of the water. Millipedes, on the other hand, are more heavily armored and generally aren't as closely tied to damp places. Millipedes are usually found in leaf litter and don't invade human domiciles as commonly as centipedes do. If you want to check for yourself, it's pretty easy to tell the difference between centipedes and millipedes (and don't worry, you won't have to touch them!).
You can generally tell them apart if you can see them walking. Millipedes lumber along at a slow, even pace (like miniature bulldozers) whereas centipedes have that sort of quick, scuttling-along motion that gives people the heebie-jeebies. In a nutshell, if it moves quickly enough to startle you, it's probably a centipede.
Now, if you care to look more closely, you'd see that centipedes have a single pair of legs on each body segment, while millipedes have two pairs of legs per segment:
Since the critters that are climbing out of drains into homes are probably centipedes, you should be aware that centipedes do have a venomous bite, and they don't hesitate to use it. I don't think that any centipedes in the US have venom that is potent enough to be truly dangerous, but you should still avoid handling them. If you come across a millipede you can handle it safely, although some millipedes may exude noxious or mildly caustic substances if they feel threatened. Despite this defense, millipedes have become quite popular as pet animals. A foray through just about any petstore will give you a chance to see live millipedes up close.
For more information on centipedes and millipedes, click here. Any invertebrate zoology textbook will also be informative. Here are some that I use:
Brusca and Brusca. 2003. The Invertebrates. Sinauer Associates, Inc.
Ruppert and Barnes. 1994. Invertebrate Zoology. Saunders College Publishing
I hope this answers your question!
Allison J. Gong
Try the links in the MadSci Library for more information on Zoology.