The larval forms of insects vary greatly both in their form and in the completeness of their metamorphosis. They may be divided into two classes, the cruciform and the campodea form. The former include those which are worm-like, such as the caterpillars (q.v.) of moths and butterflies, the grubs of beetles, the maggots of flies and the like. They are the most numerous and conspicuous forms, and are active and voracious, and do nearly all the damage to be attributed to injurious insects. The campodeaform larva are these which nearly resemble the parents, such as the nymphs of the dragon-fly and related groups. The sec ond stage of larval life among insects is a very different existence, usually stationary and qui escent, and is called the pupa stage (see PUPA). Some larva among insects and elsewhere may breed,— a phase of reproduction called pado genesis see PARTHENOGENESIS.
The value of the different forms and habits of life assumed by animals in passing through the larval stage or stages is that it tends to prevent the extinction of the species, since if at any moment all the adults were swept out of existence, the young living in a different station would continue to represent and revive the species. ((This law is seen to hold good among the insects," as Packard points out, ((where many species are represented in the winter-time by the egg alone, others by the caterpillar, others by the chrysalis, while still others hibernate as imagoes. Again, in the
marine species, the free-swimming young are borne about by the ocean and tidal currents, and in this what in adult life are the most sedentary forms become widely distributed from coast to coast and from sea to sea.* On the other hand, the larval forms of fixed marine animals serve as food for fishes, espe cially young fishes and numerous invertebrates, which, without this resource, would starve; and larva of insects are the principal resource for food of birds in the breeding season, when all the small, inland birds must feed their nest lings on caterpillars and grubs, for the most part, even when they are seed-eaters as adults.
Certain larva enter extensively into the food; supply of many mammals, and are even eaten; or become otherwise useful to man. Some are artificially cultivated for his service, the silk-worms, for an example.
Among vertebrates larva are rare, and ap pear only in the lower forms, those of the lamprey (Ammaccetes), of eels (Leptocephali), of certain salamanders (asoloils), and of frogs (tadpoles), being the most noticeable.