VI. Insect Study

NSECTS are among the most interesting and available of all living creatures for nature-study. The lives of many of them afford more interesting stories than are found in fairy lore; many of them show exquisite colors and, more than all, they are small and are, therefore, easily confined for observation.

While the young pupils should not be drilled in insect anatomy, as if they were embryo zoologists, yet it is necessary for the teacher, who would teach intelligently, to know something of the life stories, habits and structure of the common insects. Generally speaking, all insects develop from eggs. To most of us the word egg brings before us the picture of the egg of the hen or of some other bird. But insect eggs are often far more beautiful than those of any bird; they are of widely differing forms, and are often exquisitely colored and the shells may be ornately ribbed and pitted, sometimes adorned with spines, and are as beautiful to look at through a microscope as the most artistic piece of mosaic.

From the eggs, larvæ (sing. larva) issue. These larvæ may be caterpillars, or the creatures commonly called worms, or may be maggots or grubs. The larval stage is always devoted to feeding and to growth. It is the chief business of the larva to eat diligently and to attain maturity as soon as possible; for often the length of the larval period depends more upon food than upon lapse of time. All insects have their skeletons on the outside of the body; that is, the outer covering of the body is chitinous, and the soft and inner parts are attached to it and supported by it. This skin is so firm that it cannot stretch to accommodate the increasing size of the growing insect, thus from time to time it is shed. But before this is done, a new skin is formed beneath the old one. After the old skin bursts open and the insect crawls forth, the new skin is sufficiently soft and elastic to allow for the increase in the size of the insect. Soon, the new skin becomes hardened like the old one, and after a time, is shed. This shedding of the skin is called molting. Some insects shed their skins only four or five times during the period of attaining their growth, while other species may molt twenty times or more.

After the larva has attained its full growth, it changes its skin and its form, and becomes a pupa. The pupa stage is ordinarily one of inaction, except that very wonderful changes take place within the body itself. Usually the pupa has no power of moving around, but in many cases it can squirm somewhat, if disturbed. The pupa of the mosquito is active and is an exception to the rule. The pupa is usually an oblong object and seems to be without head, feet or wings; but if it is examined closely, especially in the case of butterflies and moths, the antennæ, wings and legs may be seen, folded down beneath the pupa skin.

Many larvæ, especially those of moths, weave about themselves a covering of silk which serves to protect them from their enemies and the weather, during the helpless pupa period. This silken covering is called a cocoon. The larvæ of butterflies do not make a silken cocoon, but the pupa is suspended to some object by a silken knob, and in some cases by a halter of silk, and remains entirely naked. The pupa of a butterfly is called a chrysalis. Care should be taken to have the children use the words—pupa, chrysalis and cocoon—understandingly.

After a period varying from days to months, depending upon the species of insect and the climate, the pupa skin bursts open and from it emerges the adult insect, often equipped with large and beautiful wings and always provided with six legs and a far more complex structure of body than characterized it as a larva. The insect never grows after it reaches this adult stage and, therefore, never molts. Some people seem to believe that a small fly will grow into a large fly, and a small beetle into a large beetle; but after an insect attains its perfect wings, it does not grow larger. Many adult insects take very little food, although some continue to eat in order to support life. The adult stage is ordinarily shorter than the larval stage; it seems a part of nature’s economic plan that the grown-up insects should live only long enough to lay eggs, and thus secure the continuation of the species. Insects having the four distinct stages in their growth, egg, larva, pupa and adult, are said to undergo complete metamorphosis.

But not all insects pass through an inactive pupa stage. With some insects, like the grasshoppers, the young, as soon as they are hatched, resemble the adult forms in appearance. These insects, like the larvæ, shed their skins to accommodate their growth, but they continue to feed and move about actively until the final molt when the perfect insect appears. Such insects are said to have incomplete metamorphosis, which simply means that the form of the body of the adult insect is not greatly different from that of the young; the dragon-flies, crickets, grasshoppers and bugs are of this type. The young of insects with an incomplete metamorphosis are called nymphs instead of larvæ.

