Insect respiration is independent of its circulatory system. Therefore, blood does not play a direct role in oxygen transport.
Insects have a highly specialized respiratory system called the tracheal system, which consists of a network of tiny tubes that carry oxygen throughout the body.
The tracheal system, the most direct and efficient respiratory system inactive animals, has tubes made of chitin polymeric material.
Insects and some other invertebrates exchange oxygen and carbon dioxide between their tissues and the air through air-filled tubes called tracheas.
The basic respiratory system of insects consists of a series of rigid tubes, which are connected and open outwards through small holes or pores called spiracles. Typically one pair per segment on the sides of the thorax and abdomen.
Air enters the system through the blowholes, and the tracheae are filled with air. In the case of the grasshopper, there is a blowhole on each side of the first and third segments of the thorax.
Another eight pairs of spiracles are arranged in a line on each side of the abdomen. The blowholes are protected by muscle-controlled valves that allow the grasshopper to open and close them and hairs that filter dust as air enters the blowholes.
Spiracles can often be opened and closed, leading to short tracheas that enter a pair of longitudinal tracheal trunks, the primary tracheal tubes.
Smaller tracheas branch off from these lateral tracheas that supply the tissues with air. This supply is especially rich in the most active tissues, such as muscles, nervous tissues, and the intestine.
The windpipes also extend into the wings, running within the wing veins.
The ramifications of the trachea reach a diameter of 2 to 5 thousand millimeters. Then stellate tracheolone cells ( transitional cells ) often enter from which they emerge as more delicate branches called tracheae, with diameters of less than one micrometer.
These tracheae end inside the tissues, almost always as open or blind tubes about 200 millionths of a millimeter.
Each cell in the insect’s body is adjacent to, or very close to, the end of a windpipe.
In some of Drosophila’s flight muscles, the tracheoles even penetrate their T-tubules bringing oxygen right next to the muscle-powering mitochondria.
Tracheal system ventilation
Among the smaller or less active insects, gas exchange through the tracheal system is by simple diffusion.
However, water vapor and carbon dioxide diffuse outside the animal, which could pose a problem in dry environments.
Drosophila avoids risk by controlling the size of its blowhole opening to match its flight muscles’ need for oxygen.
When the oxygen demand is lower, it partially closes its spiracles, thus conserving body water.
Giant, active insects, such as grasshoppers, forcefully ventilate their windpipes. They contract the abdomen muscles, compressing the internal organs, and the force expels the air out of the windpipes.
The abdomen returns to its average volume as the muscles relax and the air is sucked in.
Large air sacs attached to portions of the primary tracheal tubes increase the effectiveness of this bellows-like action.
Gas exchange in aquatic insects
Even aquatic insects use a tracheal system for gas exchange.
Some, like mosquito larvae, get their air by pushing a breathing tube connected to their tracheal system through the surface of the water.
Others carry an air bubble from which they obtain the oxygen they breathe. Others have spiracles mounted on the tips of the spines. They pierce the leaves of underwater plants and get oxygen from the bubbles formed by photosynthesis inside the leaves.