| Locust handbook |
|2. Desert Locust-Schistocerca gregaria|
The basic anatomy of an adult locust is described here and illustrated by photographs of the Desert Locust (see also Plate 2).
The body can be divided into three main parts: head, thorax and abdomen (Fig. 8).
On the head can be seen:
1 A pair of jointed antennae or feelers which the locust uses to recognise things by touch or smell.
2 A pair of large compound eyes which give the locust a wide field of vision and enable it to detect movement easily. It is not known how many colours locusts can recognise but it has been shown that they react to green.
3 The mouth which has several parts that can easily be separated and identified. These are the upper lip, a pair of hard, black, serrated jaws which move sideways to cut through plant food, a pair of secondary jaws which help in holding the food, and a lower lip.
4 Jointed appendages which are called palps. These are used for tasting food.
This is the part of the body which contains the muscles for walking, jumping and flying, and to which the wings and legs are attached.
On the thorax can be seen:
5 A sheath covering the top and sides of the front part of the thorax. It is called the pronotum.
6,7, 8 Three pairs of legs, the hind legs being large and used for jumping. Each leg has three main parts, the femur (9), the tibia (10) and the tarsus or foot (11).
12 Two pairs of wings. At rest the harder front wings cover and protect the softer hind wings which are folded fan-wise.
On the abdomen can be seen:
13 The ear, on the first section of the abdomen just behind the first joint of the large back legs. This is where the locust receives sounds. Locusts can hear one another up to about 2 m apart.
14 The ovipositor valves in the female. These are two pairs of short, curved, black hooks which form the tool with which the female locust digs a hole in the soil when the eggs are laid. This is how the female can be distinguished from the male, as the male does not have these hooks (Figs 9 and 10).
Along the sides of the thorax and abdomen are small openings called spiracles (15). These are holes through which the locust breathes. The largest spiracle on each side is just above where the middle pair of legs joins the body and can be seen opening and closing if a live locust is examined. The spiracles lead to very fine tubes which carry air directly to all parts of the locust's body; these tubes, which appear as slender silvery threads when you examine the internal organs, constitute the tracheal system.
The locust is covered with a special kind of skin which is referred to as the cuticle. It has three different layers. The layer nearest the inside of the body is soft and flexible; then comes a harder layer and on the outside is a thin layer of wax. This wax makes the skin waterproof and its presence also means that insecticides required to kill locusts by contact action should be dissolved in oils which will penetrate the wax. The hard part of the skin serves as the skeleton of the locust and is thinner at the joints so that movement can take place.
On many parts of the skin very fine short hairs can be seen. These are connected to nerves inside the body and serve in many ways to make the locust aware of the conditions in which it has to live. Those on the face detect air movement so that the locust can take off and land into the wind.
Inside the locust's body can be seen a dark-coloured tube running from the mouth at the front to the anus at the hind end (Fig. 11). This is the food tube or gut. There is always some material in the gut and it is not possible by merely looking at the gut to decide whether the locust has been feeding or not. The more feeding a locust does the quicker the food passes through the gut. The usual length of time is from 0.5 h to 2 h, but it can take 3-4 days if there is little food available. This allows the locust to withstand long periods of starvation.
The front part of the gut is wider than the remainder; it is concerned with grinding and storing food. The middle part is where digestion takes place and the hind part is where the water is absorbed. The waste is passed out at the anus as small dark-coloured faecal pellets, 4-5 mm long and 2 mm thick. These can often be seen in vast numbers beneath bushes and trees where locusts have been feeding and can provide evidence of the recent presence of locusts even though the locusts themselves have departed. After digestion some of the food material is stored in the body as fat, which can be seen inside the abdomen when a locust is cut open. It is a soft, yellow, shapeless mass, and it provides fuel for activities such as marching and flying. Flying locusts use up this fat at the rate of about 14 mg (0.8% of the body weight) per hour. A complete lack of fat inside the body of a fully grown (not a fledgling) locust means that it has probably been flying a long time without feeding. A large amount of fat means that it has probably been feeding considerably without much flying.
Many very fine silver-coloured tubes can also be seen inside the body. These are the air tubes or tracheae already mentioned.
In a mature female locust the yellow eggs are conspicuous and arranged in rows in the ovary. When fully grown the eggs of the Desert Locust are each about 7 mm long.
If red spots can be seen in the ovary this usually means that the female locust has already laid at least one egg pod. It can, however, mean that some eggs started to develop and then stopped because of unfavourable conditions.