The body cavity of an insect (haemocoel) extends throughout the head, thorax and abdomen and also reaches into the appendages. It is filled with a pale, often colourless, blood-like fluid called haemolymph. This bathes the internal organs and tissues, and is circulated by a tube-like heart which runs mid-dorsally from the head to the tip of the abdomen.

The nervous system consists of a brain, with close connections to the compound eyes, the antennae and the mouthparts, and a central nerve cord that extends back mid-dorsally through the thorax and abdomen. The nerve cord includes a series of swellings (ganglia) from which arise various lateral nerves. The brain occupies much of the head and is the main co-ordinating centre of the body; the ganglia, however, control many activities (such as movement of the appendages) independent of the brain.

The alimentary tract is, essentially, a long, often much modified, tube stretching from the mouth to the anus. There are three main sections: foregut, mid-gut and hindgut, located mainly within the abdomen. The foregut includes a crop within which recently ingested food accumulates. Digestion and absorption of nutriment occurs within the mid-gut, whereas the hindgut is concerned with the absorption of water and the storage of waste material prior to defaecation. The insect gut includes a large number of long, whitish, blind-ending tubules (Malpighian tubules), which arise from between the mid- and the hindgut; these tubules collect waste material from the body fluids and pass them into the gut. The haemocoel also contains an often large organ, known as the fat body, which forms whitish, yellowish or brownish groups or layers of cells. The fat body is concentrated mainly in the abdomen and serves various functions, including the synthesis and storage of fat, glycogen (= carbohydrate) and protein.

The respiratory system includes a series of small branching tubes (tracheae) and microscopic tubules (tracheoles), which maintain contact with the internal body organs and tissues. The tracheal system may either be open or closed. The former opens to the outside through a series of valve-like pores (spiracles), which occur along either side of the insect; the spiracles are sometimes located on characteristic respiratory processes. Various types of respiratory system are recognizable, including:

• amphipneustic - spiracles present on the prothoracic and anal segments only (typical of many dipterous larvae);

• apneustic - spiracles absent, i.e. tracheal system closed (typical of aquatic insects which breathe through gills);

• holopneustic - spiracles present on the mesothorax, metathorax and abdominal segments 1-8 (typical of most adult insects and many nymphs and larvae);

• metapneustic - spiracles present only on the anal segment (typical of certain dipterous larvae, including leatherjackets, mosquito larvae and syrphid larvae);

• propneustic - spiracles present only on the prothoracic segment (as in mosquito pupae).

Some insects are devoid of both spiracles and a tracheal system (e.g. Collembola and larvae of certain endoparasitoids); these forms are termed atracheate.

In females, the reproductive system is composed of a pair of ovaries, each subdivided into numerous egg-forming tubules called ovarioles. Other features include a pair of colleterial glands (often called cement glands) and a sac-like spermatheca in which, after mating, sperm is stored. The ovaries unite to form a central oviduct that opens to the outside through a genital pore on the ninth abdominal segment. Eggs are usually deposited through an ovipositor but in some insects the tip of the female abdomen is constricted into a tube-like oviscapt and an ovipositor is wanting. In some insects (e.g. bees and wasps), the ovipositor has lost its egg-laying function and, instead, serves as a sting. Male insects possess two testes, each of which opens via a long duct (vas deferens) into a seminal vesicle in which sperm is stored. The seminal vesicles (vesicula seminalis), along with a pair of accessory glands, open into a single ejaculatory duct which extends to a gonopore located on the eighth abdominal segment.

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