“Dragonflies love the heat; it makes them pop” says Kevin Munroe, who is leading a field trip as part of the Dragonfly Identification Workshop, a course in the Northern Virginia Audubon Society’s Master Naturalist Program. Kevin is working on a book describing the dragonflies of Fairfax County, Virginia and he can tell you at a glance what dragonfly species is flying past.
Dragonfly spotting is a relatively young “sport” among naturalists. Birders, who had pretty much filled up the check boxes in their field guides (nobody ever gets them all), began looking for something to observe while waiting for the elusive what-ever to show up in their lenses. They hit upon butterflies. Butterflies are colorful, fly during the day (no pesky getting up before dawn), and exhibit enough variety to give most people a challenge. Add in the Skippers, a butterfly-like Lepidoptera group somewhere between butterflies and moths (and which all look alike), and a small cottage industry of field guides and web sites was hatched.
Birds, check. Butterflies, check. What else was out there to experience? Cue the dragonflies. North America boasts nearly 700 species; about the same number as bird species. Northern Virginia has 70 species, divided unevenly among 7 larger groups, a manageable number, with life histories and migration patterns to rival those of birds and butterflies.
“Wandering Glider” says Kevin, pointing at a dragonfly-shaped dot hovering fifteen feet above our heads. Straw yellow body, red eyes and clear wings; this is the albatross of dragonflies. Wandering Gliders and their closely-related cousins, Spot-wing Gliders, cross whole oceans, their broad hind wings locked in glide mode, steering with the front wings. Following fronts and weather systems, gliding thousands of feet up, the two species, known collectively as “rain pool gliders” place their eggs in ephemeral rain pools following a storm. The eggs hatch and mature in just a week or two, taking advantage of the temporary water which being temporary, lacks predators. The Wandering Glider is found on every continent but Antarctica and is the only native dragonfly to Hawaii. The Spot-wing Glider makes it to the Galapagos Islands and throughout the Pacific. Rain pool gliders cue on the ultraviolet reflection from standing water, the same reflection given off by a newly waxed car. Check out a parking lot on a hot summer afternoon; dragonflies will be laying eggs on the hoods of blue and white cars.
Dragonflies have an ancient lineage. Fossils from Permian coal beds going back a third of a billion years show insects very much like dragonflies, reaching up to two feet in wingspan. Modern dragonflies, while smaller, have a few additional features such as a bend in the leading edge of the front wing for better maneuverability. And maneuver they do. Dragonflies can hover, fly backwards, and turn on a dime, sending power to each set of wings and to each wing individually as needed. Antennae have been reduced to tiny hairs act as air speed indicators. Heads are mostly eye with several thousand facets in each one, allowing for exquisite sensitivity to movement. In fact, dragonflies are so attuned to movement; it is possible to capture a perching insect by hand. Just move slowly enough and it literally does not see you—you are not there. How slow is enough? If it takes off with your fingers still several inches away from the wing, it wasn’t slow enough. Dragonflies can see better than we can at the far end of the spectrum; something that looks black to us like the Slaty Skimmer, may in fact be screaming purple to a dragonfly.
“Dragon Hunter” says Kevin, reaching in his net to extract a green dragonfly which, if not two feet across, is still pretty impressive. Over three inches long, this beast looks like it includes a healthy dose of steroids in its diet. Dragon Hunters specialize in preying on other dragonflies. This is prey that can bite back and this particular Dragon Hunter looks like it’s been through some real tussles. Its wings are tattered and the claws are missing from one of it its long hind legs. Kevin shows it around and releases it. It rests for a moment on a branch, then takes off, wings clattering, abdomen curling down in a J, looking for all the world like a helicopter gunship all weapons pods signaling “loaded”.
Dragonflies spend the first part of their lives in water. Eggs hatch into tiny ogres who crawl the bottom and rocks, using a wicked looking retractable lower jaw to snag insects, worms, and even small fish. The speed of the strike is among the fastest movement recorded in the animal kingdom. Dragonfly nymphs (technically naiads since they are aquatic) are top predators, the Great White Sharks, of the pool. Naiads may spend up to five years growing and lurking until one fine day, they climb out the water up a stalk or branch, split down the middle, and step out like Athena emerging from the head of Zeus. This process may take several hours, but at the end, a dragonfly has emerged, wings crumpled and shining like an enamel pennant. The insect pumps blood into the wings to expand them. Once expanded, the blood is withdrawn back into the body, and the hollow veins act as struts and braces (tubes are stronger than solid members) just like on a canvas and wood World War I biplane.
Just as birders are bedeviled by "Confusing Fall Warblers" and the myriad of sparrows ("just list it as an LBJ--little brown job”), and butterfly aficionados are made crazy by skippers (Peck's, whirlabout, or common? They all look the same), so too are dragonfly spotters beset by a whole group of look-alikes; the damselflies. Of the three groups, two (the broad wings and spread wings) are relatively easy to identify and contain manageable numbers of species. The group that makes people tear their hair out is the pond damsels. All pond damsels are small, all are blue, and all were put on this planet to humble the most discriminating of taxonomists. They can tell each other apart, but the luckless spotter is reduced to standing ankle deep in stagnant water, field guide in hand trying to remember if the light blue abdomen on the long-gone bug had nine or ten (or was it ten and a half) dark blue rings (that gets it down to only ten species or so). Audubon did his birding with a shotgun; they ought to come up with something similar for damselflies. You may still not get an ID, but you will have managed to waste the little bugger which is almost as satisfying.
No comments:
Post a Comment