Class 3. Insecta. 251 



Tlie larva^ in many species, forms a similar protective case, a cocoon, 

 before pupating ; it consists of spun silk, or of various particles 

 bound together, and tte pupa rests witbin it (Lepidoptera, Hymenop- 

 tera, some Coleoptera, and others) . 



In some of the Hymenoptera, a peculiar developmental stage is inserted 

 between the larva and the pupa, or more correctly, there are two pupal stages. 

 The fully-grown larva first forms a so-called pseudopupa, with just the 

 heginnings of wings, legs, and so on, and not till later does the insect enter the 

 true pupal stage, characterised by larger wings and legs, and generally by a 

 greater approximation to the imago. 



The metamorphosis is indicative of a definite division of 

 labour in the animal's life. The larval period is a time for 

 eating and growing; the life of the imago is devoted to 

 reproduction; at metamorphosis growth terminates, the animal 

 usually takes no more food than is necessary to make good the loss 

 ■due to vital activity, and it dies when reproduction has been accom- 

 plished. In some cases, the distinction between the two periods is 

 especially well-marked ; the imago takes no food at all, and thus 

 the vegetative and reproductive periods are sharply separated. 

 Even if the perfect form does feed, the fact that it no longer under- 

 goes ecdyses shows that growth has really ceased. The few Insects 

 (Termites) in which the abdomen of the adult increases enormously 

 in size, in consequence of the great development of the ovary, form 

 in some sort an exception to this. 



According to this account the metamorphosis of Insects is in marked contrast 

 to that of other animals, e.g., of the Crustacea, in which the conclusion of larval 

 life, and the cessation of gi'owth are by no means coincident. 



The duration of life in Insecta is almost always sharply 

 limited and fairly short. Usually the whole life (including the 

 phases of egg, larva, pupa, and imago) lasts only one year ; in not a 

 few, e.g., in the Aphides it is a matter of months only : in others 

 (many larger forms) it continues during several, usually a fairly 

 definite number, of years (the Cockchafer, for instance, lives usually 

 for four years). Of the whole life the larger part is generally 

 ■occupied by the larval stage, and only a small portion by the 

 imaginal period; the imago usually lives for only quite a short time, 

 not infrequently for a few days or even for a few hours. Only 

 exceptionally do instances of longevity (several years) in the imago 

 occur. Honey bees have been observed to live five years in captivity. 

 Ants even twelve. 



The members of this group afford an emphatically terrestrial 

 type of animal, their organisation being closely adapted to life on land 

 or in the air. Not a few are, however, modified, so that they may live 

 for their whole life, or during the larval stage only, in fresh water. 

 Few are marine, e.g., larvee of flies which occur in the mud on flat sea- 

 shores. Some (Coleoptera, Heteroptera and others) live on the coast 

 at spots which are only dry at ebb-tide, whilst at the flood they are 



