274 CLASS V1IT. 



tortuous in the pupa, and shorter in the perfect Insect ; the nervous 

 ganglia become less numerous, from some of them first approxi- 

 mating to each other and afterwards coalescing, whilst others 

 entirely disappear ; the first ganglion especially, which is situated 

 in the head, increases in amplitude. The sexual organs, of which 

 the germs already existed in the larvse, become developed as well 

 in respect of complex structure as of size. In the pupa new organs 

 also come to view, of which before there was no trace, such as the 

 wings, which are seen folded and rolled together internally at the 

 thorax. The dorsal vessel undergoes less change than most of the 

 other organs. 



There is in larvge, moreover, a peculiar fatty mass present, of 

 which we have already spoken above; the secretion of this fat 

 constantly increases, the nearer the larva approaches the condition 

 of pupa ; in this condition the fat is consumed again, and in that 

 of the perfect Insect, when it also takes food, fat is no longer 

 secreted. 



The fatty secretion is thus obviously necessary to supply nutri- 

 ment to the pupa, and to afford the material for the development 

 of the organs of the perfect Insect. Yet the pupaa of Insects 

 which undergo a complete metamorphosis, take, as stated above, 

 no food, and are in connexion with the external world through 

 respiration alone 1 . The condition of pupa therefore may be com- 

 pared with that of hybernating animals, which are very fat in the 

 autumn, during their sleep take nothing, and in the spring come 

 out of their retreats in a very emaciated condition. The larva? of 

 Insects eat more than is necessary for their own growth ; they are 

 therefore usually inactive ; deficient motion and superfluous food 

 favour, as is known, the secretion of fat 2 . The reason why inil 

 perfect Insects no more fat is secreted, is found as well in the ji 

 development of the sexual functions as in the greater activity of life ' 

 and rapidity of motion, which are peculiar to them. 



1 On this account they lose in weight. This loss is, however, at first very small* 

 and only becomes remarkable on the approach of the last change. See NEWPORT i 

 TODD'S Cyclop, n. pp. 879, 880. 



2 That this fat is not necessary for the life of the larva, is shewn by the examples 

 of caterpillars in which ichneumons have deposited their eggs ; the larvse from these 

 consume the fat of the former, which die from the robbery of their stock of food only 

 at the time they should change into pupae, or have changed. 



