Trachea

Tracheas are thin branched tubules of ectodermal origin. In the embryogenesis of animals, they arise as deep invaginations of the integument. The walls of the trachea consist of a single-layer epithelium and are lined with cuticles . The latter consists of a thick chitin-protein procticle and a thin cuticular layer of the epicuticle. In insects, the prokutikule forms spiral thickenings (tenidia), which prevent the adhesion of the walls of the trachea. Outside, the tubes open in pairs with openings - spiracles or stigmas. Each spiracle of insects serves three transverse tracheas, interconnected by three pairs of longitudinal tracheas. Branching out from these main trunks to tissues and organs. They end with thin tubes with a diameter of 1-2 microns - tracheoles. The ends of the tracheol either lie on the surface of individual cells, or enter into them.

Oxygen from the tracheol directly diffuses into the cells, and carbon dioxide from the tissues into the tracheola. Oxygen from the air filling the tracheal system is transported to individual cells of the body by diffusion. In cells, it is immediately consumed, therefore, in the tracheal system, a partial oxygen pressure gradient arises inward to the body. In addition, many insects are characterized by active ventilation of the trachea. In winged insects, it is carried out in flight due to the action of wing muscles, which simultaneously with the fluctuations of the wings pump air into the trachea and push it from the air sacs to the muscles. Large arthropods often perform special respiratory movements: some pump air due to the rhythmic expansion of the abdominal segments, others telescopic approaching them one on top of the other. In most insects, some spiracles open when inhaled, others close, and when exhaled, vice versa. In the intervals between inhalation and exhalation, all spiracles are closed. The number of respiratory movements per minute varies depending on the temperature of the environment, the physiological state of the insect and its species - from 5-6 to 150 or more. Closing the stigma between respiratory movements helps reduce water evaporation. Due to the development of the tracheal system, the circulatory system of the tracheal breathing is deprived of the gas exchange function. Direct transport of gases through the trachea to tissues and cells is significantly less energy intensive than a multi-stage respiratory system of the vertebrates (respiratory organs - blood - intercellular fluid - tissues). However, such a structure of the respiratory system is effective only for small body sizes, since with an increase in mass, the muscles are not able to pump enough air into the cells. It is thanks to the direct delivery of oxygen to the cells that asynchronous wing muscles can function. In vertebrate muscles, oxygen deficiency during increased work leads to fatigue. In spiders, the end of the trachea is washed by hemolymph, so the circulatory system takes on the function of gas exchange. The number of respiratory movements varies in different insects and in a different state: a domestic bee makes 120 respiratory movements per minute during work, at rest - 40; with increasing air temperature, their number in locusts increases from 6 to 26 or more ^[1] .

Depending on the presence of spiracles, the tracheal systems are divided into open and closed. An open system is characteristic of terrestrial arthropods that breathe atmospheric oxygen. A closed system is provided by water larvae of insects, in which the trachea is closed externally, and the spiracles connect the internal tracheal system with the trachea, branched in thin-walled film or branching outgrowths - tracheal gills. These tracheas are also filled with air, and gas is exchanged with water through the gill surface.

Insects

In insects, two pairs of stigmas are located respectively on the mesothorax and metathorax. In the first eight segments of the abdomen there is also a pair of stigmas, however, their number may decrease. Stigmas of insects have a rather complicated closure apparatus, which is serviced by one or two muscles and has a special air filtration system built from numerous branched bristles. In well-flying insects, extensions — air sacs — form on the longitudinal tracheal trunks. They do not have tenidia and can change volume. Air sacs participate in the ventilation of the wing muscles during flight and perform an aerostatic function, helping to reduce the specific gravity of the body. In many endoparasitic larvae, the tracheal system is partially or completely reduced, and breathing is carried out through the integument. Some parasitic insects connect their tracheal system with the trachea of ​​the host animal, others break through its integument, exposing the spiracles to the outside.

Depending on the number of spiracles, the tracheal system of insects can be one of the following types:

1. holopneustic - there are 2 pairs of thoracic and 8 pairs of abdominal spiracles, characteristic of most adults and larvae of hemimetabolic insects;
2. hemipneustic (holometabolic insects, their larvae and pupae). In turn, it distinguishes several subtypes:

• • peripneustic (on the chest 1 pair of spiracles);
  • amphipneustic (1 pair of chest spiracles and 2-3 pairs in the anterior segments of the abdomen);
  • metapneustic (only 1 pair of spiracles at the end of the abdomen)
  • apneystic (no spiracles and air penetrates through the surface of the whole body or with the help of tracheal gills - water larvae, larvae of riders and flies-tahin).

Millipedes

In leg-footed centipedes, the trachea system is well developed. A pair of spiracles is located on each segment of the body. Twigs of tracheas from different segments are interconnected, forming a common network within the body. In two-legged millipedes, the tracheal system begins with spiracles, which are located at the base of the legs in each segment (except for several front ones). A stigm of two pairs per segment, like legs. Each stigma leads to a bag-like expansion, from which more or less branched trachea, supplying oxygen to the organs of the corresponding segment, depart. Unlike the sea-legged and insects, the diplopods do not form a single tracheal system. Symphiles have only one pair of spiracles located on the head behind the antennae. Tracheas extending from stigma are weakly branched; their branches extend only into the head and three anterior segments.

Arachnids

In arachnids, the shape, number and location of stigmas, the thickness of the tracheal trunks and the degree of branching vary greatly between groups. The tracheal system of the salpug is best developed. The main tracheal trunks are opened in them by several pairs of spiracles on the cephalothorax and abdomen and one odd spiracle on the IV segment of the abdomen. Tracheas, moving away from the spiracles, are combined into powerful longitudinal trunks that are connected by jumpers and diverge in numerous branches to the organs. In the walls of the trachea, the salpug has spiral cuticle thickenings, similar to insect shades.

Most spiders, along with the lungs, also have trachea, although they have no four-lung trachea, and some tropical species breathe only trachea. Most often, they have only one abdominal spiracle, from which two pairs of blindly closed unbranched tubules, lined with a thin cuticle without spiral thickenings, depart. Tracheas are washed by hemolymph, so that gas exchange is carried out with the participation of the circulatory system.

In hayfields, the respiratory system is represented by well-branched tracheas that penetrate the entire body. They open with spiracles on the first and second segments of the abdomen.

• Scherbak G. I., Tsarichkova D. B., Verves, Yu. G. Zoology of invertebrates: A textbook in 3 books. Prince 2. - M .: Education, 1996. - S. 320. - ISBN 5-325-00663-0 .