Wild Side: Insects of water and air

What does evolution offer to those who can make the transition?


I’ve been thinking a lot lately about a gaping void in my natural history knowledge: the biology and ecology of insects that have aquatic larval states. This is a huge group of species, functionally related, but all over the taxonomic map.

With such a diverse assemblage exhibiting this partly aquatic life cycle, it seems certain that the combination of water-dwelling larvae and terrestrial adults must offer powerful ecological advantages. And in evolutionary terms, this must be a life history that has been lost and regained many times in the long history of insects, which, again, wouldn’t happen without good reason. I want to know more.

Life under water offers larvae protection from many types of predators (though, to be sure, it exposes them to other types). Aquatic life offers a reasonably predictable environment of assured wetness and limited temperature swings. By maturing into winged adults, these insects ensure some ability to disperse and colonize new areas. In many cases, these adults are short-lived, and may not even be capable of eating; biologically, their only functions are to mate and move around a bit. But that can be sufficient. In other cases, adults are powerful, long-lived aerial predators capable of flying hundreds of miles. As I said, a diverse bunch of critters!

Biologist Greg Whitmore has done fantastic work on Vineyard aquatic invertebrates, with much of his research associated with the ongoing study of the Mill Brook, which West Tisbury has coordinated. Other stream systems, and our various ponds unassociated with streams, have received less attention, which raises the possibility that discoveries remain to be made. I’m still familiarizing myself with existing information compiled by Greg and others. The focus of my own searching will likely be on adults rather than larvae. But my curiosity has been piqued, and I hope my casual methods can help illuminate these fascinating insects.

Members of this vast functional group range from easily studied to utterly inscrutable. At one extreme, there are the Odonates — dragonflies and damselflies. These have been well-studied, both on the Vineyard and more generally. Both larvae and adults are, for the most part, relatively easy to identify, and we know a good deal about their ecology. While this order exhibits an aquatic larval stage, its adults are highly functional insects.

At the other extreme, I’d place the midges, a large and disparate group within the order Diptera (that is, flies). For many midges, adults are literally unidentifiable. Much of the ID work within this group has focused on the aquatic larvae, and looking at the nearly indistinguishable adults, it’s impossible to say with certainty what larval species it came from. Since I’ve never learned the specialized methods of aquatic sampling and larval ID, midges leave me helpless.

Also daunting are the caddisflies (that’s the order Trichoptera). With some 15,000 species in 27 families worldwide, this group just boggles my mind. Again, this is a group with a lot of very similar adults, and it’s probably best studied by focusing on the aquatic larvae (though adults can turn up in swarms at lights). Ephemeroptera — the mayflies — offer a similar situation, though with somewhat less imposing diversity. There are also beetles (Coleoptera), and true bugs (Hemiptera) with aquatic larval stages.

As a starting point to begin learning about insects with this life history, I’ve zeroed in on two other orders, the stoneflies (Plecoptera) and alderflies/dobsonflies/fishflies (Megaloptera). Especially on the Vineyard, diversity here seems to be low enough to be manageable for a beginning amateur. Fairly good ID resources are available for both larvae and adults. And conveniently for someone who has yet to learn about aquatic sampling, the winged adults of both orders are relatively easy to find, and often make a naturalist’s life easier by being attracted to light.

Among the Megaloptera, alderflies appear to be rare on the Vineyard, and the same appears true of dobsonflies (as far as I can tell, only one species of the latter group, the stream-dwelling Eastern dobsonfly, occurs on the East Coast). Our two fishfly species, in contrast, seem to be fairly common, associated with ponds rather than streams, and likely less fussy about water quality than their relatives. Fishflies are capable of dispersing considerable distances: I’ve found them on the window screens of our Oak Bluffs house, 500 yards from the nearest plausible breeding site.

Stoneflies, spending that all-important larval stage in fast-moving, well-oxygenated streams, are represented here by at least a half-dozen species, spanning several families and genera. The few Vineyard records of adult stoneflies that I’m aware of come from right next to stream reaches suitable for breeding; the weak-flying adults may tend to stick close to home.

So there you have it: several lifetimes of inquiry waiting for me, piled on top of the multiple other lifetimes I’ve already gotten wrapped up in. So many bugs, so little time!