ARTHROPODS
The arthropods are a large group of invertebrate animals which include insects, spiders, millipedes, centipedes and Crustacea, All arthropods have a hard exoskeleton of Culicle, Segmented bodies and jointed limbs.
Crustacea are arthropods with five or more pairs of walking legs, They have compound eyes but differ from insects in possessing two pairs of sensory antennae. The majority of erustacea live in the sea (eg prawns, crabs and lobsters), or in fresh water (e.g. water fleas and crayfish), though a small number of species live on land (eg. woodlice) The aquatic crustacea breathe either through the entire body surface or through gill-ike structures
Insects, The body of an insect is divided into three regions, head, thorax and abdomen. There are only three pairs of legs, restricted to the thorax and, in nost cases, there are one or two pairs of wings also attached to the thorax, Some primitive species have no wings while other, parasitic forms such as fleas, have lost their wings in the course of evolution. In beetles, the first pair of wings is modified to form a hard covering over the second pair when not in flight. Insects have compound eyes, one pair of antennae and breathe by means of a tracheal system.
Arachnids, These are the spiders, scorpions, mites and ticks. Their bodies, like those of crustacea, are divided into two regions, the cephalothorax and abdomen, They have four pairs of limbs on the cephalothorax but no antennae.
In place of antenne there is a pair of chelicerae which are used to hold prey, In spiders the chelicerac pierce the prey and paralyse it with a poison secreted by a gland at their base, There are usually several pairs of eyes but they are not compound Breathing is by internal, gill-like structures called lung books, or by trachea, or by a combination of both.
Myriapods. These are milliepedes and centipedes. They have a head and a segmented body which is not divided into thorax and abdomen. There is a pair of legs on each body segment. As the myriapod grows, additional segments are formed. The myriapods have one pair of antennae, a tracheal system and simple eyes.
General characteristics of insects
Some of these will now be treated in more detail.
Cuticle and ecdysis. The value of the external cuticle is thought to lie mainly in reducing the loss from the body, of water vapour through evaporation, but it also protects the animal from damage and bacterial invasion, maintains its shape and allows rapid locomotion. The cuticle imposes certain limitations in size, however, for if arthropods were to exceed the size of some of the larger crabs, the cuticle would become too heavy for the muscles to move the limbs. While they are growing, the arthropods periodically shed the outer layer of their cuticle when it has become too small. A new cuticle then forms over their expanded bodies. In insects, this moulting, or ecdysis, takes place only in the larval and pupal form and not in adults. In other words, mature insects do not grow. The number of moults in any one species is usually constant, the forms existing between each moult being called instars.
Breathing. Running through the bodies of all insects is a branching system of tubes, tracheae, which contain air. They open to the outside by pores called spiracles and they conduct air from the atmosphere to all living regions of the body. The tracheae are lined with cuticle which is thickened in spiral bands. This thickening keeps the tracheae open against the internal pressure of body fluids. The spiracles, typically, open on the flanks of each segment of the body, but in some insects there are only one or two openings. The entrance to the spiracle is usually supplied with muscles which control its opening or closure.
This tracheal respiratory system is very different from that of the vertebrates, in which oxygen is absorbed by gills or lungs and conveyed in the blood stream to the tissues. In the insects, the oxygen diffuses through the tracheae right up to the organ concerned, and the carbon dioxide escapes through the same path.
Blood system. The tracheal supply carrying oxygen to the organs makes the circulatory system rather less important to insects than it is in vertebrates, and it is generally little more than a single vessel running dorsally in the body cavity.
External muscles propel the blood forward in this vessel and maintain a sluggish movement in the blood which surrounds the organs in the body cavity. Apart from this single, dorsal vessel the blood is not confined in blood vessels but occupies the free space between the cuticle and the organs in the body cavity.
The blood, therefore, serves mainly to distribute digested food, collect excretory products and, in addition, has important hydraulic functions in expanding certain regions of the body to split the old cuticle and in pumping up the crumpled wings of the newly emerged insect.
Life history and metamorphosis. Insects lay eggs which hatch into larvae. These larvae are usually quite unlike the adult and are called grubs, maggots or caterpillars according to the species of insect. Generally the larva is the feeding and growing stage, eating voraciously, shedding its cuticle repeatedly and growing rapidly. When it has reached full size, the larva becomes inactive, neither moving nor feeding, and extensive breakdown and reorganization takes place within its body, giving rise eventually to the adult or imago form. The stage in the insect's life when these changes take place is called the pupa and the changes are called metamorphosis. The adults then mate and lay eggs. The habitat, behaviour, locomotion and feeding habits of the adult are quite different from the larva's. In some insects, such as dragonflies, the nymphs live and grow in water for a year or so but live only a few hours as adults; long enough to mate and lay eggs.
