GENERAL ZOOLOGY 

 Adult 



Pupa Embryo Fig. 15.24. Life cycle of a holometabolous insect. 



Larval 

 stages 



wastes, representing excreta which have accumulated during the period of 

 pupal life. The life cycle just described, consisting of embryonic, larval, 

 pupal, and adult stages and including complete metamorphosis, is of the 

 holometabolous type (Fig. 15.24). 



Consideration of the life cycles of insects raises significant questions con- 

 cerning the controlling influences regulating the far-reaching sequential 

 changes involved in molting and metamorphosis. We have already noted 

 (p. 435) that molting and other important systemic processes in the crayfish, 

 and in crustaceans generally, are under the control of hormonal substances, 

 produced by certain neurosecretory cells and transported by the circulating 

 blood. Analysis of the processes of molting and metamorphosis in insects has 

 shown that an analogous mechanism of control operates in these animals. In 

 general terms, a group of neurosecretory cells in the brain of the insect 

 secretes a hormone which acts specifically to activate a structure termed the 

 prothoracic gland, apparently homologous with the "Y-organ" of crustaceans 

 (p. 435). The prothoracic gland, in turn, produces a hormone which sets in 

 motion the complex processes of growth and differentiation which lead to 

 molting. A third endocrine organ, the corpus allatum, produces a conserva- 

 tive factor which has been termed the juvenile hormone, the general effect of 

 which is to prevent changes of form at molting. In the presence of this last 

 factor, a nymph molts to become a larger nymph, not an adult; and a larva 

 transforms into a larger larva, not a pupa. At certain times in the life cycle, 

 the corpus allatum ceases to produce its hormone, and in paurometabolous 

 insects the adult stage is attained; in holometabolous forms pupation occurs. 



A correlation between cyclical climatic changes and the timing of events 

 in the life cycle is evident in many insects. For example, in many moths 

 pupation occurs in late summer, and the pupa remains dormant until the 

 following spring, when metamorphosis takes place. In some species it can be 

 demonstrated that pupal dormancy continues indefinitely unless the insect is 

 exposed to low temperature for a sufficiently long period of time. Low tem- 

 perature, then, exerts some effect which sets in motion the initial steps in 

 the process of metamorphosis. It has been established experimentally that the 



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