LIGHT 



123 



nette and Wilson, 1939), and of two 

 weasels" can also be controlled by manip- 

 ulating the daily light-dark ratio. For 

 weasels, prime winter pelts can be produced 

 in summer, despite high temperature, by 

 reducing the length or the intensity of the 

 lighted period. Thermo-induction is at most 

 a minor factor in this reaction. With birds, 

 it was shown before the modern period that 

 moulting of the scarlet tanager and bobo- 

 link could be controlled in part by reducing 

 illumination and that, even in mid\\'inter, 

 the males could be made to break out in 

 their spring songs if gradually brought into 

 the light for a week or two and meal worms 

 were added to the diet (Beebe, 1908). 

 More recently. Host (1942) has controlled 

 plumage color in the willow ptarmigan 

 (Lagopus lagopus) by changes in the pho- 

 toperiod, irrespective of temperature. 



The length of day and night is approxi- 

 mately equal at the equator throughout the 

 year. At 6 degrees from the equator annual 

 differences may amount to forty minutes, 

 and at 10 degrees the differences may be 

 as much as seventy minutes. Small weaver 

 finches, Etiplectes {=?ijromelana) francis- 

 cana among them, were transferred to north 

 latitude 42 degrees from near Senegambi in 

 tropical Africa. In the experimental labora- 

 tory, the birds showed an annual rhvthm in 

 color pattern that could be controlled by 

 varying the length of the lighted period. 

 Fifteen or sixteen hours of light daily were 

 more effective than fourteen hours or less. 

 E. f. pusilla is in plainer plumage in south- 

 em Ethiopia from December to February; 

 in the Sudan, the same species (typical 

 franciscana) is in nuptial plumage from 

 August to January. The two populations are 

 geographically distinct. They are physiologi- 

 cally isolated by differences in their respec- 

 tive breeding seasons, yet the scanty evi- 

 dence available suggests that each might 

 assume the breeding habits of the other if 

 transplanted to its habitat. The ability to 

 respond to difference in length of days is 

 apparently inherent in these tropical birds 

 (Brown and Rollo, 1940; Friedmann, 

 1937). 



Wings and Sexual Reproduction in Aphids 

 The marked and regular autumnal short- 

 ening of length of day is apparently a major 



" Mustela frenata noveborecensis and M. 

 cicognanii cicognanii (Bissonnette and Bailey, 

 1944). 



factor in the production of bisexuality in 

 the root louse. Aphis jorhesi, of the straw- 

 berry. The change from parthenogenetic 

 forms normally occurs in November, but 

 was brought about in May by subjecting 

 aphids to only 7.5 or eight hours of day- 

 light during twenty four hours. The aphids 

 were kept in a ventilated dark chamber out 

 of doors. Temperature was not a factor, 

 although wing formation for some aphids 

 is suppressed by high temperatures. Short- 

 ening days may also produce alate forms 

 reminding one of short-day plants. In the 

 rosy apple aphid. Aphis sorbi, in which 

 alate migrants appear in any generation 

 after the third one in spring, they can be 

 produced by experimentally lengthening the 

 daily exposure to light; such changes sug- 

 gest the reactions shown by long-day plants 

 (Marcovitch, 1923; Shull, 1942) . Other en- 

 vironmental controls of wing production in 

 aphids are known; one of these, the effect 

 of crowding, is discussed on page 347. 



Genera! Considerations 



It must be emphasized that many other 

 factors affect seasonal periodicities of ani- 

 mals. Stickleback fish respond to changes in 

 temperature, but not to light. Vitamins, 

 proteins, fats, and salts as elements of diet 

 are frequently important. With field mice, 

 light, food, temperature, rainfall, and local- 

 ity all affect the seasonal cycles, and tem- 

 perature, as well as lighting, is impor- 

 tant for hedgehogs (Baker and Ransom, 

 1933; Allanson and Deansley, 1934). While 

 our knowledge of the ecology of seasonal 

 activities of animals is far from complete, 

 we now know that photoperiodicity is a 

 factor of major importance. Many of the 

 cycles formerly thought to be under the 

 control of temperature, or of an imanalyzed 

 internal rhvthm, have since been shown to 

 be primarily controlled by the length of 

 day. In evaluating these advances in knowl- 

 edge, we need to remember that the 

 changes produced bv experimental manip- 

 ulation modifv times of expression of in- 

 herent potentialities rather than "hange 

 the potentialities themselves. 



The evidence indicates that the eyes are 

 the chief receptors for the stimulation that 

 produces photoperiodicity among animals. 

 As with plants, low intensities are effective; 

 an increase of as little as 1.7 foot candles 

 can produce an increase in the activity and 

 size of the testes in the starling, Sturnus vul- 



