The inside of insects

Fig. 3.2. Insects interior, schematic. Boas-Thomsen. 1,2,3: the three pairs of legs are located on the thorax joints, a: hindgut, c: brain, ch: midgut, e: anus, h: dorsal tube (heart), k: crop, m: mouth, n: thoracic ganglion, o: oviduct (or testes), o ‘: genitalia, s: digestive tract, sp: salivary gland, t: foregut, tr: peritrophic matrix, where the peritrophic membrane is formed, u: malphigian tubule which corresponds to our kidneys.

Insects have similar organ systems to the one we know from people, but they are often built in different ways and may lie elsewhere in the body. Insect central nervous system is placed just opposite to vertebrates.
The intestinal tract is basically a tube that leads from mouth to anus. A few large salivary glands end in the oral cavity. Food passes through the alimentary canal which is usually extended with a salivary reservoir that acts as a reservoir for food. The rear part of the foregut is often developed as a muscle and reaches into the midgut like a funnel. In the midgut, or intestine, digestion occurs by enzymes produced by cells in the midgut. The midgut is separated from the rectum by a sphincter. In the rectum dietary water content is absorbed. The so-called malphigian tubules correspond to our kidneys. It is a sort of glands that absorb waste products from the blood and leads them out through the rectum.

Insects' and mites' foreplay

Fig. 3. 3. Even the smallest of insects and mites have preludes to the actual mating. During the prelude, or foreplay, the insects exchange signals that are specific to their species. The male fruit fly, Drosophila melanogaster, which is shown here in black, places itself in a certain way when the female is near. It performs a complicated dance that ends with palpation of the female and finally the transfer of seamen takes place. If the male makes the wrong movements in the dance, the female will perceive him as if he were a different species and reject mating. (Manning 1965).

Transport systems for nutrients and oxygen are among the organs that are very different from the equivalents in vertebrates. Insect blood does not run in veins, but runs freely around in the body. The heart is built like a vein of muscles that provide some circulation of blood. Blood transports nutrients and hormones around to different parts of the body, but the insect blood will not distribute oxygen! A system of fine tracheas lead oxygen directly to the organs where it is needed, such as brain and muscles, from breathing holes in the surface of the insect. Carbon dioxide and water leak out the same way. The oxygen transport in trachea happens mostly by diffusion, but some insects aid the ventilation by muscular pumps in the abdomen.

A typical insect has a few breathing holes on each of the two rear thorax segments and the eight anterior segments of the abdomen. (This means that an insect cannot be drowned by holding its head under water). The breathing holes are provided with filters and with complicated closing mechanisms, so that they can be opened and closed as required. In dry environments the breathing holes can be closed so the insect avoid excessive fluid loss through breathing holes. The need of oxygen varies from species to species and is according to conditions in which the insects live. The higher the temperature and the more active the insects are, the more oxygen is needed. Insects can survive at low oxygen levels for long periods at a time.

The insect nervous system has exactly the same function as ours. The brain is in the head just above the oesophagus and is connected to the subesophageal ganglion by two nerve strands surrounding the oesophagus. From the subesophageal ganglion the nervous system reaches into the rear of the insect, typically in the form of two nerve strands which connect a number of ganglia. The female reproductive organs include a pair of ovaries, where eggs are formed. The oviducts meet in the vestibulum or the vagina. In the vestibulum, there is normally a tiny area, the spermatheca, which stores sperm. The sperm is stored in the spermatheca during mating and from here the sperm is later released and will fertilize the eggs one by one when they pass through the vestibulum. The vestibulum is also connected to glands that can give off sticky matter on egg surfaces. The male reproductive organs consist mainly of testes where sperm is formed. From each testis, a sperm tube leads to the seminal vesicle, which stores sperm until it is discharged into a female. Deferent ducts connect in the ejaculatory duct, which leads to the penis.