Incomplete metamorphosis. The type of metamorphosis described above is sometimes called complete metamorphosis, to distinguish it from another kind of life history such as that of the cockroach or locust. With these insects the egg does not hatch into a larva but a nymph which, though still very different from the imago, more closely resembles it than does a larva. The nymph has three pairs of jointed legs, compound eyes and rudimentary wings. At each moult, changes occur which bring it nearer to the adult form. There is no prolonged "resting stage in a pupa, though the final ecdysis usually reveals drastic changes that have occurred in the final weeks of the nymph's development.
THE DESERT LOCUST (Incomplete metamorphosis)
Mating. The male locust mounts the back of the female, applies the tip of his abdomen to hers and passes sperms into her reproductive tract. The sperms are stored in a sperm sac in the female's abdomen, and as the eggs pass down the oviduct during laying the sperms are released and so fertilize the eggs.
Eggs. After mating, the female lays her eggs in warm moist sand following a rainy spell. She pushes her abdomen down into the sand, extending the membranes between the segments, and burrowing to a depth of 70 or 80 mm. In this burrow, 50 to 100 eggs are laid and mixed with a frothy fluid, which hardens slightly and may help to maintain an air supply round the eggs. In 10 to 20 days depending on temperature and moisture, the eggs hatch and the nymphs make their way to the surface.
Nymphs. As the nymphs grow they shed their skin five times. The stage between each moult is called an instar and each in- star lasts 4 to 5 days, except for the fifth which occupies about 8 days. The first instar nymphs are about 8 mm long, with three pairs of legs but no wings. At each successive moult the wings grow larger until by the fifth instar they are quite prominent.
There is no resting stage or pupa, but the changes in form which take place after the fifth moult are more pronounced than any previous changes. During the 26 days from hatching until the final moult, the nymphs or "hoppers move by crawling or leaping with their long and powerful hind legs. They feed on vegetation, biting off pieces of leaf and stem with their jaws.
Adult. The adult may live for up to six months. It can crawl or leap, but also has two pairs of wings which enable it to fly. Like the nymphs, it feeds on vegetation using its biting jaws. The adults are sexually mature within a few weeks of the final ecdysis and the female may lay three or four batches of eggs during her lifetime.
Swarming. If food is in adequate supply and the hoppers are not forced to crowd together when they emerge from the eggs, the locusts live their lives separately as do other grasshoppers. I1, however, the hoppers are crowded together for one reason or another, they enter a gregarious phase of activity. The hoppers tend to keep together in a band and move forward together. The crowding effect also results m a change of colour from the normal green, buff or brown to a striking black and yellow (or orange) coloration. There are also structural differences from the solitary form. The bands of hoppers vary in size from hundreds to millions, covering a few square metres or several square kilometres, depending partly on the age of the hoppers and how many bands have combined. As hoppers, they migrate only a few kilometres each day, basking in the early morning sun until their body temperature rises to a level which allows them to move off and eat all the vegetation in their path. When the temperature drops at night they climb bushes and plant stems and remain immobile.
After the final ecdysis, the adult locusts take to the wing, and after a few days of short flights set off on extensive migrations, settling at night and in the middle of the day when it is hottest. A medium-Sized swarm may contain a thousand million locusts and cover an area of 20 square kilometres.
Such a swarm will consume some 3000 tonnes of vegetation per day; and so if a swarm lands on agricultural crops, the locusts will strip them of every vestige of leaf and edible stem. The swarms may travel many hundreds of kilometres from their place of origin, e.g. from Africa to India, and the females will lay eggs during their journey, so leaving the nucleus of successive swarms within a few weeks.
Methods of control. The range of the adult locusts is so great that international cooperation is essential for effective control.
A swarm may originate in India but cause devastating damage to crops in Africa. Sixty countries in Asia and Africa are threatened by swarms of the desert locust.
The main method of control is by spreading poisoned bait, for example bran containing BHC insecticide, in the path of the migrating bands of hoppers. The BHC kills them by being eaten and by its contact with their bodies. Poisoned bait can only be used when the locality of the hoppers is known, and a careful watch must be kept over wide areas so that swarms are discovered as soon as possible after they emerge. The information is then sent to anti-locust centres, e.g. in Nairobi or London, and trucks and personnel are mobilized to take the bait to the appropriate location.
Other methods employed are to spray insecticides over swarms of hoppers or settled adults using aircraft or motor vehicles, or to spray the vegetation in the path of the hoppers. If the region of egg-laying is known, the vegetation in the area can be sprayed with an insecticide which kills the hoppers at their first meal. The danger with all spraying techniques is that the chemicals used may be poisonous to man and other animals, particularly if used on food crops.
Two other species, the red locust and the migratory locust, have been held in check for many years by effective control measures, but the desert locust still constitutes a major threat. Constant vigilance and international cooperation are needed if crops are to be protected against this insect.