732 



Comparative Animal Physiology 



velopment of pigments of quite different chemical composition, it would ap- 

 pear doubtful that the substances actually become a portion of the chromo- 

 phore molecule. 



We see from the preceding discussion that in the production and opera- 

 tion of gene hormones we have a most favorable opportunity to investigate 

 in some detail instances of the production and action of hormones in terms of 

 the metabolic transformations proceeding within cells. 



Molt, Pupation, and Metamorphosis in Insects. The post-embryonic de- 

 velopment of insects comprises an orderly series of stages in the course of 

 which the insect becomes transformed from a larva to an adult or imago 

 (Fig. 279). The process involves growth by means of a series of molts, and 

 a metamorphosis in which the larval characters are lost and imaginal ones 



HEMIMETABOLOUS DEVELOPMENT — " 

 NYMPHAL STAGES , OR 



Molf / IN8TAH \ 



Ej. ORTHOPrEHA 



WalKm^ sticks 

 HEMIPTKH* 



Bed buj^ 

 OS ON ATA 



LAHVAL STAGES 

 f-lOLOMETABOLOUS DEVELOPMENT 



Fig. 279. Diagram representing the principal stages and processes involved in the 

 development of hemimetabolous and of holometabolous in.sects. 



differentiated. In the hemimetabolous insects, such as the orthopterans (grass- 

 hoppers, cockroaches, walking sticks, etc.), the hemipterans (bedbugs, Rhod- 

 nixis, etc.), and Odonata, the transition from larval to adult condition is typ- 

 ically a gradual one, in which the developing young, typically known as 

 nymphs, in succeeding molts come graduallv to resemble more and more 

 nearly the adult. In such hemimetabolous insects, however, the last nymphal 

 molt is usually associated with by far the most striking transformation and 

 hence is commonly -considered the metamorphic moit. In holometabolous in- 

 sects, such as the Lepidoptera (moths, biflterflies, etc.), Diptera (Droso- 

 phila, blowflies, etc.), and Coleoptera, ^'ery little change of form in the di- 

 rection of the adult occurs during the larval series of molts, these being 

 primarily growth changes, but the last larval molt is associated with the 

 formation of a pupa. During the pupal stage the larval organism becomes 

 completely transformed into an adult, a spectacular metamorphosis. There is 

 no fundamental difference between the two foregoing types of development, 

 the observed difl^erences being primarily the result of differences in the 

 times at which the processes of imaginal differentiation occur. These latter 

 are extended to a considerable extent over the whole larval period in the 



