Insect identification > Insect internal structure

Insect internal structure

Insect internal structureFew of the internal structures of insects are of any great importance from the standpoint of control methods, but some knowledge of them and their arrangement is desirable.

Digestive Organs (Fig. 22). - The alimentary canal extends from the mouth through about the center of the body to the anus at the hinder end. In those insects whose food is most concentrated (Fig. 23), it is in its simplest form and is but little, if any, longer than the body. In those which feed on less concentrated food (Fig. 24), the necessity for a greater digestive and absorptive surface has resulted in an increase of its length and the accommodation of this within the body by the production of loops and coils.

In the embryo the alimentary canal forms as three separate sections which connect later. One of these is an ingrowth from the surface where the mouth is to be; another and similar ingrowth occurs where the anus forms; and a third, forming earlier than the other two, arises as two masses of cells, one near each end of the embryo, which move inward and toward each other, unite, and surround the yolk. Later, when this has been absorbed, a space is left with which the two ingrowths already mentioned connect, the hollow centers of all three joining to form the tube through which the food travels. The ingrowth from the mouth is usually called the fore intestine, the central portion the mid intestine, and the ingrowth from the anus the hind intestine. The first and last of these begin to grow inward from the surface of the body after that surface has begun the formation of its chitinous exoskeleton, and aecordingly they also have this power, and line the inside of the parts of the canal which they form, with chitin. In that portion of the canal termed the mid intestine, however, this power does not appear to be present, and the mid intestine is without this lining.

The midintestine forms the stomach of the adult insect; the fore intestine forms those parts of the alimentary canal from the mouth to the stomach; and the hind intestine those from the stomach to the anus. Each of these sections may sometimes have portions differing in structure, producing a greater or lesser number of subdivisions. Thus the fore intestine, by differences of structure, may sometimes consist of a mouth cavity, oesophagus, crop and proventriculus; the stomach may develop side pouches or gastric caeca; and the hind intestine is often separable by differences of structure into an ileum, colon and rectum.

Lined as these parts are by chitin which often bears rough, tooth-like projections and spines, some persons have suggested that in insects where these structures are present in the fore intestine, the food is masticated more thoroughly and mixed with digestive juices before it reaches the stomach. In the stomach digestion is probably completed and absorption at least begun, but the length of the hind intestine in many insects suggests the idea that absorption in those cases has not been completed when the food leaves the stomach but continues in the hind intestine. Opening into the mouth is a tube leading to the salivary glands, which generally lie in the front of the thorax and appear to have a similar function to those in man. In some cases other glands for different purposes are also present in the head or front of the thorax and open into the mouth. Some of the poisons used in control measures are swallowed by the insect, passing to the stomach and there are dissolved by the digestive juices. Thus dissolved, they set up inflammation of the stomach walls and finally cause death. Poisons acting in this way are called "stomach poisons."

Breathing Organs. - Respiration in insects is accomplished by a method which is nearly unique. The oxygen needed, instead of being drawn into lungs and there being taken up by the blood and carried to the parts of the body where it is needed, as in man, is carried directly to those parts by a system of air tubes which open along the sides of the body (Fig. 25). Here the air enters the tubes and proceeds through them to where it is utilized. The openings by which the air enters are called spiracles, and these occur in pairs on some of the thoracic and most of the abdominal segments, varying somewhat in number and in position on the segment in different insects. The spiracles often have valves by which they can be more or less completely closed at will.

Each spiracle opens into a short tube or trachea which, with the others of that side, soon joins a similar tube running along the side of the body and quite close to its surface. From these longitudinal tracheae, branches pass off in various directions and in turn branch again and again until every part of the body is reached by its air supply. The tracheae frequently enlarge here and there, forming so-called air sacs.
The tracheae are lined by chitin con­nected with that of the surface of the body. In these tubes, however, it is formed with spiral thickenings which act like a spring, keeping the tracheae open when not under pressure. There is probably considerable pressure on them temporary variations in diameter aid in the circulation of air in these tubes. Not only are the tracheae of use in carrying oxygen to all parts of the body, but they also receive the carbon dioxid gas produced by the activi­ties of the cells and permit it to escape through the spiracles from the body, thus performing both of the functions which the blood, so far as gases are concerned, accomplishes in man. Blood then, in insects, does not have any important respiratory function.

