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[R] Articulate animals, covered with an exoskeleton (integument) and belonging to the phylum Arthropoda. This links them to the arachnids (spiders, scorpions, mites, etc.), the crustaceans (including woodlice) and the myriapods (symphilids, diplopods, chilopods).
Body typically divided into three parts:.
-the head, bearing the mouthparts, the antennae (one pair) and the eyes (compound in the adult);.
-the thorax, composed of three segments, each bearing a pair of jointed legs, the second and third segments usually possessing wings;.
-the abdomen, normally formed of 10 segments and containing the reproductive organs, the terminal segments sometimes bearing appendages (cerci) and/or reproductive parts used in mating (genitalia) or egg laying (ovipositor).
The development of insects and of all other arthropods is punctuated by a series of moults. Each moult marks the beginning of a new instar.
The egg hatches into either a larva or a nymph which passes through several instars (called L1, L2, L3, L4, L5, etc. or N1, N2...) until (at least in larvae) metamorphosis takes place. The larva or nymph is devoid of wings (or possess only rudimentary ones) and is sexually immature.
The adult (or imago) is characterized by the ability to reproduce and by the possession of wings in the winged species or forms. The adult does not moult (except in certain primitive insects).
In hemimetabolous insects, the final-instar nymph is transformed directly into the adult.
Two moults are necessary in holometabolous insects, where there exists an intermediate stage, the pupal stage, resulting from the larval-pupal moult. During pupation, the larva undergoes striking changes (e.g. the larva of Lepidoptera changes into a butterfly or a moth). The pupa does not feed and most are immobile.
The development of insects is controlled by hormones and often interrupted (embryonic development, growth, feeding, locomotion and, occasionally, reproduction are stopped); there are two types of arrested development :.
- quiescence, directly controlled by the thermic conditions prevalent in the individual's living environment (insects are poikilothermic animals: a body temperature regulation system is lacking).
- diapause, triggered by an external event.
Reproduction in most insects is bisexual. However, parthenogenesis (the female produces asexually young without the male taking part) can be the rule. It is also characteristic of certain phases in the insect's cycle (e.g. in aphids) but does not always occur.
Certain insects have a brief life (a few days) and several generations follow one another in the year (multivoltine insects). The life cycle of most species lasts a year with just one generation (univoltine insects). A few live several years. Many have complicated and variable cycles.
The insect egg is relatively large. Its envelope is termed the chorion. The yolk, situated inside it, provides the embryo with food in the course of its development. Certain groups of insects lay eggs in masses, i.e. in "egg masses"; others deposite them in a protective envelope (o÷theca or egg bag).
Insects have varied diets.Insect mouthparts are either adapted for chewing solid substances (biting mouthparts of Orthoptera, Coleoptera, larvae of Lepidoptera, Hymenoptera, etc.), sucking liquids (piercing and sucking mouthparts of Heteroptera, Homoptera and certain Diptera) or lapping up exposed liquids (sucking and lapping mouthparts of Lepidoptera, many Diptera etc.).
The class Insecta numbers about one million classified species and is divided into approximately thirty orders. In crop protection, the most important families are : the Coleoptera, the Lepidoptera, the Diptera, the Hymenoptera, the Heteroptera, the Homoptera, the Thysanoptera, the Orthoptera and the Planipennia.
The majority of these orders includes potentially harmful species, competing with man for his resources (pests of crops and trees, depredators of food products, enemies of Man, domestic animals or game) and beneficial species (natural enemies of pests, pollinators, decomposers, etc. and insects otherwise valuable to Man, e.g. the honey bee, the silkworm, etc.).

[R] Images

* Typical insect (adult) ()
1: head (*) .
a: antenna; b: compound eye; c: mouthparts.
2: thorax.
e: fore leg (*) ; f: mid leg; g: hind leg; h: fore wing; i: hind wing.
3: abdomen.
j: abdominal tergites ; k: abdominal sternites; l: cerci; m: ovipositor (female).

* Insect head (INRA)
After a primitive insect, with chewing mouthparts.
a: antenna; b: ocellus; c: compound eye; d: prothorax; e: clypeus; f: labrum; g: maxillary palpus; h: mandible; i: labial palpus.

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