Onset and Termination of Insect Diapause' 



HOWARD A. SCHNEIDERMAN 



Deparlmeni of Zoology, Cornell University, Ithaca, Ne"u' York 



IN MANY INSECTS at a Specific stage in their life history, development suddenly 

 ceases and the insect enters a state of dormancy, called diapause, which 

 enables it to over-winter or survive unfavorable conditions. The particular 

 stage at which diapause occurs is characteristic for a species but may vary even 

 in closely related forms. Whenever it interv^enes during embryogenesis, larval 

 or pupal life, it results in developmental arrest, and growth and molting cease. 

 When it occurs in adult life, activity and the maturing of gametes are brought 

 to a standstill. The end of diapause is signaled by the onset of development. 

 Thus the diapausing insect possesses a mechanism for turning off and on the 

 normal processes of growth and presents to the experimenter an exquisite 

 example of natural growth regulation. Teleologically, the adaptive value of 

 diapause is manifest; physiologically, its mechanism is still in doubt. 



Although the end result of diapause in all cases is the cessation of growth, 

 the physiological events that bring about embryonic diapause are very likely 

 different from those acting in post-embryonic life. The notion of a unitary 

 theory to explain all diapause appears wishful (but cf. 4, 18). In the present 

 discussion we shall consider primarily post-embryonic diapause, more particu- 

 larly the larval and pupal diapause of Hymenoptera and Lepidoptera. These 

 forms are especially amenable to physiological and biochemical analysis and 

 provide insight into general mechanisms underlying post-embryonic diapause. 

 In these insects diapause is controlled by neurosecretory cells in the brain 

 which release a tropic factor stimulating the prothoracic glands to secrete a 

 growth hormone, PGH (48, 9). Normal nervous connections are unnecessary 

 for neurosecretory activity, since implantation of isolated brain fragments 

 containing neurosecretory cells is effective (50). When neurosecretion ceases, 

 growth soon stops and diapause supervenes. 



Three principal problems are outstanding in the study of diapause: /) what 

 shuts off the neurosecretory activity of the insect's brain? 2) what turns it on 

 again? 3) How does PGH react with the tissues to terminate diapause and 

 initiate growth and molting? These questions bear directly on the more general 

 questions of triggering of endocrine activity and of mitosis and the biochemical 



iThis study was aided by Grant H-1SS7 from the National Heart Institute, U. S. Public 

 Health Service, and the Sage and Sackctt Funds of Cornell l^niversily. It is dedicated to 

 Professor Albert Kiihn of Tubingen in honor of his seventielii birthday. 



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