492 COMPARATIVE ANATOMY CHAP. 



with concealed extremities. They are generally quiescent, immovable, and not 

 capable of taking food, and are often surrounded by special protective envelopes 

 cocoons. The best known cocoons are those of the Lepidoptera, which are spun by 

 the caterpillars from the secretion of their spinning glands. At the end of pupal 

 life the envelope is opened and the imaginal insect (beetle, butterfly, fly, etc.) issues 

 from it. 



This complete metamo'rphosis evidently proceeds from an incomplete or gradual 

 metamorphosis. The wingless larva adapts itself to various conditions of existence, 

 or is provided by its parents with excessive nourishment. It accumulates so much 

 reserve material in its fat body that the further larval stages do not need to feed 

 independently. By the suppression of numerous ecdyses these successive larval 

 stages are abbreviated into one stage, the stage of the outwardly quiescent pupa, 

 within which the imago attains development at the expense of the reserve nourish- 

 ment. 



The larvae of insects with complete metamorphosis vary much in their organi- 

 sation, each being adapted to its own special surroundings. Two principal groups 

 can, however, be distinguished : (1) with feet, e.g. the larvae of the Neuroptera, the 

 "caterpillars" of the Lepidoptera, the larvae of the Coleoptera and Trichoptera ; (2) 

 without feet, maggot -like larvae of the Diptera, larvae of most Hymenoptera and 

 Siphonaptera. The former by possessing legs are the least removed from the 

 Thysanura-like larval form of other winged insects ; they move freely, and with few 

 exceptions feed independently. Many are carnivorous, living either on land or in 

 water ; many feed on plants, on the leaves (caterpillars) or roots (cockchafer grubs). 

 Among the Hymenoptera the larvae of the Tenthredinidce are vermiform, and like 

 the butterfly caterpillars possess parapodia - like appendages on several abdominal 

 segments in addition to the thoracic feet. 



The modes of life of the footless larvae of the Diptera and most Hymenoptera are 

 very varied. They sometimes live free and are carnivorous, generally living then in 

 water ; sometimes they are parasitic in the bodies of other animals or in plant tissue ; 

 sometimes in putrefying matter, dung, etc. ; sometimes inside cases or cells which are 

 filled with nutritive material ; sometime they are fed by the adults, etc. etc. Head- 

 less maggots without feelers or ocelli and with reduced mouth parts are distinguished 

 from larvae which have heads with these organs. 



The larvae of insects with complete metamorphosis are all originally peripneustic. 

 By adaptation to aquatic or parasitic life they may become amphipneustic, meta- 

 pneustic, or even apneustic, and in the last case may develop trachaeal gills. The 

 mouth parts of the larva may differ greatly from those of the imago. This 

 difference is best known and most striking in the Lepidoptera, whose larvae have 

 masticating mouth parts, while their imagines have sucking mouth parts. 



The more specialised the larva on the one hand and the imago on the other, and 

 the greater the difference in organisation between them, the more far-reaching natu- 

 rally are the transformations by means of which during the pupal period the larval 

 organisation becomes that of the imago. For instance, in the bee, the larva does 

 not pass direct into the pupal stage, but first into the pre-pupal stage. 



In certain Coleoptera several larval stages differing very much from one another are 

 met with. The Coleopteran genus Sitaris (Fam. Meloidea) lives parasitically on a 

 bee (Anthophora). The larvae of this beetle, which are hatched from the egg, are 

 active little animals with thoracic legs. They lurk in flowers in order to spring 

 upon the bees coming to gather honey. They are thus carried to the hive, where 

 they seize upon the eggs of the bee as soon as these are laid in the honey of the 

 cells, and devour them. They afterwards moult and appear, after ecdysis, as meta- 

 pneustic maggot-like larvae with reduced feet, floating on the surface of the honey, 

 the mouth placed below, and the posterior end on the surface. When the honey is 



