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.

The egg of the cotton moth, greatly enlarged.  From Manual for the Study of Insects.

The forest tent-caterpillar shedding its skin.  Photo by M. V. Slingerland.

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.

Full-grown caterpillar of the luna moth.  Photo by M. V. Slingerland.

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.

A luna cocoon cut open, showing the pupa.  Photo by M. V. Slingerland.

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.

A butterfly chrysalis.

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.

On the left: A young grasshopper, enlarged.  The line shows its actual length. On the right: The adult of the same grasshopper,  natural size.

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.

Insect brownies; tree-hoppers as seen through a lens.