Segmentation Head Limbs Organs Cuticle

Segmentation

The arthropod body is composed of a linear series of segments, some or all of which bear paired
jointed segmental appendages. Most arthropods, including insects, are heteronomous, with their
segments and their appendages variously modified and grouped in regions, or tagmata, specialized for different functions.

The three tagmata of the insect body are the anterior head, middle thorax, and posterior abdomen (Fig 1). The head, which shows few external signs of segmentation, bears the eyes, antennae, mouth, and mouthparts. Its major functions are sensory reception and feeding. The thorax, whose primary function is locomotion, is larger and bears three pairs of legs and, typically, two pairs of wings. The abdomen is the largest tagma and is conspicuously segmented. It houses most of the digestive, excretory, and reproductive viscera and its appendages, when present, are specialized for copulation or oviposition.

abstn abdominal sternite, el elytron; eppl epipleuron; fres frontoctypeal suture; frs frons; gl galea; hymr hypomeron; lbrcly labroclypeus; msstm mesosternum; mtfe metafemur; mtstm metasternum;
mttb metatibia; prn pronotum; prtb protibia; pyg pygidium; scl scutellum

Limbs - Wings

Most beetles have two types of wings - elytra and hind wings. Beetles use only their hind wings for flight. The hind wings typically fold to fit under the elytra when not in use. In some beetle species that live on the ground, the hind wings may be missing altogether. The elytra cover the abdomen when the insect is not in flight. The two elytra meet in the middle of the abdomen, forming a character- istic straight line down the abdomen. The elytra protect beetles from drying out, from predators that bite, and from other kinds of damage. Many of the beetles that do not fly have fused elytra that insulate the abdomen and protect the insect from extreme heat or extreme cold. As a beetle prepares to fly, it opens the elytra, unfolds its hind wings, and uses its legs to jump into the air. Muscles embedded in the thorax enable the wings to beat. Some beetles can open their wings and jump into the air from a flat surface. Larger beetle species must climb onto a plant or other platform for takeoff or sit in the sun to warm their thoracic muscles before they are able to take off and fly. Beetles active primarily at night must vibrate their thoracic muscles to help raise the body temperature before flight is possible.

Organs - Nerves

The nervous system of an insect centres on a nerve cord that runs from the head to the abdomen along the underside of the body.
Typically the cord is equipped with a pair of ganglia, or nerve centres, for each segment of the body.
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. The brain receives stimuli from the antennae and from the eyes. 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.

The sense organs of beetles consist of eyes, auditory organs, organs of touch, organs of smell, and organs of taste.
Each of the two compound eyes, which are usually situated directly behind the antennae, contains from 6 to 28,000 or more light-sensitive structures, called ommatidia, grouped under a lens or cornea that is composed of an equal number of hexagonal prism-shaped facets. These structures permit only light that is parallel to their axes to reach the nerve endings, and thus build up an optical image.

Organs - Circulation

The internal anatomy of insects is characterized by an open circulatory system, a multitude of breathing tubes, and a three-chambered digestive system. With the exception of a heart and an aorta, there are few blood vessels; insect blood simply flows around inside the body cavity.
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.

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.

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. 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 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.

Organs - Reproduction

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. In the female (Fig. 9) 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. 10) 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.