Summer is the season for insects, and it would be hard to think of a group of insects that is more prominent on the Vineyard, and more interesting, than the dragonflies. Predatory aerialists ranging in size from about an inch and a half long to five inches, our dragonflies are diverse, abundant insects, and major players in our ecology because they eat everything from gnats up to other dragonflies. Nearly 40 species are known to occur on the Island.
While it’s the wings, often boldly patterned, that are easiest to see, dragonflies possess spiny legs for catching and holding prey, and robust jaws for dismembering and swallowing their victims. (While larger species can give a noticeable nip to a human finger-tip, dragonflies can do no serious harm to people.) Highly visual insects, dragonflies possess large compound eyes. Exquisitely sensitive to motion, a dragonfly’s vision serves it well whether it is on the hunt or trying avoid becoming prey itself, to a bird or a larger insect.
Like their smaller, daintier relatives, the damselflies, which perch with their wings folded over their backs instead of spread to the side, dragonflies have an interesting two-parted life history. Laying eggs in or near water, dragonflies spend much of their lives in an aquatic larval state, feeding on smaller invertebrates that happen within reach. After overwintering, the mature larvae climb from the water onto a plant and emerge from their larval exoskeleton as an adult dragonfly. If you look carefully, you can find the larval husks left behind, still stuck to the vegetation.
Though it appears to me that none of our dragonflies breed in moving water, as long as the water is still, most species seem willing to settle for anything from a puddle to a pond. Dragonflies have been seen attempting to lay eggs on surfaces like the roof of a car, presumably mistaking the smooth, shiny surface for water.
A few dragonflies, however, are highly specialized in their choice of breeding habitat. The most obvious example would be a medium-sized dragonfly called the seaside dragonlet. As its name suggests, this elegant species tolerates (and perhaps even requires) salty conditions; it is restricted to salt marshes, where its larvae mature in pools and ditches. Adults are easy to find over a marsh in early summer, but it is rare to find an adult that has strayed even a few yards past the edge of its preferred habitat.
In any case, laying eggs is only the first step for successfully reproducing: a host of obstacles plague the developing larvae. Once it has hatched and made its way to the sediment on the bottom of the body of water it’s in, a dragonfly nymph has to be lucky to avoid predators such as fish and turtles. Luck is required, too, for capturing enough food to power the growth to maturity. And of course, water levels can drop due to drought: if a dragonfly nymph finds itself in a pond that dries up, it will die because, until it emerges as a flying adult, it has no way to move to a wetter environment.
The fortunate minority that survive to adulthood mate to repeat the process, and often disperse in search of new locations to breed. So although dragonflies are generally most common near water, wandering adults turn up, often in large numbers, in dry habitats as well, often far from water.
In some dragonfly species, the tendency to disperse takes the form of long-distance migration. It’s a tribute to the strength and speed of these insects that they can travel hundreds of miles from where they emerged as adults. The typical pattern is for dragonflies to move north in spring or summer, the females laying eggs in suitable sites as they go. Some species, such as the green darner, also make a return migration, heading south in late summer and fall.
Because they don’t need to wait for water at our latitude to warm up in order to reach maturity, migratory species, especially the green darner, are generally the earliest dragonflies I find each year: it’s not unusual to find green darners in mid-April on the Vineyard, and in the fall, the last few individuals typically straggle through in late October. But it is not long after the first migrants arrive that our non-migratory species begin emerging from ponds: certainly by late May most years, dragonflies are easy to find around nearly any body of water on the Vineyard.
Traveling dragonflies have a knack for finding areas where wind currents or topography concentrate prey; one frequent result is a “feeding swarm” of scores or hundreds of dragonflies swooping and gliding as they work a productive area. Currently, the spot-winged glider, a heavy-bodied, reddish dragonfly about two inches long, is migrating through our region in large numbers, potentially visible anywhere on the Vineyard. (Regrettably, the spots this species is named for are small ones at the base of the wing, often not visible even when the insect is perched.)
Field guides are readily available that will help you identify at least our most common dragonflies. But serious study is optional: these insects are fascinating to watch simply for their markings, which are sometimes elaborate, and for their agility and speed in the air. They deserve some of your attention this summer.