Because they are distinguished from the preceding classes by the absence of red blood, in place of which they have a white fluid. In recent times, on account of the absence of vertebrae and ribs, they have received the name of Invertebral Animals.
Among the crustaceous animals, as the lobster and shrimp, the blood is white ; while, among some insects, as the grasshopper, and white caterpillar, it is green. - Hewson, Phil. Trans.
Because, both in their perfect and larvae state, they have a membranous tube running along the back, in 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.
Because they are furnished with air-vessels or trachea, which ramify over most of their body. These tracheae 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.
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 red-blooded animals, and many in mephitic atmospheres, so fatal to others, and in which animal and vegetable substances become putrid.
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 larvae, whilst in the state in which they escape from the egg. They are mostly very small on their first appearance, so that a full-grown 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 meatfly, at the end of twenty-four hours, is 155 times heavier than at its birth. Larvae are incapable of propagating ; they merely feed, increase, and change their covering several times.
The larvae become nymphae. Many can move about and take food in this state. Others, on the contrary, are covered up, as pupae, (chrysalides, aureliae) 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.
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.