How INSECTS LIVE AND GROW

How INSECTS LIVE AND GROW Since their bodies are encased in rigid shells, insects cannot grow gradually as we do. Instead, they grow in stages. They feed until their coat becomes too tight, then they stop eating and rest while it splits open. Meanwhile, a new shell has formed beneath the original one so, when the insect crawls out of .its old armor, it is already protected. This is called molting, After molting, it feeds voraciously until it outgrows this new shell, then molts again. In this way, it gradually increases in size. The form of the insect between each molt is called an instar. Most insects pass through three to six instars. However, depending upon temperature and food supply, there may be up to 30 iinstars before the insect reaches maturity. Insects are not born with all their adult features. They acquire these during certain moltings and pass through a series of “form changes." This process is known as metamorphosis. True bugs; grasshoppers, crickets. and mantids; earwigs; thrips; whiteflies; aphids; and scales pass through three basic forms egg, nymph, and adult. This is called simple or incomplete metamorphosis (fig. 2). This is because the nymph more or less resembles the adult form, but it does not possess fully developed wings and its coloring may be quite different. After a series of molts in which it increases in size, it molts a final time, acquiring wings and fully developed antennae. lt emerges as a fullfledged adult.

Most other insects, including bees and wasps; beetles; butterflies and moths; flies; and Iacewings have complete metamorphosis (fig. 3) in which they pass through four distinctly different life stages egg, larva, pupa, and adult. Immature forms known as larvae bear no resemblance to the adult form. They are more or less wormlike and may be legged or legless. Sometimes they are covered with spines or tufts of hair. They have chewing mouthparts, even if the adult counterpart has sucking mouthparts. They molt several times, increasing in size but very little in form.

After the molt of the last instar, the larva changes into an inactive form called the pupa. It stops feeding and may even stop moving. Some insects spin a cocoon or web or roll a leaf around their bodies to protect themselves during this period of vulnerability.

During pupation, the insect changes profoundly from larva to adult. Some structures and tissues such as the plump little false legs dissolve and new features such as legs, wings, and antennae develop. When the adult features have been formed, the insect splits its pupal chamber and emerges. Later, wings dry, pigmentation develops, and armor hardens.

The adult stage of insects with either type of metamorphosis is usually a relatively short one, geared entirely toward reproduction. Some adult forms such as male scale insects do not even have mouths since they live such a short time and do not need to eat. They mate and die almost immediately afterward.

Although adulthood is usually the shortest period of the insect's life, it is often the one in which the insect ‘is most aware of the world around it. The nervous system is fully developed and vision is at its best. Respiration, circulation, and, if the insect has mouthparts. digestion, are in full swing. Often, insects are most mobile during adulthood.

Breathing. insects do not have lungs. in almost all species, the blood contains no hemoglobin and is not used to carry oxygen. Instead, insects breathe through tiny holes or spiracles in the thorax and abdomen (fig. 1 on page xii). Oxygen passes through a system of branching tubes to all parts of the body. Sometimes the insect pumps its abdominal muscles in order to encourage ventilation.

Circulating the blood. Insect blood is a yellowish green or clear, thick fluid. it carries food to body organs, stores proteins and water, and picks up waste materials for excretion. It is pumped by a simple, tubular heart located in the middle of the back, just beneath the body wall. Since there are no arteries or veins, the blood sloshes through the insect's body cavity, percolating back and forth from the head to the abdomen, bathing every organ. It moves into the wings through rigid tubes.

Eating and digesting. l nsects feed on all kinds of things -dead and living plants and animals, paper, trash, and fabrics. Some dine only on blood or eat just a particular layer of cells in a particular kind of leaf. Others consume anything and everything.

The mouthparts needed to break down these foods are complicated and specialized. There are two basic types -those that chew food and those that suck it. insects that chew their food have sturdy jaws which move Sideways. 3