HAVING learnt something of the ways in which an insect can live and react to its environment, it is now possible for us to turn to the types of life-history and its stages. Primitive wingless insects like the springtails, which are commonly found amongst the grass of certain meadows, hatch out from the egg in a form very like the parents, so that there is no metamorphosis. The complications of metamorphosis arose with the development of wings. There are two types of wing development: first, that in which they develop gradually as external buds, and second, where they develop internally during the early stages of the life-history and are later everted and remain in a half-grown condition for a definite period when, after a sudden change, they become fully grown.
There are two orders of insects, the lice and the fleas, which are wingless and are yet included amongst the winged insects in classification, the reason being that their winglessness is a secondary modification due to their mode of life : their ancestors were winged. As already pointed out, in the true wingless forms like the springtails and bristletails, the life-cycle is direct, the egg hatching out into a young insect not very dissimilar from the adult into which it will grow. There is no metamorphosis.
In the case of insects whose wings develop as external buds, there is a partial metamorphosis and the life-history consists of three well-defined stages—the egg, nymph and adult or imago. A nymph is the name given to a young insect which is hatched in a fairly advanced state of development but whose wings are only partially developed. The nymph possesses compound eyes and tissue identical with that of the adult. The change from nymph to adult is really a very slight one. This three-stage life-history occurs amongst cockroaches, crickets, stick insects, earwigs and bugs.
The life-history of those insects whose wings develop internally is more complex and is made up of four stages : the egg, larva, pupa and adult or imago. Young insects within the egg feed on the yolk and, where this is scarce, as it is in the eggs of many of the higher insects, the young have to emerge much earlier than those which have plenty of yolk on which to feed. I’he result is that they hatch out at a much earlier stage and consequently differ very considerably from the parents in general shape and make-up. This young stage is called a
larva, and it differs from the imago or perfect insect in the possession of simple eyes and special larval tissue differing from that of the adult. Metamorphosis could be defined as the change from larval to adult tissue. The larval tissues ultimately break down, and buds which give rise to adult tissue are found throughout the body.
A very active larva with well-developed legs is characteristic of the beetles; the soft, cylindrical and much less active caterpillar of the butterfly and the well-known maggot of the fly are both larvae. They vary in shape, size and habit, but they all have this in common, that they are the effective feeding and growing stage of the life-history.
Following on the larval stage comes the inactive pupa or chrysalis, a stage of internal reconstruction, during which the adult tissues are being built up. This is a helpless phase, often seeking protection in a silk or earth-cemented cell or cocoon. Pupa? vary as much as larva? : the legs and wings may be free from the body as in beetles and bees; they may be closely attached to the body and the pupal case rigid, with the exception of the abdomen, as in butterflies and moths, or a definite puparium or capsule is formed from the larval skin as in flies.
The final stage of the life-history is the imago or perfect insect whose principal function is that of reproduction. It may become so specialised to this end that the feeding mechanism degenerates altogether. In some cases the life of an adult varies from one hour to a fortnight, but, in the case of insects which emerge from the pupa in the autumn, the whole winter may be passed in hibernation until the spring heralds the time for egg-laying. In many examples very resistant eggs are laid at the end of the summer or in early autumn, which hatch in the spring and serve to carry the species over the inclement winter months.
The imago has rarely any personal interest in the food plant of its larval days, and yet she invariably chooses a similar kind of plant on which to lay her eggs, thus malting provision for her offspring. The Cabbage White butterfly does not eat cabbage but her larvaj do, and so she always lays her eggs on cabbage leaves.
The larva and nymph, then, are the stages of growth, while the imago is essentially the unit in the life-history which makes for increase both in numbers and range—for its wings enable it to cover much wider areas than a wingless form could negotiate and so a wider distribution of eggs is possible.