H. A. SCHNEIDERMAN 47 



mechanism of a hormone action, subjects which are considered by Dr. Szego 

 and Dr. Sawyer in this symposium. 



Two insects have proven especially useful in attacking these problems — the 

 giant Cecropia silkworm, Plalysamia cecropia, and the diminutive parasitic 

 chalcid wasp Mormoniella vilripennis. In C^ecropia, diapause is obligatory and 

 occurs in each generation immediately after pupation. Mormoniella, by con- 

 trast, has a facultative larval diapause which occurs at the end of the last larval 

 instar in occasional individuals. 



Turning to the first question, what turns off neurosecretion in the brain and 

 triggers the onset of diaf)ause? We find a wealth of specific details but few 

 generalizations. For example, in insects with facultative diapause, inactivation 

 of the brain may be triggered by temperature, absolute or changing photoperiod, 

 humidity, nutrition, etc. Thus in the parasitic wasp, Tritiieptis kliigii,- exposure 

 of larvae to low temperature causes the larvae to enter diapause (19), while in 

 the tomato moth, Dialaraxia oleracea, short photoperiods during the last larval 

 instar result in pupal diapause (45). Perhaps it is simplest to believe that an 

 insect like Cecropia, which enters diapause in every generation, possesses a 

 facultative diapause which is triggered by stimuli which are always present 

 in the environment. 



In some cases of diapause, notably where photoperiod is the stimulus, it is 

 clear that the stimuli reach the brain via sensory pathways and in some manner 

 influence neurosecretion (45, 46). In the obligatory diapause of Cecropia, how- 

 ever, there is evidence that nervous connections are unnecessary for inactivation 

 of the brain after pupation (54, 44). We should note in this connection the 

 important observation of Van der Kloot that in Cecropia the cessation of 

 neurosecretion is accompanied by the disappearance of electrical activity in 

 the entire brain, a finding which suggests that the factors that turn neuro- 

 secretion on and off are ''. . . the same (factors) which regulate the activity of 

 neurons" (43). Recognizing this fact, increasing attention is being given to the 

 role of the rest of the nervous system in triggering neurosecretion. 



It is not difiicult to comprehend how various triggering stimuli might inhibit 

 neurosecretory cells and cause diapause. But the true dimensions of this prob- 

 lem first reveal themselves when we recognize that in most insects the triggering 

 stimulus acts at an early stage in the life cycle while diapause is not manifest 

 until much later. For example, in Mormoniella exposure of female wasps to low 

 temperature during oogenesis causes diapause in their offspring at the close of 

 the last larval instar (29). There are numerous other examples in the literature 

 (3). In cases where diapause appears in the final larval instar after having been 

 triggered at an earlier stage it is clear that whatever the mechanism which 

 inactivates the neurosecretory cells in the last instar, it has not interfered with 



2 Kindly supplied by Dr. A. Wilkes and Dr. J. C. Martin, Entomology Laboratory, Belle- 

 ville, Ontario, Canada. 



