Wild Side: Unsolved mystery

Why was a caddisfly in an Island garage in early January?

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Unsolved mystery: An unidentified caddisfly in Oak Bluffs, Jan. 3. — Matthew L. Pelikan

Ah, there are few things better than a natural history problem solved! A tough ID nailed down, a droll bit of ecology revealed. Such a feeling of accomplishment!
About the only thing better is an unsolved mystery — a question that unfolds into other questions, and into still more beyond that. Nothing makes me appreciate the elegance of nature more than being utterly baffled. And that’s why there are caddisflies.
Last Friday evening, I stopped by a pre-Christmas Bird Count party in an Oak Bluffs garage. It was fun to hang out with birders, inhale a grilled bratwurst, and admire the premium-grade cellar spiders hanging out near the garage’s ceiling. And I found a caddisfly.
Perched on the wall, this insect was nearly an inch long, sporting the long antennae threads typical of this group, and featuring dark brown, vaguely mottled wings. I took a few cell phone photos, figuring that a caddisfly active on Martha’s Vineyard in early January would be a cinch to identify. How many choices could there be?
I was blissfully, intriguingly, encouragingly wrong. It’s probably true that only a handful of species are possible at that time and place. But the life histories of these insects are not well understood, their seasonality turns out to be highly variable and complex, and the identification of adults poses gnarly problems even for experts. I failed miserably, and while I’ve been able to rule some groups out, I haven’t even been able to pin down the family this critter belonged to.
Caddisflies, or Trichoptera, are an order of insects, a taxonomic rank equivalent to that of beetles (Coleoptera) or flies (Diptera). It’s a large group, with some 15,000 known species worldwide. North America hosts about a tenth of that diversity. “Trichoptera” means, literally, “hairy wing,” referring to the powdery texture of the forewings of adults of this order.
Caddisfly adults are medium-size insects, most of them between a quarter-inch and an inch in length. When perched, they hold their furry wings tentlike over their abdomens. The wings themselves are generally drab brown or gray, sometimes dully patterned, only rarely distinctively colored. To put it another way, caddisflies all look about alike. Vast tracts of species in this group look indistinguishable to me.
Adults being so nondescript, caddisfly classification rests largely on the aquatic larval stage. Here, real differences exist, though it may take a microscope to see them. Fine details of anatomy, such as the number of hairs on a particular body part, allow experts to sort the larvae out. But in many cases, easily observed behavior suffices. Caddisfly larvae, famously, wrap themselves in movable shelters made of sand, gravel, or organic debris. The nature, size, and setting of the shelter, or whether the larva constructs one at all, are powerful identification clues.
Also helpful is the nature of the body of water a larva occupies: Caddisflies are notoriously finicky about water quality and chemistry, depth, velocity, temperature, the composition and texture of the bottom, and more.
I’m unaware of any comprehensive study of caddisflies on the Vineyard, though some good aquatic biologists have gathered some limited data. Island species diversity may not be especially high (if I had to guess, I’d say in the scores of low hundreds); we don’t have the extent and variety of freshwater habitat to support huge numbers of species. But some of the caddisflies we do have are plentiful, sometimes, for example, dominating the contents of ultraviolet light traps set for moths. Abundance surely gives Trichoptera considerable ecological importance, the larvae as aquatic predators and both larvae and adults as prey.
So what was that caddisfly in that garage, and what was it doing there in the first week of January? It was a large species, and clearly a species able to come to adulthood in mild-early winter weather. It may well have been one of the relatively few that like brackish water (the garage is not far from Sengekontacket Pond). All that seems helpful.
But there are also some small bodies of freshwater within range. Anyway, while adults are short-lived (in some species, they don’t even eat), they fly perfectly well, and can disperse heroically. The possibility of an individual accidentally imported from off-Island can’t be ruled out. Many species, it turns out, show some tolerance of low temperatures. And, well, a lot of caddisflies look very much alike.
So I haven’t a clue what it was. But in the process of failing to ID this critter, I’ve soaked up information about Trichoptera anatomy and ecology. I’ve developed a nascent sense of some field marks that may help ID at least a few of the families. If nothing else, I’m less ignorant than I was about what features matter for caddisfly ID, and next time, I’ll know some angles and body parts to observe and photograph. In short, I find myself getting fond of these drab, curious, frustrating little insects.
Success: Another mystery unsolved!