CHAP. HI.] METAMORPHOSIS. 31 



highly developed, the full-fed larva usually spinning a cocoon of 

 silk or of fragments of vegetable matter spun together with silk, 

 in the shelter of which it pupates and passes its pupal existence; 

 when the larva lives within a ease or within a burrow, however, 

 a true cocoon is not formed as a rule and those larvae which 

 pupate in the soil generally dispense with a silken covering. 

 In tin' more highly-specialized groups of Lepidoptera the cocoon 

 is sometimes dispensed with, the pupa being left exposed and 

 trusting to its procryptic coloration to elude discovery by ene- 

 mies; most of the butterflies, tor example, have naked pupae 

 which are suspended by the tad by peculiar cremastral hooks 

 (Nymphalida) or attached at the tail and girt by a loop around 

 the middle (Papilionidee, Pieridce); in such cases there is often a 

 marked degree of individual colour adjustment to agree with 

 environment, and this power is also seen even in the case ol 

 some cocoons. 



Turning now to the heterometabolic groups, in which meta- 

 morphosis is slight, we find that the immature insect, on emergence 

 from the egg, leads .inactive existence and is (broadly speaking) 

 similar to the adult except as regards the absence of wings, and 

 these may be permanently absent or imperfect in some forms. In 

 these groups the immature insect generally consumes the same food 

 as the adult, whereas in holometabolic insects the food of the larva 

 generally differs from that of the adult insect. 



In all cases, whether larva or nymph, the function of the 

 immature active stage is to assimilate nourishment and to grow, 

 and this is often done at an astonishing rate. The immature insect 

 being encased in a more or less hornj outer covering which is only 

 capable ot expansion within limits, growth is usually accomplished 

 by throwing off this outer skin, such process being termed a moult 

 or ecdysis and the periods between moults being called stadia 

 (singular, stadium) or instars. The frequency of moulting differs in 

 different groups of insects and sometimes varies (as in some 

 grasshoppers) in the two sexes. Usually there are about five moults 

 but the number may be decreased in insects undergoing rapid 

 metamorphosis (e.g., House-fly. with about three moults) or increased 

 in the case of insects whose immature condition is greatly prolonged 

 (e.g., some Cicadida, with 25 — 30 moults * spread over a period of 

 fifteen years or longer). The process of moulting, which i> not 



iervations on the number of moults in long-lived Cicadas have been made in 

 America and it has recently been staled that this number is nut so large as hail hecn 

 supposed. Riley's observations, however, were usually accurate ami may provi 

 in this case of individuals "r races and I therefore leave the above paragraph a. originally 

 written. T.B.l . 



