590 PHOTOPERIODISM IN INVERTEBRATES 



MODE OF ACTION OF CYCLES OF LIGHT AND DARKNESS 



There seems little doubt that the majority of arthropods respond to 

 the length of the light-dark cycle as such and not to gradual changes in 

 either component. For example, this statement is certainly applicable 

 to the moth Grapholitha (Dickson, 1949) and the mite Metatetrany- 

 chus (Lees, 1953a,b). The critical role of the dark period in the meas- 

 urement of time provides a further parallel with the photoperiodic 

 mechanism in plants. Several Lepidoptera including Acronycta (Dani- 

 lyevsky and Glinyanaya, 1949, 1950), A ntheraea (Tanaka, 1950) and 

 Grapholitha as well as Metatetranychus, are now known to have a long 

 night requirement for diapause induction. But besides an adequate 

 dark period Grapholitha also requires a light period of appropriate 

 length. Indeed the larva will enter diapause only with days of 7 to 15 

 hr duration and nights of 11 to 16 hr; all other cycles are inoperative. 

 Failure to initiate diapause in permanent darkness (a common feature 

 in arthropods) is a special case of this response. 



Some differences are apparent in the behavior of Metatetranychus. 

 Although long dark periods are always strongly inductive and long 

 light periods noninductive, the former are far more potent. Thus a 12- 

 hr dark period can completely annul an accompanying light period of 

 nearly 36 hr. It follows that the combinations of hght and dark capable 

 of inducing diapause are much wider than in Grapholitha. 



There are also obvious contrasts with the mechanism in plants. The 

 failure of light "breaks," even of several hours duration, to reduce the 

 effect of inductive dark periods is a noteworthy feature of the response 

 in arthropods. Moreover, the interruption of long light periods by 

 fairly protracted intervals of darkness is also inoperative. The impres- 

 sion conveyed is of the light and dark reactions increasing in rate with 

 time. Nevertheless, it may well be that their products must reach a 

 threshold before becoming active. A further point is that some reactant 

 in the system must be accumulated, since one or two inductive cycles 

 are usually without effect. Indeed, some twenty or thirty cycles are 

 required by the larvae of some Lepidoptera. 



In all cases yet examined photoperiodic induction has proved to be 

 independent of light intensity, provided the latter is adequate for 



