Insects: The Cokroach

My Article: The Cockroach

    Insects are the most successful group among the Arthropoda and have considerable economic importance in our daily lives. The characteristics of insects are as follows: they have a jointed and chitinous exoskeleton. The body is segmented and the legs are jointed. Their bodies are divided into three regions: the head, thorax, and abdomen. The head bears a single pair of antennae, while on the thorax are three pairs of walking legs. The abdomen, however, has no walking legs. Spiracles just as in millipedes and centipedes. While in some insects the young emerge from the egg in a similar form to that of the adult, in most insects the young ones pass through a series of distinct forms or stages before assuming the adult form. This process of transformation is called metamorphosis.

                                                       The Cockroach

  We shall describe the cockroach to illustrate the essential characteristics of insects as outlined above. The cockroach is quite a convenient insect for this purpose since it is fairly large, easy to obtain, and has a rather simple structure when compared with a number of other types of insects. Many species of cockroach are found in the tropics, but a common one, called periplanta americana, is the one on which the present description will be based.

    External Features: The cockroach is a dark brown insect with a flattened body, about four centimeters long. The body is divided into three regions: the head, the thorax, and the abdomen. The head and thorax are joined together by a narrow soft region known as the neck. The head is made up of six segments, although there is no sign of this externally; the thorax, however, consists of three distinct segments on which are borne the legs and the wings; some seven segments may be counted on the abdomen, but these are legless. The head is small and flattened from front to back and is held almost at right-angles to the axis of the body. At the side of the head are the two large black compound eyes. These are similar to those of crustaceans but are not carried on stalks. The eyes are able to record quick movements. From just below each eye arises a long whip-like feeler, or antenna made up of many joints. The antennae are sensitive to smell and touch and are always being moved, particularly when they receive stimuli.

    The mouth-parts of the cockroach have a simple structure. The insect is omnivorous -that is, it feeds on plant or animal material. The mouth, as in crustaceans, is wide and there is a pair of biting jaws or mandibles. These are so shaped as to be suitable for biting any kind of food. Just outside the mandibles are two jointed structures called maxillae, each with two blades, together with another jointed structure called the palp. There is a sort of lower lip called the labium below all use its mouth-parts for picking up pieces of food and carrying them to the mouth. Mouthparts are adapted in different insects according to their mode of feeding, and we may remark here that, while the biting mandibles are large in the cockroach, they are small in the bee, and are virtually non-existence in the butterfly.

     The thorax consists of three segments, each of which bears a pair of jointed walking legs with hooked claws. The second and third segments also each bear a pair of wings. The three segments are called prothorax, mesothorax, and metathorax. In the cockroach, the prothorax is very large and its large covering protects the head by overhanging it. It also bears a breathing pore or spiracle. The first pair of wings are thick and horny, while the second pair, which is folded like a fan under the first one when the insect is at rest, are membranous.

     The abdomen consists of ten segments, each of which has a chitinous plate separated from the next one by a soft membrane. This acts as a joint and allows movement of the abdomen. At the side of each abdominal segment, excepts the last two, are pairs of breathing pores, or spiracles. The anus is on the last segment and on each side of it is one of a pair of a structure called the cerci.

It is possible to distinguish between the male and the female cockroach by examination of the end region where structures associated with reproduction are found. In the male, these are paired rods called styles very near to the cerci. The abdomen of the male is generally narrower than that of the female. The female is distinguished by the possession of a genital pouch, below the seventh segment, which holds the egg-case before the eggs are laid in it.

      Respiration: One characteristics feature of insects is their mode of respiration. We have already mentioned the presence of paired spiracles on eight segments of the abdomen and on two of the thorax. These breathing pores lead to a series of branched tubes called trachea, or tracheal tubes, which are distributed throughout the body. Oxygen reaches the cells of insects directly through this system without being carried by a blood-system. When the cockroach is dissected it is possible to see parts of the tracheal system looking like white lines.