Insect Orders
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The Three Insect Divisions
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Below is a brief description of the three insect divisions.
Information on the thirty insect orders listed on this site can be obtained by clicking an order button underneath each of the three divisions
The Apterygotes are mostly small wingless insects living on and within decaying vegetation. The name Apterygote actually means without wings. Although it is an area of great debate among taxonomists it is generally believed that these
insects have never had wings throughout their evolutionary history. Studies of the thorax of many Apterygote insects has failed to reveal any vestiges
of flight structures such as those seen in fleas and lice which have lost their wings as an adaptation to their parasitic lifestyles. The presence
of abdominal appendages, arranged on either side of the body are believed to indicate a multi-legged ancestor. Another characteristic of Apterygotes is either no or only minimal metamorphosis. Five orders are recognised although their relationship to each other is not
clear at present. Some entomologists place the Protura and the Collembola into separate classes within the Arthropoda, believing that they are not insects at all.
The Apterygote Orders
The Exopterygotes are insects which have two pairs of wings, or
did have at some point in their evolutionary history. These insects have what is called an incomplete metamorphosis
with the young stages showing a degree of similarity with the imago stage. This division is also referred to as the
Hemimetabola, a name based on the incomplete metamorphosis. The name Exopterygotes stems from the type of wing bud
development. The wings arise from external buds on the thorax which increase in size as the young insect passes through
its nymphal stages. This type of external growth also occurs with the genitalia. This type of incomplete metamorphosis
was referred to as ametabolous or paurometabolous by Snodgrass (1954). There is often a considerably change from the last
nymphal stage to the adult imago, although these changes are not always easy to observe. In Rhodnius the nymphal stages show both numerous plaque bearing sensilla, and a
stellular (star shaped) type folding of the cuticle. After ecdysis (moulting) the adult cuticle shows no plaques, few sensilla,
and transverse, rather than stellate folds. The typical incomplete metamorphosis of an Exopterygote insect is that seen in Odonata (Dragonsflies)
from its aquatic form to its terrestrial form.
The Exopterygote Orders
The Endopteraygotes are also termed the holometabola, these insects exhibit complete metamorphosis
with a pupal stage resulting in an imago with no or very little resemblance
to the larval stages. These insects are either winged or have lost their wings
secondarily. Both the wings and the genitalia develop internally during the larval stages.
The Exopterygota division includes what are often termed the "higher insects", such as the beetles,
true flies, butterflies, moths, bees, ants and wasps. The term "higher" is used
loosely to refer to a more complex life-cycle and morphology, not to indicate that
this group is any more successful than the other two divisions, many of which have
been in exsistence a great deal longer.
© Rob Hutchinson
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