Summary of the Metamorphoses of Insects

• I. Complete metamorphosis
• Egg.
• Larva.
• Pupa. (The pupa is sometimes enclosed in a cocoon.)
• Adult or winged insect.
• II. Incomplete metamorphosis
• Egg.
• Nymph (several stages).
• Adult, or imago.

The Structure of Insects

The insect body is made up of ring-like segments which are grown together. These segments are divided into groups according to their use and the organs which they bear. Thus the segments of an insect’s body are grouped into three regions, the head, the thorax and the abdomen. The head bears the eyes, the antennæ, and the mouth-parts. On each side of the head of the adult insect may be seen the compound eyes; these are so called, because they are made up of many small eyes set together, much like the cells of the honeycomb. These compound eyes are not found in larvæ. In addition to the compound eyes, many adult insects possess simple eyes; these are placed between the compound eyes and are usually three in number. Often they cannot be seen without the aid of a lens.

The antennæ or feelers are composed of many segments and are inserted in front of the eyes or between them. They vary greatly in form. In some insects they are mere threads; in others, like the silk-worm moths, they are large, feather-like organs.

The mouth-parts of insects vary greatly in structure and in form, being adapted to the life of the insect species to which they minister. Some insects have jaws fitted for seizing their prey, others for chewing leaves, others have a sucking tube for getting the juices from plants or the blood from animals, and others long delicate tubes for sipping the nectar from flowers.

In the biting insects, the mouth-parts consist of an upper lip, the labrum, and under lip, the labium, and two pairs of jaws between them. The upper pair of jaws is called the mandibles and the lower pair, the maxillæ (sing. maxilla). There may be also within the mouth, one or two tongue-like organs. Upon the maxillæ and upon the lower lip there may also be feelers which are called palpi (sing. palpus). The jaws of insects, when working, do not move up and down, as do ours, but move sidewise like shears. In many of the insects, the children are able to observe the mandibles and the palpi without the aid of a lens.

The thorax is the middle region of the insect body. It is composed of three of the body segments more or less firmly joined together. The segment next the head is called the prothorax, the middle one, the mesothorax, and the hind one, the metathorax. Each of these segments bears a pair of legs and, in the winged insects, the second and third segments bear the wings. Each leg consists of two small segments next to the body, next to them a longer segment, called the femur, beyond this a segment called the tibia, and beyond this the tarsus or foot. The tarsus is made up of a number of segments, varying from one to six, the most common number being five. The last segment of the tarsus usually bears one or two claws.

While we have little to do with the internal anatomy of insects in elementary nature-study, the children should be taught something of the way that insects breathe. The child naturally believes that the insect, like himself, breathes through the mouth, while as a matter of fact, insects breathe through their sides. If we examine almost any insect carefully, we can find along the sides of the body a series of openings. These are called the spiracles, and through them the air passes into the insect’s body. The number of spiracles varies greatly in different insects. There is, however, never more than one pair on a single segment of the body, and they do not occur on the head. The spiracles, or breathing pores, lead into a system of air tubes which are called tracheæ (tra’-ke-ee), which permeate the insect’s body and thus carry the air to every smallest part of its anatomy. The blood of the insect bathes these thin-walled air tubes and thus becomes purified, just as our blood becomes purified by bathing the air tubes of our lungs. Thus, although the insects do not have localized breathing organs, like our lungs, they have, if the expression may be permitted, lungs in every part of their little bodies.

Summary of Structure of an Insect

• Head
• Antennæ.
• Compound eyes.
• Simple eyes or ocelli.
• Mouth-parts
• Labrum, or upper lip.
• Mandibles, or upper jaws.
• Maxillæ, or lower jaws, and maxillary palpi.
• Labium and labial palpi.
• Thorax
• Prothorax and first pair of legs.
• Mesothora and
• second pair of legs.
• first pair of wings.
• Metathorax and
• third pair of legs.
• second pair of wings.
• Wing
• veins.
• cells.
• Leg
• Two small segments called coxa and trochanter.
• Femur.
• Tibia.
• Tarsus and claws.
• Abdomen
• The abdomen bears
• ears (in locusts only).
• spiracles.
• ovipositor.

References.—Manual for the Study of Insects and Insect Life, Comstock.