288 



NATURAL HISTORY. 



\ B 



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Fig. 7. EARWIG IN ITS DEVELOPMENT. 

 A, Larva; B, Pupa; c, Imago. 



in many cases the young present a very different structure. In all Arthropoda the skin, or rather the 

 epidermis or outer layer of the skin, is a continuous formation, and as completely outside the growing 

 part of the organism as the hair or nails of quadrupeds or the feathers of birds. Perhaps the best 

 analogue is to be found in the epidermis of reptiles in both cases the outer layer of the skin is 

 continuous, and, when once formed, incapable of receiving any increase of size ; hence, as the animal 

 contained in it grows, there arises a necessity for the epidermis being thrown off from time to time. 

 In insects this moulting (or ecdysis) regularly takes place ; in some the skin is changed as often as 

 twenty times ; and in a great number the shedding of the skin is associated with a great change 

 of form. 



This, however, is not the case in all. A considerable number of insects, chiefly parasitic in 

 their habits, are hatched from the egg in a form almost exactly resembling that of their parents. 



Others, on making their first appearance in the world, 

 more or less closely resemble their parents in shape, but 

 nevertheless present certain differences, notably the absence 

 of wings, which the latter possess (Fig. 7). In these 



\ f\ s \K^J V\^y/i\I^MMk/Tl\\^' ; ^y instances the young insects in the course of growth change 

 \-J C ft "\ ^^vUHBvll^^' their skins several times, and at a certain period of their 



existence we find that behind the pronotum a pair of 

 prominences not previously present have made their ap- 

 pearance. These are the cases containing the rudiments 

 of the wings, which become fully developed after the 

 last moulting. This condition of things may be observed 

 in the Cockroaches and Crickets which frequent our 

 kitchens, in the Grasshoppers, whose shrill cry enlivens the meadows in summer, and in the 

 whole tribe of Bugs. 



In others, and indeed in the majority of insects, the change that takes place is much greater. 

 The young insect creeps out of the egg in a form totally unlike that which it is ultimately to possess, 

 and in some respects much more closely resembling that of the lower Annulosa, commonly known as 

 Worms. Nearly all are soft, fleshy creatures, with the body divided into segments ; some are entirely 

 destitute of limbs and of any distinct head ; others have no limbs, but are furnished with a horny 

 head ; others have jointed legs attached to the first three segments of the body ; and others, again, in 

 addition to these, possess a larger or smaller number of pairs of fleshy feet (prolegs) appended to 

 some of those segments which represent the abdomen. In common parlance, the headless and footless 

 forms are called Maggots ; those with a head and no feet, and some of those which possess legs, are 

 known as Grubs ; and the forms with legs and prolegs are generally termed Caterpillars. The term 

 larvce is applied to all the different forms by naturalists, as also to the young insects above referred 

 to, which resemble their parents in most respects except the entire absence of any trace of 

 wings. 



At the close of this so-called larval period of existence, however, there is a very great difference 

 in the course of events in the two series of insects. As already stated, at the last change of skin, or, 

 in some cases, at a somewhat earlier one, the larvae which resemble their parents in general form 

 acquire rudimentary wings, enclosed in cases which lie upon the sides of the body, behind the pronotum 

 (see Fig. 7, B) ; but in other respects, as has been seen, the insect retains the same form as before, and 

 continues to run about and feed like the larva. This goes on until the final moult, when the wings 

 are freed from the case enclosing them, and speedily acquire their full size and development. 



In the case of the more or less worm-like larvaa (Caterpillars, Grubs, Maggots, &c.), affairs go 

 on very differently. When the larva has acquired its full growth and the last change of skin takes 

 place, the result of this operation is the production of a creature wholly unlike the larva, and 

 generally presenting no more than a distant resemblance to the perfect insect. Where this 

 resemblance is closest, the product of this change of skin is a creature showing the division of the 

 body into the three regions head, thorax, and abdomen characteristic of the perfect insect, but 

 which was wholly wanting in the larva ; the legs and antennae, and the wings in an undeveloped 

 condition are also distinguishable, but all these parts are enclosed in a skin, which closely covers 



