PHOTOPERIODISM IN INSECTS AND MITES 597 



periods. The resemblance of this curve to those relating photoperiod 

 and diapause needs no emphasis (cf. Fig. lA). 



The question of male determination in aphids is instructive for 

 various reasons. In the first place it provides an example of sex deter- 

 mination by external factors, including photoperiod. Secondly, this 

 system of control is unusual in that the operation of an alternative 

 stimulus (long or short photoperiods) controls the production of 

 three morphological types. It is well known that parthenogenesis in 

 aphids is of the diploid type so that both virginopara and ovipara may 

 be expected to preserve the full complement of chromosomes. Their 

 determination is therefore entirely "physiological." On the other hand, 

 the immediate cause of maleness is genetical, sex determination being 

 of the usual XX-XO type with the male egg losing one X chromosome 

 during the single nonreductional meiotic division. Since it is also 

 clear that in many aphids (e.g., A. pisiim) the production of males 

 is entirely suppressed in long days (Kenten, 1955), this chromosomal 

 change must be under physiological and ultimately environmental 

 control. The second problem is met in such species as A. pisum 

 and Megoura viciae where, under inductive conditions, males and 

 oviparae both arise from the same parent. Why is the sex of the off- 

 spring not uniform? Records show that it is rare for the male progeny 

 of one mother to exceed one-quarter of the total. Further, serial collec- 

 tions of the offspring indicate that the probability of a given egg 

 becoming a male embryo depends on its time of release from the 

 germarium, the chances being greatest near the middle of the sequence 

 in Megoura and at the end in AcyrthosipJwn. The eggs must there- 

 fore differ in their proneness toward chromosome loss, although this 

 is only expressed if the physiological conditions favor the production 

 of sexual forms. Whether these conditions are the same that induce 

 female embryos to become oviparae is at present uncertain. 



From this brief survey of arthropod photoperiodism it will at least 

 be apparent that a wealth of material is available for study by the 

 insect physiologist. Although some regularities in the pattern of 

 response are becoming evident, new variations are still being dis- 

 covered as the list of examined species grows. Moreover, with regard 

 to one of the central problems, namely the photoperiodic measure- 

 ment of time, little more than a beginning has been made at the 



