Dispersal strategies among biological organisms are fascinating. Pull out a field guide for any kind of organism, and, if it’s a good guide, it will show you a range map. These maps plot the expected or documented occurrence of the species under discussion. Some species, say Charadrius vociferus (our common North American Killdeer), have very large ranges:
This bird can pop up just about anywhere in North and Central America, and some spots in South America.
Other species, particularly those with limited dispersal powers (no wings, no legs, required to remain permanently rooted in the soil to survive) have much more restricted ranges. For example, Torreya taxifolia, one of the world’s rarest plants, has a range map that looks like this:
No matter how determined you are, and no matter how good you are at plant identification, you are unlikely to find the Florida Torreya anywhere outside its known range: the bluffs over the Apalachicaola River in Florida’s Torreya State Park, and the immediately adjacent river bluffs in Southern Georgia.
Insects, with less robust flight powers than birds, but not required to stay in one spot for their entire lives like plants, fall somewhere between these two extremes. And it’s a rare field guide to insects that shows range maps; none of mine do. Part of the problem is that there are just too many species; it’s hard enough to include all 29 orders of Insecta, let alone all the genera; you need a specialized field guide, like Grasshoppers of Florida, just to deal with each family in one restricted locale (also available in PDF format here).
Some insects do have storied powers of dispersal; the Monarch butterfly’s famed migration is one example. They merit their own range map. But they, and the other insects with large seasonal movements (dragonflies come to mind, and I’m sure there are other examples as well) seem to be the exception rather than the rule among insects. Most of them, on an individual basis at least, tend to stay fairly close to home, even if they are distributed across a wide range.
But most insect species that are found across wide ranges rely on human assistance to disperse, these days. Lady beetles and lacewings, for example, are sold via mail order catalog and the Internet, and can occur almost anywhere there is commercial demand.
This morning, though, on my way to take Eric to summer school, I discovered that one species of lacewing (the same one I wrote about the other day) isn’t relying on the commercial distribution methods anymore. It’s taking matters into its own hands, so to speak:
Those egg stalks are on the door of our minivan, which has been parked in the driveway instead of the garage for the past couple of nights. I didn’t see them yesterday, though, so I’m pretty sure they’re fresh from this morning.
I can just see the “thought process” of mama lacewing: “the aphid pickings are slim in this yard, so we’ve got to get moving. Now, wherever these guys go, my babies will go: guaranteed!” And she’s right—at least until the next time we wash our car!