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Insects are an extremely diverse group as can be observed from the various groups outlined below. The class Insecta lies in the phylum Arthropoda. All constituents of this class possess distinct body regions: head, thorax and abdomen. They also all possess three pairs of legs. These distinguish insects from other members of the phylum Arthropoda such as spiders, crabs and centipedes. Insects also possess a pair of sensory organs on the head known as antennae. The majority of species in the adult form possess wings - this is not the case for any other arthropod or invertebrate.

There are two forms of insect life cycle: incomplete and complete metamorphosis. Both life cycles begin as an egg. In the case of exopterygote metamorphosis (e.g. grasshoppers, dragonflies, bugs), the nymphal stage resembles a small adult, generally with reduced wing buds and grows as a result of moulting the exoskeleton. The external exoskeleton is composed of chitin; this tough exterior requires moulting (ecdysis) once the old exoskeleton has been outgrown. Moulting continues until the adult size is reached. Due to wing development occurring outside the body these insects are termed exopterygotes. In contrast the larval stage of endopterygote metamorphosis bears no resemblance to the adult form (e.g. butterflies, wasps, beetles). The larvae increase hugely in size undergoing numerous moults until finally they pupate. Within the pupa the larval structures are degraded into a liquid by predetermined biochemical reactions and surviving cells divide and reassemble into adult structures. Eventually the pupa splits and the adult emerges. As the wings are developed internally these insects are termed endopterygotes.

Wing venation - Explained

The prehistoric insect wing had 8 pairs of main veins. Each pair diverged from wing base into anterior convex and posterior concave sector (e.g. MA and MP). The two sectors often fused into one veinal stem near the wing base. In evolution insect wing is in most case modified in reduction of veins. The various veins are labelled:


(i) Precoasta (PC) - This vein is fused with costa in all extant insects, mostly unrecognisable.

(ii) Costa (C) - at the leading edge of the wing, strong and marginal, extends to the apex of the wing, it is unbranched.

(iii) Subcosta (Sc) - the second longitudinal vein, mainly the subcosta posterior sector (ScP). Sc is reduced or fused with R in most Hemiptera.
(iv) Radius (R) - the third vein, usually the strongest vein on the wing, with branches usually cover the largest area of wing apex. RP is often referred to as radial sector (Rs) and the end branches as R1-5.
(v) Media (M) - the fourth longitudinal vein, MA and MP usually with 4 branches each. In some insect groups MA fused with R so only MP on the medial area. In this case the MP1-4 are often referred as M1-4.
(vi) Cubitus (Cu) - fifth longitudinal vein, CuA may branch to 4 or fewer veins. CuP is unbranched, lies near the claval fold and reach the wing posterior margin.
(vii) Anal veins (A) - veins behind the cubitus, AA and AP are usually separated by the anal fold. In Neoptera, AA is always fused with Cu or CuP. In the hind wings of most orthopteroid insects, there is a large anal area where anals branch several times to form a fan-like folded wing.
(viii) Jugal (J) - small veins in the jugal area, found only in Neoptera.

The black pterostigma is carried near the wing tip, between RA1+2 and RA3+4. It may coloured in some insects e.g. dragonflies, sawflies. Cross-veins are transverse veins joining longitudinal veins. Their names are based on the position relative to longitudinal veins, e.g. r-m is the cross-vein between radius and media.