which alternate dilatations and contractions may be observed; but it is closed at both ends, and no vessels can be perceived to originate from it. Why are not insects concluded to take in air through the mouth? Because they are furnished with air-vessels or trachea, which ramify over most of their body. These tracheæ are much larger and more numerous in the larva state of such insects as undergo a metamorphosis (in which state also the process of nutrition is carried on to the greatest extent) than after the last, or, as it is called, the perfect change, has taken place. Why is the mode of respiration observable but in few insects? Because they in general breathe not by the mouth, but by many spiracula, or pores. The greater number of them can live in a vacuum much longer than redblooded animals, and many in mephitic atmospheres, so fatal to others, and in which animal and vegetable substances become putrid. Why is the metamorphosis of insects so called? Because there is not any winged insect which escapes from the egg as such, but all, as well as many insects which have not wings, must first undergo a kind of change, at a certain period of their existence. Such insects are called larva, whilst in the state in which they escape from the egg. They are mostly very small on their first appearance, so that a fullgrown caterpillar of the willow moth, for instance, is 72,000 times heavier than when it first issues from the egg. On the other hand, they grow with great rapidity, so that as an example, the maggot of the meat-fly, at the end of twenty-four hours, is 155 times heavier than at its birth. Larvæ are incapable of propagating; they merely feed, increase, and change their covering several times. The larvæ become nymphæ. Many can move about and take food in this state. Others, on the contrary, are covered up, as pupa, (chrysalides, aureliæ) and pass this portion of their life in a state of torpor, without eating or moving. A great change is, however, going on, by which the animal quits its larva state, and leaves its prison a perfect insect. In popular language, a caterpillar or grub is furnished with feet, and a maggot or gentle is without feet. Why is the metamorphosis of insects so remarkable a branch of their economy? Because, by this, not only their external form, but also at the same time their internal structure, contrary to common opinion, is altered in a certain degree. Blumenbach remarks, if the moth existed already formed in the caterpillar, we should at least expect that similar moths should be produced by similar caterpillars. But many American caterpillars, which resemble European ones in the closest manner possible, give origin to moths having totally different forms; and, on the other hand, many remarkably similar moths of both these parts of the world are developed from caterpillars altogether unlike. Why do certain larvæ form an exterior covering or cocoon? Because the pupa may be lodged with greater safety. This covering is in some composed of threads of silk. Sometimes only one or two threads are required to keep the pupa in a proper position; in others, the silk is woven into cloth, or so inatted together, as to resemble paper. The matter, of which these cases or cocoons are fabricated, is prepared by two long tubes, which take their rise in the abdomen, enlarge as they approach the head, and terminate by a duct, which opens under the labium, or lower lip. By pressing the orifice of this duct to one place, and then to another, the larva draws out the tenaceous threads.Fleming. Why is the pupa so called? Because the larva, when so enclosed, resembles an infant in swaddling bands. From the pupa of many of the butterflies appearing gilt as if with gold, the Greeks called them Chrysalides, and the Romans Aurelia, and hence naturalists frequently call a pupa chrysalis, even when it is not gilt. Why do certain insects neither eat nor move when in the pupa state? Because they derive their nourishment from their stores of fat. Why are the larva more voracious than the perfect insects? Because their digestive organs are of much greater dimensions than when arrived at maturity; and in the condition of larvæ, insects possess a variety of members, as legs, suckers, hairs, and even stigmata, (respiratory organs) which they do not possess in their maturity.— Fleming. Why have most of the rings of the abdomen an open pore placed laterally? Because they may serve as breathing holes, through which the fluids of the animal become aërated. The three portions of the body of insects, the trunk, head, and abdomen, in the different tribes, exhibit very remarkable combinations. In some of the crustaceous animals, these portions are incorporated in the dorsal (or back) surface of the body. In some of the arachnide, (or spider genus) the head and trunk are niched, while, in others, the head appears to be distinct, while the trunk and abdomen are incorporated. These modifications are extensively employed in the methodical distribution of the groups. Why do insects attach their eggs to certain substances? Because the young, being hatched, are destined to feed on those substances. Thus, the butterfly attaches her eggs to a leaf; the flesh-fly deposits her's upon |