The destruction of insects by fumigation is accomplished by the substitution of a gas, destructive to life, for the air, and this gas enters the spiracles and follows along the tracheae to the living tissues, which take it in place of the oxygen usually received in this way, and the insects are killed. It was formerly supposed that certain materials called contact insecticides, which kill insects by contact with their bodies, caused death by entering the spiracles and closing them up, thus producing suffocation. This has now been proved to be incorrect in most cases.

Insects which in their early stages live in water cannot, of course, breathe air into their bodies through spiracles during that period of their lives. These are closed in such cases and the animal obtains air usually through special structures called tracheal gills. These will be described in connection with the insects which possess them. In a few small water-inhabiting forms, the chitin covering the surface of the body is so thin that oxygen present in the water can pass directly through it into the body and to the parts there which need it, and carbon dioxid passes in the reverse direction.

Circulatory Organs. - Insects have only an incomplete system of blood vessels. A tube lies in the middle of the body close beneath the back, beginning near the hinder end of the animal and extending forward into the head (Fig. 26). In the abdomen this tube is constricted, forming chambers, and the chambered portion is called the heart. There is a pair of openings on the sides of each chamber through which blood can enter, and valves there which prevent its going out again. The walls of the heart contain muscles and these contract one after another, forming a sort of wave of contraction which begins at the hinder end and travels forward. Blood in the heart, being unable because of the valves to pass out at the sides, is pressed forward by this contraction wave and at the front end of the heart finds itself in a tube without chambers or valves, called the aorta, through which it is led to the head where the aorta may divide into a few short branches or may be unbranched. In either case, at this point the blood pours out of it into the body, the system of blood vessels coming to an end. There is now no definite and particular path for the blood to follow, but it would, in theory at least, remain near where it escaped from the aorta or gradually pass into any spaces it might find unoccupied between the different structures in the head. With each heart-beat, however, more blood is poured out of the aorta, increasing the pressure upon that already in the head. It therefore is gradually forced backward and to other parts of the body, each particle probably taking the path where there is least resistance to its passage. In this way a general backward direction is given to the flow.

As it approaches the heart, another influence appears. During each contraction of the heart, it occupies less space, which leads to less than normal pressure near it, and blood close by naturally flows closer to it. Upon its expansion again and the opening of its valves, the direction of least resistance is now through the valves and into the heart. As the blood passes back through the body, a given particle may at one circuit go over certain organs, and at the next over entirely different ones. All the internal organs, however, have their surfaces bathed by blood and this as it passes over the stomach or other parts of the alimentary canal will pick up any food which having been digested has passed through the canal walls. Likewise in passing over any organ needing this food, it is given up to those organs. The blood therefore serves as a distributor of food from the place where it is digested to all the parts which need it.

We have already seen that the living parts of the body - the cells - need oxygen and as the result of their activities give off carbon dioxid gas, but that this exchange is accomplished by the aid of the tracheae. In a somewhat parallel way, the cells which need food obtain it from the blood. The cells by their activities produce not only carbon dioxid gas but also waste material nitrogenous in nature which must be removed, like all wastes, from the body. This nitrogenous waste is picked up at the cells by the blood and carried along, perhaps for sometime, before a place to dispose of it can be found. Sooner or later, however, a particle of blood containing this waste material will wash over certain structures called Malpighian tubes, to be described in the next section, and the cells which form these tubes have the power to collect this waste material from the blood as it flows over them, thus purifying it.

The blood itself is usually a colorless, yellowish, reddish or greenish fluid, in which are corpuscles resembling the white corpuscles of human blood. It appears to serve to carry food to the tissues, and waste matter from them, and therefore has no need of structures in it like the red blood corpuscles of man, the work of which in insects is done mainly by the tracheae.

Excretory Organs. - The organs which eliminate the nitrogenous wastes from the body and correspond in function to the human kidneys are known as Malpighian tubes (Fig. 27). These are blindended tubes, the walls of which consist of a single layer of cells surrounding a central channel which at one end opens into the hind intestine, usually near its front, just behind the stomach (Fig. 22). When blood containing nitrogenous waste matter washes over the outer surface of a Malpighian tube, the cells of which it is composed have the power of taking this matter out of the blood into their own substance and passing it through themselves into the channel between them, down which it moves until it enters the hind intestine, from which it is finally expelled at the anus.

The Malpighian tubes may be few or many, long or short (see Figs. 22, 23, 24). They show a tendency to collect in groups and to unite near the hind intestine, so that their outlets into this are much fewer than the number of tubes. It seems possible that a certain amount of poison entering the body by way of the stomach can be eliminated by the Malpighian tubes, which may explain the varying degree of resistance to such poisons by different insects.

