The science of life : fully illustrated in tone and line and including many diagrams

BOOK 2

man—which surpasses them in this respect. With the division of labour and consequent specialization of individuals that is found in bees, ants, and termites, and with the fascinating problem of the insect mind we shall deal in later chapters; as we shall be returning to these and other questions about them, our present treatment of the class may be brief.

There is one consequence of the possession of wings that is worth noting. ‘The arthropods, as we have seen, have to moult in order to grow, and the insects are no exception to this rule. But it is apparently impossible to moult the skeleton of a wing and then to grow it again. An insect’s wing is a delicate, temuous structure; it consists of little more than a double film of chitin with an invisibly thin layer of living tissue in between. If the stiff outer film were cast off the inner layer would be quite unable to retain any sort of shape. For

© 8 Brouwer

Fig. 84. Peripatus—a link between Arthropod and Worm.

this reason, the winged stage of an insect’s life is invariably its last. For most of its life it simply creeps and eats and grows ; once it has developed its wings it cannot moult and its growth therefore ceases. During its long childhood the insect is called a larva ; in its often brief final stages, when it flies and reproduces its kind, it is called an adult or imago. The difterence of duration between the two periods may be very striking; in some of the may-flies the larva crawls and feeds for two years or more as a preparation for one day of winged loye-making.

In some insects, such as the earwig or the dragon-fly, the transition from larva to adult is comparatively gradual. The larva in such forms is often distinguished as a “nymph.” The dragon-fly larva lives at the bottom of a pond, snatching and devouring tadpoles and other creatures by means of the hinged “ mask” that projects from its face. From time to time it moults, and after the third or fourth moult there appear on its back two small backwardly directed lobes,

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which are the rudiments of the wings. During the later moults these rudiments increase in size, and at the same time a . system of trachez develops in the body. Finally, often as long as a year after hatching, the fully grown nymph crawls up a weed or some other object that projects from the surface of the pond into the air and the final moult occurs. When it has found a suitable place, it pauses for hours, sometimes even for a whole day; then abruptly the nymph-skin splits down the back and the tender body of the imago comes out. There is then a period of almost volcanic expansion before the outer skeleton dries and stiffens. The wings, at first small and moist and crumpled, expand and straighten very quickly ; the body alters its shape. In a few hours the body is well formed and rigid, and after a few tentative movements, the adult dragon-fly sails off to hunt and devour other insects and to breed until the chill of winter puts an end to its life.

Tn other insects, such as butterflies, beetles, bees, and house-tflies, the metamorphosis from larva to imago is more abrupt. The buttery larva, or caterpillar, is a voracious soft-bodied grub with hardly a trace in its body of any of the structures that it will possess when it is adult. The fully grown caterpillar, when it is ready to moult for the last time, generally encloses itself in a bed of silk and then undergoes a series of profound changes. First it moults and assumes a form resembling more nearly that of the adult. In this stage, as a pupa, it remains for weeks or months, and during this time its organs are reconstructed very drastically indeed, undergoing almost complete liquefaction in the process. Finally, as with the dragon-fly, a limp and crumpled butterfly emerges, to expand and dry its wings and then take to the air for its culminating experience of life.

§ 3d Centipedes and Mullipedes

The class Myriapoda includes a number of land-living arthropods, breathing by means of trachee, and having long serpentine bodies with from nine to over a hundred and fifty pairs of legs. In the arrangement of the appendages on the head, im their