Caracteristics/Anatomy



Many characteristics distinguish Odonata from other groups of insects:

Dragonflies and damselflies share many common features, however there are a number of distinguishing characteristics. First of all the eggs have unique morphologies such as dragonflies (Anisoptera) are short and round while damselflies (Zygoptera) have long thin eggs. The larvae of the two groups are also readily distinguishable. A larval damselfly abdomen is longer and narrower with three fin-like gills projecting from the end of the abdomen. Dragonfly nymphs are shorter and bulkier, and the gills are located inside the abdomen.

 

 

 

 

 Generalized Dragonfly (Anisoptera) Larva & Egg

 Generalized Damselflies (Zygoptera) Larva & Egg.

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Larvae
It is among the larvae that the greatest range of form is found. Most of a dragonfly's life is spent in the larval stage where it moults from 6-15 times. Larval development varies from the common 1-2 years to as many as six years. Rates of development depend on a combination of inherited mechanisms, and environmental factors such as altitude and latitude, food abundance. The larvae of some species can tolerate quite a wide rage of conditions, while others are highly adapted to specific habitats. Factors which can effect the distribution of larvae include water pH, amount and type of aquatic vegetation, and whether the water is still of flowing. Dragonfly larvae possess a highly specialized mouthpart, known as the labial mask, which can be extended rapidly to grab prey such as small animals. Odonata larvae detect their prey mainly by sight and touch (vibrations).
Towards the end of the larval stage of the life cycle, the compound eyes develop rapidly, the wing buds expand and the flight muscles increase in size. The respiratory rate rises, and 1-2 days before emergence the larvae stop feeding and the tissues are withdrawn from the terminal part of the labial mask. Finally the gills cease to function, and the spiracles (pores for air intake during respiration) begin to open. Unlike fully metamorphic butterflies and beetles, dragonflies and damselflies do not have an intermediate pupal stage before becoming an adult. Therefore, Odonata are known as hemimetabolous insects, which undergo an "incomplete" or "gradual" metamorphosis.

Flight

The flight system of the dragonfly is primitive in the arrangement of flight muscles and the type of wing. Despite this dragonfly flight performance is highly efficient. The complex venation system provides a light yet stiff structure. A nodus, or break halfway along the leading edge of the wing, allows the wing to be twisted along its axis. This is a specialization of dragonflies giving them higher manoeuvrability. Most dragonflies also develop a prominent spot, the pterostigmma, near the wing tip. This serves an aerodynamic function by adding weight to the end of the wing, which it thought to assist the twisting of the wing at the top and bottom of the stroke. The fore and hind wings are controlled independently, and normally beat out of phase allowing for high manoeuvrability. Dragonflies dramatically increase their metabolism during flight. Therefore there is a high demand for fuel and oxygen. Stored carbohydrates fuel most flight, however migratory flight will also be sustained by fat tissue. Oxygen is supplied to the flight muscles through a highly specialized tracheal system (tubular network used for respiration).

 

 Dragonfly (Anisoptera) Wings

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Vision

 
 Dragonflies have very large compound eyes relative to their body size. In addition there are three simple eyes ocelli on the upper part of the head. More than 80% of their brain is devoted to processing visual information. The compound eye is made up of thousands of ommatidia, each ommatidium is a visual unit consisting of a lens system and a photoreceptor. This highly developed visual system allows prey to be caught with great efficiency.

 Compound Eyes



Author: Andre Butler
Last updated: 20th November 2005
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