Nervous System. - The nervous system of insects is located along the middle line of the body quite near its under surface (Fig. 22). As in animals generally, it is

The former are for the most the Maipighian tube of part gathered together in clusters which are called ganglia, a fly, greatly enlarged: k, cell nucleus; l, lumen and from each of the cells in a ganglion one or more nerve of the central canal; tr, fibres pass out, to connect either with some other nerve t r a o h e x .

are really bundles of these fibres running side by side like the wires of a telephone cable.

Apparently each segment of the insect body once had a nerve ganglion, but with the fusion of the segments many of these have also fused, reducing the separate ganglia in adult insects to a smaller number, which varies in different kinds. This fusion has been produced by the hinder ganglia moving forward until in some cases none is found in the abdomen. Different degrees of this are shown in Fig. 28.

Each ganglion is connected to the one in front and the one behind by one or two bundles of nerve fibres which are called commissures. Each consists of numerous fibres and these taken together form the means of communication between the different parts of the system.

In the head, in front of or above the oesophagus, is the largest ganglion of the body, called the brain, produced by the fusion of several ganglia. In addition to its two commissures, which connect it with the ganglion next behind, it has nerves which lead to the eyes, to the antennae and to other parts of the front of the head.
Below or behind the oesophagus is a second ganglion, also in the head, called from its position the suboesophageal ganglion. As the oesophagus lies directly between this and the brain, the commissures connecting the two do not lie close together but separate far enough to permit the oesophagus to pass between them. The suboesophageal ganglion, besides being connected with the brain in front, and the first thoracic ganglion behind it, by commissures, sends nerves to the mouth-parts and other nearby regions of the head.

The thoracic ganglia may be more or less separate or fused and may have fewer or more of the abdominal ganglia added. Commissures, however, connect all separate ganglia, and these also send out nerves to all the parts of the segments to which they belong, no matter what their final location may be. In this way, the wings, legs, muscles and other parts receive their nerve supply. A small "sympathetic nerve system," also present, appears to be concerned chiefly with the nerve supply of the alimentary canal and tracheae.

Sense Organs. - All the more evident senses possessed by men appear to be present in insects, but not in all cases in the same individual. Thus some cave­inhabiting insects have no eyes. It is almost certain that insects may have other senses not possessed by man.

Reproductive Organs. - Insects are of distinct sexes, male and female. In many cases, however, individuals occur incapable of reproduction, their sexual organs not having become fully developed, and such insects may be termed neuters. Most of these appear to be really undeveloped females, though undeveloped males are also known. They are found in colonial insects where division of labor occurs, as in the honey bee, ants, termites, and are known according to their duties, as workers or soldiers or by other names. Conventional signs for the various forms of insects as a convenience, are: male; female; worker.

In the female (Fig. 29) the eggs are produced in a pair of ovaries located in the upper front part of the abdomen. Each is a cluster of ovarian tubes whose walls are cells. Some of these cells grow and separate from the others to lie in the central cavity of the tube and then pass downward, growing till they reach its hinder end, which connects with the similar ends of all the ovarian tubes of that side to form a single tube called the oviduct. This extends downward and backward around the side of the alimentary canal, below which it joins with a similar oviduct from the other side of the body to form a single duct, the vagina, which lies below the alimentary canal and extends backward to its outer opening which is located, in most cases, in front of the next to the last abdominal segment. Surrounding this opening may be external structures (an ovipositor) for the purpose of together making holes in some object (the ground, wood, etc.) in which to deposit the eggs. A side pouch (seminal receptacle) connected with the vagina is for the storage of the sperms which fertilize the eggs; a gland-producing material, which forms the egg shell and is known as the shell gland, also opens into this portion, and other glands similarly connected with the vagina may also be present.

In the male (Fig. 30) the arrangement of the organs closely corresponds to that in the female. A pair of spermaries or testes is present in the upper front part of the abdomen, each consisting of a rather closely coiled mass of tubes in which the sperms are produced. The tubes on each side unite to form a single tube, the vas deferens (plural, vasa deferentia). These differ from the oviducts usually, in being much longer and coiled or twisted. They pass downward and backward, however, and unite on the middle line of the body below the alimentary canal, forming a single tube, the ejaculatory duct, corresponding to the vagina in position, which leads backward to an opening in front of the last segment. An enlarged portion of the vas deferens is often present, for the temporary storage of the sperms, and is termed the seminal vesicle. Accessory pouches opening into the ejaculatory duct appear to be, in part at least, for the production of mucus and secretions to mix with the seminal fluid.