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.

Inescts: The House-fly

                                                         My Article: The House-fly

The house-fly, Musca domestica is an insect belonging to the same group (Diptera) as the mosquito and the tsetse fly. As mentioned above, these insects are noted for possessing only one pair of wings. Their mouth-parts, such as the labium, which usually forms a long proboscis, are also adapted for sucking or piercing.

   There is probably no insect more commonly encountered in daily life in the tropics than the house fly, which interferes so much with our food. The body of the fly is grey in colour with black streaks and is covered with bristles that give it a hairy look. It is divided into the three parts of head, thorax, and abdomen, as with all insects. The compound eyes are particularly large and in addition, there are three small simple eyes on top of the head. The two antennae are rather short. The mouth-parts are much modified, forming a tube of proboscis for sucking.

   On each of the three-segment of the thorax, there is a pair of jointed walking legs, with two small claws at their tips which help in walking on rough surfaces. Two small pads are found between the claws and these secrete a sticky substance which attaches the fly firmly to its support when walking on smooth surfaces, even when it's body is up-side-down. The single pair of wings are found on the second segment of the thorax. The single pair of wings are found on the second segment of the thorax. These wings are held horizontally along the abdomen when the insect is resting. The abdomen is rather short and appears to have only four segments.

Mode of Life: The house-fly lives on fluid foods and its mouth-parts are adapted to the nature of the food. The long proboscis is not simply pointed at its tip but has two flaps at its end with grooves along which the fluid from the food runs into the main tube. Although the fly seems adapted only to fluid foods, it can also tackle solid foods which are fairly soluble, such as sugar. In this case, by exuding a drop of fluid from the alimentary canal through the proboscis, the fly first dissolves the sugar at one spot and then sucks up the resulting solution. The habit of 'vomiting' fluids from the gut and the fact that the fly may feed on decomposing or faecal matter, explains why the house-fly is such a disseminator of diseases, particularly those of the alimentary tract.

Life-History: 
   The female fly lays her eggs in any decomposing matter, such as a rubbish-heap, and dive or six batches each of about a hundred eggs may be laid. The eggs are small and white and, after about a day, they hatch out into the larvae or maggots. These have neither head nor legs and the body, which consists of twelve segments, tapers from the hind end to the front. A hooked structure situated on the first segment enables the larva to crawl along and to feed, not only on liquid food but also on the solid organic matter which is first liquefied by enzymes. The larva makes it's way farther into the rubbish -heap and is able to breathe by means of a pair of spiracles situated at each end of the body.

    At pupation, the body of the larva contracts and the skin hardens and turns brown. One peculiar feature here is that larva, after the second of the two moults which take place, the skin is not shed but continues to enclose the pupa. This is the brown covering called the pupa which after about four days changes into an adult, or imago, inside the puparium. The imago has to free itself from this covering and to do this it uses a bladder-like structure filled with blood which projects from the front of its head. After breaking through the puparium the adult fly emerges.

Economic Importance: As already mentioned, flies will feed on any fluids, including decomposing organic material and even faeces. Thus, before settling on our food, a fly may already have been contaminated with filth. This may be on the sticky pads of the feet or any part of the hairy body since particles of dirt easily cling to its surface. Apart from its alimentary canal food, the fly can also 'vomit' out the filth from its alimentary canal on to our food when it wishes to feed. This habit can bring all sorts of bacteria to us when we eat food contaminated in this way, and diseases like diarrhoea, typhoid fever, cholera, and dysentery are all spread by the house-fly.

     The female house-fly seeks dead organic matter in which to lay her eggs and, in order to reduce the fly population in our area, we should make sure that we destroy any rubbish, preferably by burning, since it is not effective merely to bury it. Food should not be left exposed but should always be kept covered until required. When there is any food leftover it should not be thrown away haphazardly but should be placed in covered refuse Tubs. Faeces should be disposed of immediately, and even in latrines, the holes should always be covered after use. Fly-traps are also very useful if well maintained.