Life of the Dragon Fly
This is another word for transformation and, in the insect world, it is applied to the changes that take place during the lifetime of a single individual. During the development of some insects there is virtually no change in form (except for size) from the moment of hatching out of the egg, to the fully mature adult. For instance, a newly hatched cricket larva is, in appearance, a tiny wingless replica of its parent.
At the other extreme, there are species that enjoy a complete metamorphosis: in the Order Lepidoptera (butterflies & moths), for example, the egg hatches into the caterpillar (larva) which pupates into the chrysalis (pupa), from which emerges the adult (imago).
Between these two forms of insect development is that of those subject to partial change and it is here that we find odonates. The pupal stage is omitted but the differences between larva and imago are considerable.
Odonates have two different methods of laying eggs and the basic shape of the egg depends on the method used. All damselflies and, among dragonflies, the members of Aeshnidae (Hawkers), Neopetaliidae (Redspots) and Petaluridae (Petaltails) are endophytic which means that, having well formed ovipositors, they insert their eggs into plant material above or below the surface of the water; this is the more primitive method. Most other dragonflies are exophytic: these lack functional ovipositors and merely deposit their eggs directly onto the surface of the water or into mud at the water’s edge. The eggs of endophytic species are long and cylindrical, while those of exophytic species are broad and elliptical.
At one end of the egg there is a minute hole through which the sperm enters just before oviposition and from which the larva will later hatch. In some species the eggs are surrounded by a jelly-like substance that enables the eggs to attach themselves to the leaves of plants or to stones and rocks under the water, so preventing them from sinking into the mud or being swept away by fast-flowing water. Although some species over-winter as eggs, most eggs start to develop soon after they have been laid and the larvae hatch out one to three weeks later.
This is the growth stage of an insect’s life. Like all Arthropods, the developing larva must repeatedly shed one outer casing in order to grow a new one. Periods between these moults are called ‘instars’ and the number of instars necessary to complete development varies between eight and fifteen. By the time the aquatic larva reaches its final instar, it will have developed all the organs and other attributes that will be needed to sustain life as a winged terrestrial insect.
Odonate larvae are aquatic and breathe through gills. In most damselflies, these take the form of three leaf-like appendages at the tip of the abdomen, whilst the gills of dragonflies are projections within the rectum. Both respiratory systems also serve the larvae as means of moving around in the water: damselflies’ appendages are used as rear paddles and the pump that ventilates the dragonflies’ rectal gills provides a spectacular jet propulsion.
The larvae of dragonflies have robust bodies that are somewhat bullet-shaped if they live amongst water plants, or flattened if they live in bottom deposits. Damselfly larvae have slender, cylindrical bodies ending in the three conspicuous leaf-like gills which sometimes bear striking patterns. Their chief predators are fish and frogs while their main source of food is fish-spawn, tadpoles and the larvae of smaller insects. The larval stage can vary in duration from about three months to four or more years.
The larva having completed its growth and development will, when circumstances are right, leave its aquatic environment and start a new life, almost completely divorced from water. Although some species of odonates can emerge on a flat surface (Gomphidae for example and most damselflies), the majority need a vertical one; the larva climbs up the stem of a reed or other plant, until it is well out of the water, and affixes itself firmly by means of its claws. After a pause, the larval casing breaks at the back of the head and, slowly and laboriously, the adult insect emerges. ‘Blood’ is then pumped strenuously round the body, an action that expands the body and also the wing-buds, transforming them into the beautiful lace-like wings which the insect will soon use to fly away from the water.
The Imago (adult).
The necessary growth having been achieved during the larval stage, the imago can concentrate on ensuring the continued existence of its species: it is the stage of dispersal and reproduction.
(i) Dispersal period.
Immediately after emerging, young adults instinctively head away from water and fly off into the surrounding countryside. The dispersal period, which lasts from a few days to two or three weeks, is important in two ways. First, as will be seen later on, it has probably accounted for the survival of the Order Odonata. Secondly, it is the period during which the newly emerged insects attain full coloration and sexual maturity: they will not make their way to water until they are ready to mate..
(ii) Reproductory period.
This period lasts around two to three weeks in dragonflies and one to two weeks in damselflies, although the period is sometimes considerably extended.
The mating of odonates is virtually unique in the animal kingdom: although his sperm is produced (as is normal in all insects) near the tip of his abdomen, the male has an accessory organ on the underside of his second abdominal segment to which, prior to mating, he must transfer his sperm. When a male encounters a receptive female he will use the appendages at the extreme tip of his abdomen to clasp her securely behind her head (dragonflies) or neck region (damselflies) and both partners will curve their abdomens so that the female’s genitalia engages with the male’s accessory organ, the pair thus forming the characteristic ‘wheel’ position. Depending on species, mating may last from a few seconds to more than an hour. Before actually ejecting his own sperm, the male will sometimes remove any that may have been deposited by a previous suitor. Promptly after mating, the female will commence egg-laying and will then fly away from the water until she has another batch of eggs ready for fertilisation.
In some species this is carried out in tandem, the male continuing to hold his mate’s head or neck while she deposits her eggs. In other species, the pair separates but the male will hover over the female, thus discouraging other males from mating with her. In a third group, the female oviposits totally unattended but, in these cases, she will do so in secluded places, often under a bank or among thick vegetation.
(iv) Life expectancy.
The average life expectancy of the adult odonate depends on the part of the world in which it lives. Generally speaking, in temperate zones the largest portion of an odonate’s lifetime, which may amount to several years, is spent in the larval stage while the adult phase is one or two months. In species common to the tropics and subtropics, however, larval development may be reduced to a few months and the adult stage may last a full year.
In temperate zones, adult odonates that survive the vulnerable period between commencement of emergence and successful maiden flight, have an average life expectancy of 4-6 weeks (dragonflies) or 1-2 weeks (damselflies).
Generally speaking, dragonflies are larger and more robust than damselflies. Other differences between them are as follows:
Differences Between Dragonflies & Damselflies
|Generally strong fliers||A weak, fluttery flight|
|Eyes (apart from Gomphidae & one or two others) touch on top of the head||Eyes are well separated|
|Fore- & hindwings are of different shape||Fore- & hindwings are of similar shape|
|At rest, the wings are held away from the body at an angle of approximately 180°||At rest, the wings (apart from Lestidae and one or two others) are held close to the body|