Sources of pheromones in insects

The source of pheromone in insects can be individual secretory cells scattered throughout the body of the insect, or groups of secretory cells that form a special organ – the pheromone gland (Fig. 2). The excretory ducts of the pheromone glands open on the surface of the body or in cavities in communication with the external environment. Often, insects have various devices that ensure the active release of pheromones into the environment (spraying and evaporation).

Pheromone gland cells are formed during ontogenesis from hypodermal cells. These cells are polar, have a large nucleus of irregular shape, located in the basal part; the more intensive is the synthesis and release of pheromone, the larger is the size of the nucleus (Fig. 3).

The cytoplasm density of different pheromone gland cells is not the same: cells with a lower cytoplasm density are conventionally called light, and cells with a higher cytoplasm density are called dark. In pheromone glands of insects, light cells predominate. There are more mitochondria in dark cells, and generally pheromone gland cells are rich in mitochondria. Dark drops of secrets contain lipids.

The structure of pheromone gland cells is very diverse (Fig. 4), but almost always there are microvilli on the outer surface of these cells, which increase the surface area to release secretions. Pheromone cells can be with or without an outlet duct. The outer part of the glandular cells is covered with a porous cuticle. The cuticle has a complex structure and consists of several layers. In the outer layer, pores are clearly distinguishable. Studies on females of the apple codling moth have shown that during the isolation of the pheromone, the pores are filled with filamentous formations, and under them are small vesicles. When the pheromone is not secreted, these formations are absent in the cuticle. The channels (threads) that penetrate the entire multilayer complex cuticle are not only a vehicle for secretion, but also a place for further processing.

The fact is that in many insects the pheromone gland can consist of more than one group of two-system formations, that is, the structure of the gland is more complex. Thus, females of the apple codling moth in the pheromone gland have light and dark cells (groups of two-system formations). Representatives of some termite families have pheromone glands consisting of three types of cells: columnar, basal, and ordinary, that is, there are already three groups of two-system formations.

There are cases when systems are located in different parts of the body of an insect. For example, males of some types of butterflies place special “tassels” to obtain the final pheromone product — groups of hairs located at the end of the abdomen in pockets located on the wings.

These are the so-called functionally separated glands; they were formed from two groups of glandular cells located in two different parts of the body of the insect. Each group of cells secrets its individual pheromone components. To obtain pheromone, the insect mechanically mixes these components. Most likely, in addition to mechanical mixing, chemical reactions involving the components of pheromones and enzymes also occur. This is evidenced by the fact that in the pockets, regardless of whether there was contact with the brush or not, there are about 10 μg of the main component of the pheromone. However, on the brush after contact with pockets, its amount reaches 400 mcg. These data allow us to understand the reason why it was not possible to detect pheromone in the glandular cell or inside the pheromone gland. The final product of biosynthesis – pheromone – is formed on the surface of the gland, and not inside it. Functionally undivided pheromone glands comprise cells localized in one place, for example, sternal glands in termites or Nasonov iron in working individuals of honey bees that secrete trace and aggregation pheromones.

Pheromone glands of various insects also differ in the presence or absence of a reservoir for secretion. Pheromone glands with a reservoir, as a rule, secrete a relatively large amount of pheromone – tens, hundreds of micrograms. So, in the pheromone gland of a female codling moth (without a reservoir), up to 8.7 x 10 ~ 3 can be containedmicrograms of trans-8, trans-10-dodecadiene-1-ol, the main component of the sex pheromone, while the mandibular glands of the queen bee (a gland with a reservoir) contain about 200 micrograms of trans-9-ketodecene-2-acid, the main component of the pheromone uterus, or the so-called uterine substance. Glands with a reservoir are usually located in the body cavity; their reservoirs in public insects are covered with cuticles. Glands are isolated whose secretory cells form part of the walls of the reservoir (for example, Dufour iron, Pavan iron and the hibibular tibial gland in ants, pubic gland in some termites), and glands whose secretory apparatus is connected to a reservoir distant from it (for example, mandibular glands in honey bees, the anal and poisonous glands in some ants). These glands secrete pheromones that signal danger

The location of pheromone glands in the body of an insect is diverse. In single insects, pheromones produce secretory cells and pheromone glands located mainly on the integument of the body, less often in the hind gut. In the back of the digestive tract, pheromones are secreted by representatives of the orders of Orthoptera (Orthoptera), Equine-winged (Homoptera), beetles (Coleoptera), termites (Isoptera), Hymenoptera (Hymenoptera), Diptera (Diptera), cockroaches (Blattop.

So, the pheromone of the American cockroach ( Periplaneta americana L.) is secreted by the epithelium of the intestinal tract of the insect and is excreted out. The sex pheromone of the pseudotugous beetle ( Dendroctonus pseudotsugae Hopkins) enters the environment from drill flour, and is secreted in the female’s back gut. In caddis flies, pheromone is produced by the sternal glands located between the IV and V segments of the abdomen. In females of the cockroach Prusak C Blatella germanica L.), the sex pheromone is contained in the wax on the surface of the body. In males of the North American bark beetle Ips confususLeConte pheromone is formed in malpighian vessels. Sources of pheromone (androconia) in males of some species of Lepidoptera are located on the hind wings and look like dark spots. 

During sexual arousal, pheromone from androconium is transferred to the brush – a group of hairs at the end of the male abdomen, from where it then dissipates into the surrounding space. In some sawflies, pheromone is formed by the cells of the fatty body, and then, when dissolved in hemolymph, it is excreted through the integument. In female Lepidoptera, pheromone glands form modified intersegmental membranes; their glandular cells are not uniform in shape and are covered with cuticle. So, the pheromone gland in a female silkworm is between the VIII and IX segments of the abdomen. The pheromone glands of the capra beetle ( Trogoderma granarium) have a similar structureEverst) and representatives of some other insect orders. Various social insects: wasps, bees, hornets, bumblebees, ants, termites, pheromones are also sources of pheromones located on the integument of the body and in the body cavity.

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