Dragonfly Life Cycle
In most species, male dragonflies are fiercely competitive over preferred breeding and mating sites. Only the dominant males will get an opportunity to mate; others will be driven away. Competitions between males include sparring, flight contests and threat displays of bright colors on the abdomen or wings. Female dragonflies are not sexually competitive, but, like many males, they will compete with other dragonflies for the best feeding grounds.
Prior to the selection of a willing female, the male will transfer sperm from his testes located on the underside of abdominal segment 9 to his hamulus located on the underside of segments 2 and 3. This is accomplished by simply arching the abdomen until the undersides of the appropriate segments make contact. Mating is normally initiated by the male who, with the grace of a professional wrestler, uses his legs to grasp the female by her head and thorax. Curving his abdomen forward, he uses his two cerci and the lower epiproct as a clamp and clasps the female by the back of the head. They are now “in tandem.” Mating is accomplished by the male arching his abdomen downward while the female arches her abdomen toward the male’s hamulus. Once connected, the pair is in the wheel position or “in copula.” Still connected, the pair will usually fly up to the safety of treetops to mate, although some species do copulate in mid flight. The male commences a purging of the female’s genital opening. He uses the hamulus to remove, squash or push out of the way any sperm that the female may still be carrying from prior matings with other males. This process ensures his genetic investment in the clutch of eggs that the female will soon lay. The time needed to complete fertilization ranges from 15 seconds to well over an hour.
Guarding and Egg Laying
After the completion of copulation, the couple may split up, or they may stay together through oviposition, i.e. egg laying. In some species, the male guards the female from the competition of other rivals and possibly even from predators. In other species the female is left alone to lay her eggs. Guarding may take several forms. The most basic form is contact guarding, where the male stays attached to the female for the entire egg-laying process. Hover guarding is another strategy in which the male hovers above or perches near the female as she is laying eggs. From this vantage point he can attack and chase away from his territory any competing suitors as well as mate with any other females that enter his territory. A hover-guarding male may attempt to protect several of “his” egg-laying females at a time. There is also a rare type of guarding known as karate guarding where the protective male actually clasps a trespassing male, much as he would grab a prospective mate, and holds him until the guarded female has laid her eggs.
There are many different oviposition, or egg-laying, strategies employed by dragonflies. Many female darners use their lance-like ovipositors to insert eggs into plant stems, sphagnum moss, rotting wood or wet soil. But most species of dragonflies possess non-functional ovipositors. Eggs must be washed off into water during flight as the female dips the tip of her abdomen into the lake, pond, river or stream. Other dragonflies have special flanges that flank the genital opening, allowing them to strike the surface of the water, purposefully splashing eggs into the water. Some females plunge the entire tip of their abdomen into mud or silt to deposit eggs. Others simply sprinkle eggs over a suitable habitat.
Among the many parasites that may infest dragonflies, by far the most common is the water mite (Hydracarina species). The larval mites attach themselves to larval dragonflies and feed off of their host’s body fluids. Very young nymphs and unhatched eggs may actually be killed by water mite larvae, whereas larger dragonfly larvae are able to survive such an onslaught and may host a myriad of water mites. Look also for the tiny red critters on the legs and thorax of adult dragonflies. Back to the top
Life as an Aquatic Larva
Many people are astounded to hear that the stately and colorful dragonfly spends the majority of its life as a drab, creepy-looking underwater larva. Unlike butterflies or mosquitoes, which undergo a complete, four-stage metamorphosis, dragonflies undergo three stages of development known as incomplete metamorphosis. There is no pupal stage in the life of a dragonfly. Incomplete metamorphosis consists of the egg, the larva (nymph) and the adult.
After the egg hatches, the free-crawling aquatic larva molts once and then starts hunting voraciously. As the larva grows, it will molt numerous times. Most dragonfly larvae mature to adulthood in one to three years. There are exceptions. The migratory Wandering Glider will take as few as four weeks to complete its development from egg to adult, whereas there are Asian species that take as long as 8 years to mature to adulthood. Some species known from the far North take longer to mature than the same species in more southerly environs. Water temperature and the length of the growing season are variables that help determine the length of maturation.
A day or two prior to emergence from the aquatic to the aerial form, the larva goes into a state of diapause, or rest, while the final changes are made inside the larval exoskeleton. They may sometimes rest with a part of their head above water to facilitate the transition to breathing air. Back to the top
Emergence, the transition from aquatic larva to adult dragonfly, usually takes place very early in the morning while clinging to a vertical or diagonal surface such as a plant stem, rock face, tree trunk, dock or bridge abutment, although many clubtails emerge from a horizontal position. When in position, the larva hooks its claws into the perch. After a short period of rest, the skin at the back of the head cracks open, and the thorax emerges from the larval skin. The split enlarges down the back, and the head, compressed wings, legs and part of the abdomen are forced out. The dragonfly rests again, arched backwards and hanging from its unreleased abdomen. During this time, the legs harden. Grabbing its shedding larval case with its “new” legs, the dragonfly pulls its abdomen free. The abdomen extends, and the wings unfurl as they fill with hemolymph (blood). After a short while, the hemolymph is drawn back into the body, and then the juvenile, or teneral dragonfly, rests, letting the wings dry for about an hour before the first flight is attempted. The discarded exuviae—the empty larval shell— will remain “perched” until wind, rain or a curious naturalist removes it. By the time a dragonfly makes its first flight, it is at full, adult size. Little dragonflies are not babies; they are full grown adults. No more molting will take place after they leave their larval case behind.
Transformation is a very vulnerable time for the dragonfly. As much as 90 percent mortality has been observed in a population due to bird predation. Many dragonflies are ready to fly soon after sunrise, a potentially successful strategy used to avoid being eaten by the “early bird.” Spiders and ants eat their fair share of young dragonflies, as well. Back to the top