Odonates---An Introduction

Odonates — an Introduction:

‘Odonates’ is a collective term for the dragonflies and damselflies – those small, slender, four winged creatures seen hovering above the grasses and water bodies, sometimes perching among the vegetation and at times in long flights above the ground. Most people get attracted to these by their super-fast flights, especially those of the dragonflies.

     The term ‘Odonate’ has been derived from the Order of these insects, which is Odonata. The Order Odonata consists of three suborders – Anisoptera (Dragonflies), Zygoptera (Damselflies) and Anisozygoptera (Relict Dragonflies). Of these, Anisozygoptera or the Relict Dragonflies are the rarest.

Above: A simulated image of the Meganeura.

©National Geographic Society.

The dragonflies’ ancestors were probably the Meganeura, of the Order Meganisoptera. In a simple way, we can describe it as a larger version of the modern dragonflies. They evolved about 250 million years ago during Devonian period and largely remained

unchanged morphologically.

     An odonate’s body is divided into three parts:

1.       The head, with two large, well-developed compound eyes and biting mouthparts.

2.       The thorax, which is further divided into two parts:

a.       The anterior prothorax (nearer to the head) bearing the pair of legs in front (the 1^st pair)

b.      The fused synthorax, bearing the other two pairs of legs (2^nd and 3^rd pair) and the two pair of wings

3.       The long, thin abdomen, divided into 10 segments (of different sizes). The 10^th segment ends in structures called anal appendages.

     Both dragonflies and damselflies have a larval stage and an adult stage (imago). The adult odonates require specific water bodies to lay eggs, in which the larvae can develop; consequently, the odonates are frequently found near water. The larval stage lasts from few months to several years; the longest reported to have been nearly 7 years. In fact, the larval stage constitutes the larger part of the lives of odonates. During this stage, the larva rapidly feeds on anything smaller, which it can manage, and moults about 9-15 times in order to grow in size. When the larva becomes mature and the wing pads are fully formed, it crawls out of water and attaches itself onto the surrounding vegetation or rocks. During this stage, its skin splits from the back and the adult odonate emerges. It is very vulnerable in this stage as the body of the adult odonate is still soft and it is unable to fly. It slowly emerges and remains there until its body and wings are sufficiently strong and dry. After this, it enters the short, teneral stage, during which the body and wings further hardens. In this stage, the odonate cannot fly for a long time. After this, it becomes the imago, and flies around, sometimes in quick, long flights in search of mates and also food. It again feeds on anything it can manage, like midges, mosquitoes, other flies and sometimes, other odonates. When males become mature, they establish their own territory, which they guard aggressively and never go beyond its limits, unless forced to. Many species of odonates, such as Crocothemis servilia, spend most of their time perching on reeds and other ‘perches’, trying to find an unsuspecting prey or a for a mate. Others like Pantala flavescens, engage in long flights in swarms consisting of both males and females.

       When the females arrive in these territories, there is stiff competition among the males. The male Calopterygid damselflies probably use wing-flashing displays to attract females, though they may also be used for warning other males or any other sort of signal (I’m not so sure). The victorious male in the competition is allowed to grab the female by the occiput (in dragonflies, close to the head) or the prothorax (in damselflies), with the legs and then the anal appendages. The male grips the female by his anal appendage and the female bends her abdomen so that her anal appendage touches the second segment of the male, which bears a secondary copulatory organ. This position is called ‘Copula’.

     After this, in dragonflies, the female breaks contact and dips her abdomen in water to lay the eggs, while being guarded by the male. In damselflies, the eggs are laid without breaking any contact. Damselflies and some dragonflies like the Darners (Aeshnids) lay their eggs on underwater vegetation (endophytic) while other dragonflies lay their eggs in open water (exophytic). And the process continues all over again.

    When the outside temperature is high, some dragonflies dip their abdomen in water to cool down. They may also adopt a posture known as obelisk posture, where the dragonfly lifts its abdomen to a near vertical position and also the thorax by a few degrees. Only the legs remain in contact with the perch and since the surface area in contact is reduced, the body temperature is lowered.