SCOTT SHALAWAY Wetland assassins are human-friendly
Naturalists who enjoy watching and studying animal behavior are often frustrated by the difficulty of finding subjects to watch. Most mammals are nocturnal and secretive. Reptiles and amphibians are hard to find. And birds so easily flee to dense vegetation. But just above the surface of most lakes, ponds, and streams live some of the most common and easily observed members of any wetland community -- dragonflies and damselflies.
These insects, members of the insect order Odonata, are relatively large, easy to see, and confine their territorial activities to a small area. Birders should find odonates especially appealing -- many are brightly colored and strikingly marked, they are diurnal and strongly territorial, and their feeding strategies mimic several familiar groups of common birds.
The basic body plan of these insects is similar -- a large head dominated by huge compound eyes, lacy transparent wings, and an elongated body.
Dragonflies are larger, have stouter bodies and at rest, hold their wings flat and perpendicular to the body. Damselflies, as the name suggests, are more delicate. They have much slimmer bodies, and at rest, hold their wings together over the body.
Pose no harm: Though all odonates are predaceous and ferocious looking, dragonflies and damselflies pose no harm to people. They do not bite or sting like horse flies or wasps. They patrol wetlands and even nearby fields and woodlots in search of prey.
Some species hunt from a perch like flycatchers; some hunt on the wing like swallows; and some fly slowly through vegetation and snatch prey from the surface of plants like kinglets and gnatcatchers. In any case, their huge compound eyes, which consist of thousands of simple eyes, enable them to spot small flying insects -- flies, gnats, mosquitoes -- in flight. .
Powered by two pairs of strong and maneuverable membranous wings, they use their legs to capture prey. Their legs are covered with spines, and when they are held just right, they form a basket or net into which they funnel their victims. After capturing their prey in the feeding & quot;basket, & quot; they either eat it on the wing or rest on a perch and enjoy a leisurely meal.
Next time you get to the water's edge, spend just 10 or 15 minutes watching the dragonflies and damselflies. See how they hunt, and notice their fiercely territorial behavior.
Defend small areas: Male dragonflies and damselflies defend small areas, again just like many birds do. When not hunting, they spend most of their time perching, patrolling and criss-crossing the territory watching for intruders. While perched, they vigilantly scan the territory for intruders. If another male approaches, the resident male gives chase.
Usually the intruder leaves, but sometimes there is a fight accompanied by a noisy clash of wings. Other times the rivals hover, face each other, and perform an aerial dance. The outcome of these ritualized match-ups determines which individual is dominant.
The reward of dominance is access to females for mating. When a female visits an occupied territory, the dominant male mates with her. After mating, the female hovers just above the surface of the water and lays eggs on floating leaves or on rocks just below the surface. Females of some species insert their eggs into the tissues of aquatic plants.
About a week later, the eggs hatch and the nymphs drift to the bottom, where they assume their role as fierce underwater predators. Dragonflies may live in the nymph stage for several years before transforming into adults. Damselflies usually emerge as adults the following year.
When the time comes to transform into the adult stage, the nymphs climb out of the water onto a piece of emergent vegetation. Before the nymph can dry out, its exoskeleton splits down the back, and an adult dragonfly or damselfly emerges. The adult rests for several hours while blood pumps through and unfurls its wings.
As soon as the wings dry out and harden, the contests for territories, dominance, and females begin.
Short-lived: Adults live for only a few weeks, but because nymphs emerge and transform into adults all summer long, dragonflies and damselflies are always abundant.
For more information, consult Dragonflies through Binoculars: A Field Guide to Dragonflies of North America by Sidney Dunkle (2000, $29.95, Oxford University Press).