482 RADIATION BIOLOGY 



cited other work indicating that the stickleback is photoperiodically 

 responsive, as is also reported for the brook trout. 



Responses other than those concerned with sexual reproduction occur 

 in animals as they do in plants. Typical of these is the production of 

 winged forms of certain aphids, shown by Marcovitch (1924) to be regu- 

 lated by the relative length of the night and the day. Different species 

 of aphids present many variations, but in general they pass the winter in 

 northern latitudes in the egg stage. The eggs hatch in the spring, and a 

 wingless stem mother that reproduces parthenogenetically develops. In 

 some species a winged migrant form appears during the late summer, and 

 upon approach of autumn the winged forms produce males and females. 

 Marcovitch observed that sexual forms of Aphis forbesi, A. rumicis, 

 A. sorbi, and Capitophorus hippophaes appear only on short days. He 

 found migrant forms to be either long-day-dependent, as C. hippophaes, 

 or short-day-dependent, as the apple aphis, A. sorbi. ShuU (1927) found 

 that many more winged individuals of the potato aphid, Macrosiphum 

 solanifolii, formed if they were reared in 24-hr cycles of 8 hr of light and 

 16 hr of dark than if they were grown in continuous light. Shull and 

 Marcovitch both concluded that the action of light is a direct one on the 

 aphid rather than an indirect one received through the host plant. Shull 

 also found that photoperiod treatments are effective in control of wing 

 production only when applied some hours before the aphid is born. 



Diapause, a physiological state of arrested development which occurs 

 at some stage in the life cycle of many insects, is photoperiodically con- 

 trolled in some instances. Dickson (1949) found that the oriental fruit 

 moth, Grapholitha molesta, did not enter diapause if held in continuous 

 darkness or continuous hght. When the larvae were subjected to 3-13 hr 

 of Hght per day, the number entering diapause increased from about 

 10 per cent at 3 hr to nearly 100 per cent on the longer light period. 

 With 14 hr of light per day, however, the percentage dropped abruptly 

 to 4, and with further increase in day length it became practically zero, 

 the response being similar to that of intermediate plants. Dickson's 

 results indicate that the length of the dark period is a critical factor 

 in determining whether the organism enters diapause. The response 

 is complicated by interaction with temperature and possibly other 

 factors. 



Other photoperiodic responses of animals include the seasonal changes 

 in plumage of birds and in hair types of certain mammals. Lyman (1943) 

 made an intensive study of coat changes in the varying hare, Lepus 

 americaniis. This animal undergoes three molts, one in the spring and 

 two in the fall. In the spring the white winter coat is replaced by a 

 brown one. In late summer this brown coat is replaced by another 

 brown one of a different type, and soon thereafter the second brown one 

 is followed by the white winter coat. Each of these molts is photo- 



