Wild Side: Erythrodiplax, the lazy dragonfly

That lives by the sea.

0
The female seaside dragonlet. —Matt Pelikan

I have a special fondness for species that have evolved to exploit conditions that would that would be inhospitable or even harmful to near relatives. Late July is prime time for one such critter: the seaside dragonlet, Erythrodiplax berenice, a dragonfly adapted to life in highly saline environments.

Seaside dragonlet nymphs, or larvae, are aquatic, like the larvae of all dragonflies. Nymphs of this species have a remarkable ability to regulate the concentration of salt within their bodies, and can maintain it at a healthy level, even in extreme conditions. They survive just fine in freshwater when placed there by researchers, despite the fact that species doesn’t seem to breed naturally in freshwater.

But at the other extreme, one intrepid researcher exposed larvae to increasing concentrations of salt in water, finding that it was only at three times the typical ocean-water concentration of salt that nymphs finally showed distress.

As with other dragonflies, adult seaside dragonlets are wary and possessed of outstanding eyesight. Thus, if you’re on the move around a salt marsh, you’ll generally spot this dragonfly skimming low over the vegetation, on the wing because you’ve scared it. You might conclude, based on such observations, that seaside dragonlets are active insects, spending much of their time airborne. Such a lifestyle is common enough among dragonflies.

But an elegant behavioral study of this insect, which involved patiently watching for long enough and from far enough away so that the subjects were undisturbed by the observer, revealed this species is actually profoundly lazy, spending 99 percent of its time perched and stationary.

Sedentary habits make perfect sense for this insect. Flight is energy-intensive, and may make you more visible to predators. From an insect’s perspective, then, flight is a liability to be minimized. But salt marshes are famously buggy environments; the density of potential dragonfly prey is generally very high, and a seaside dragonlet can count on a steady stream of targets coming by. The dragonfly simply perches and waits, launching itself only briefly to snag its victims.

Like other dragonflies, the seaside dragonlet strikes its prey in midair, catching it in what amounts to a basket formed by the dragonfly’s spiny legs. Captured prey is eaten entirely, cut up by the dragonfly’s powerful mandibles. This species, naturally, also finds time to mate, each pair locked together in flight, skimming the marsh and dipping down to deposit eggs, mostly into mats of algae.

Seaside dragonlets belong to a rather small genus, with only three members in North America, and are found along the coast from Nova Scotia south through the West Indies to portions of Venezuela. The species also occurs on the west coast of Central America and Mexico, north to the U.S., with some populations occurring around brackish ponds and playas in the desert Southwest.

The geographical range of this dragonfly spans some 35 degrees of latitude — not really rare among insects, but is still suggestive of a lot of adaptability. Throughout this range, the species is strongly coastal, associated mainly with salt marsh in northern portions of its range and mangrove swamps in the southern portion. From time to time, wandering individual seaside dragonlets turn up in upland or freshwater settings, but basically this is an insect of saline wetland edges.

On the Vineyard, the seaside dragonlet is a common species, though your odds of encountering it are not great away from its preferred breeding habitat. The Land Bank’s Pecoy Point Preserve in Oak Bluffs [bit.ly/VVPecoy] may be the easiest place to find one, since the species breeds plentifully on the good-size marsh adjacent to the preserve; the shoreline along the boat landing is a good place to look.

But seaside dragonlet seems to breed in virtually every decent piece of salt marsh around the Island, and if you can get access to the edge of a marsh, you can probably find erythrodiplax there. It is a midsummer species at our latitude, though exhibiting a much longer season — even year-round — in southern portions of its range. Late July sees its peak of abundance on the Vineyard, though I’ve found it as early as mid-June and as late as early September.

The species varies considerably in appearance, which can complicate identification. (But in its preferred salt marsh habitat, seaside dragonlet is the only medium-sized dragonfly, about an inch and a half long, that you’ll encounter.)

Adult males are nearly all deep blue or black, with clear or nearly clear wings; young males and females show varying amounts of yellow on the top side of the abdomen, and also sport an elaborate pattern of black-and-yellow striping on the sides of the thorax. This is on the drab side for dragonflies, but what it lacks in looks, the seaside dragonlet makes up for in elegant adaptation to a harsh environment.