236 LECTURE XVIII. 



The last change in the working of the procreative machinery of the 

 Aphides is essential to the preservation of the race ; the larvae of 

 these little delicate insects would be all destroyed by the winter 

 frosts ; but the cold is effectually resisted by the latent vital forces of 

 the ova, which are defended by a compact case of mucus, and are 

 instinctively glued by the parent to the sheltered nook or crevice 

 of the plant, of the inherent temperature of which they have the 

 benefit. 



Other Hemiptera and all the Orthoptera produce eggs in which 

 the development of the embryo proceeds in the order already de- 

 scribed until it attains as advanced a state as that of the viviparous 

 larva of the Aphis. The subsequent changes of these insects consist 

 in the growth of all the parts, which takes place chiefly during the 

 period of the moult and the gradual acquisition of the wings, which 

 is not attended with any loss of activity or diminution of voracity. 



The successive states of an apodal worm, of a worm with feet, and of 

 one with feet and w4ngs, being accompanied likewise with the ac- 

 quisition and perfection of the antennal and visual organs of sense, 

 and of the internal and external organs of generation, and often with 

 great changes in the digestive, muscular, and nervous systems, in the 

 development of one and the same insect have been emphatically 

 termed metamorplioses. Entomologists have defined various kinds of 

 metamorphoses under special heads, as the coarctate, obtected, in- 

 complete, semi-complete, and complete metamorphosis. 



The progress of the insect through these several stages being in 

 many species interrupted, and active life enjoyed for a longer or 

 shorter period under one or other of the immature forms, these have 

 been sooner and more prominently brought under the notice of the 

 naturalist, than if they had had to be sought for, as in the bird or 

 mammal in the early periods of the development of the minute embryo. 

 They have consequently had assigned to them a character of singu- 

 larity and exception which they do not intrinsically deserve. The 

 different stages of development have been likewise, for the most part, 

 studied only in the instances in which they are manifested by insects 

 after exclusion from the egg, and thus their minor modifications and 

 differences have attracted more attention than their essential resem- 

 blances and relations to one and the same type and course of de- 

 velopment. 



As soon as the young insect breaks through the egg-shell it is 

 called a Larva, whatever grade of development it may have attained 

 in ovo. During the period when it acquires the wings it is called a 

 Pupa. 



From the importance which has been assigned in some entomo- 



