Caracteristics/Anatomy
Many characteristics
distinguish Odonata from other groups of insects:
- Pair of short bristle-like antennae.
- Extremely large compound
eyes.
- The protothorax is relatively, small,
and the mesothorax and metathorax are fused into a robust pterothorax.
- Legs are adapted for grabbing prey.
- Long abdomen with 10 visible segments,
which terminate in clasping organs in both sexes.
- Two pairs of transparent membranous wings with many small veins,
- an aquatic larval stage (nymph)
with posterior tracheal gills, and a prehensile labium (extendible
jaws underneath the head for catching prey).
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External Characteristics
of a Dragonfly (Odonata) |
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.
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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).
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Dragonfly
(Anisoptera) Wings |
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Vision
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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. |
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Compound Eyes |
Author: Andre Butler
Last updated: 20th November 2005
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