== Insects (Insecta) ==
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=== Characters of parasitic insects of veterinary and medical importance ===
There are four Orders (a major taxonomic grouping) of insects with species of medical and veterinary importance: lice (Phthiraptera), fleas (Siphonaptera), two-winged flies (Diptera), sucking bugs (Hemiptera). The insects separated from the acarines (mites and ticks) very early during evolution and need to be considered separately, despite the words 'insect' or 'bug' sometimes being used to include mites and ticks.
The insect body consists of a series of structurally similar segments, and most of these are clearly visible. Segments of adult insects are distinctly grouped into: head, thorax, and abdomen. The head bears a pair of antennae to sense smells, and usually a pair of eyes.
Complex mouthparts are borne on the head. These include piercing tubes made of many separate cutting parts, and sometimes a sponge-like organ (labella) to collect liquid food. A pair of palps is used to sense food sources.
The thorax consists of three segments, each of which bears a pair of legs with many joints. In the two-winged flies (Diptera) the middle thoracic segment bears a pair of wings, and the posterior segment bears a pair of halteres. The halteres were by evolution derived from wings and assist the insect to fly.
There are two types of life-cycle of these insects. These are illustrated for lice and for two-winged flies. To develop from egg to adult all arthropods need to shed repeatedly their old exoskeleton to permit further growth and then mature a larger exoskeleton. The transition from one stage of growth of the exoskeleton to the next is a molt. These stages are also called instars.
Diagram of lice feeding on a cow is an example of a parasitic insect life-cycle with incomplete metamorphosis (a transformation of morphology and physiology). Lice develop from egg to first nymph stage then several more nymph stages, and finally to a mature adult, either female or male. These larvae and nymphs closely resemble the adults in structure and behavior. In this example all nymphs and adult feed parasitically.
Diagram of fly maggots and adults is an example of a parasitic insect life-cycle with complete metamorphosis, using example of two-winged flies. Fleas and two-winged flies develop from egg to first larva stage. The larva is worm-like, legless and clearly segmented, and behaves differently from the adult. The larva typically goes through two or three more molts. When the larva is fully grown it transforms into a pupa. Within the outer case of the pupa a complete metamorphosis occurs. The pupa transforms into an adult female or male then emerges from the case. In this example either the larvae or the adult may feed parasitically .
=== Glossary ===
Antenna = Paired organs protruding from head that sense odors (see Mosquitoes and similar ).
Exoskeleton = The external skeleton of insects, acarines, and similar animals.
Instar = Another term for a stage in the life-cycle of insects and acarines.
Labella = A single component of the mouthparts of insects, may be sponging or piercing (see Mosquitoes and similar )
Metamorphosis = A change in the size and/or form of an insect or acarine.
Molt = Shedding of exoskeleton to permit growth from one stage of life-cycle to next.
Order = A major taxonomic grouping, between class and family (see Introduction ).
Segment = Many invertebrate animals are basically composed of many similar functional units called segments. Insects are clearly segmented, but segmentation in acarines is obscure.(to Table of Contents)
== References ==