LIGHT AND METABOLISM 21 



environmental factors have but a slightly modifying effect upon it. A 

 good example of this is the crab, Uca, the responses of which have been 

 extensively studied i ; the diurnal rhythm of its colour change is 

 remarkably constant, and within wide limits is independent of influences 

 such as humidity and temperature, but the influence of metabolism on 

 the phenomenon is exemplified in its retardation with a lengthening of 

 the cycle on exposure to cold below 6° C (Figs. 9 and 10). 



This rhythmic mechanism operating to disperse pigment in the 

 day phase and concentrate it in the night phase of the cycle would 

 seem to be adaptive in function, partly protective against deleteriously 

 bright illumination, partly thermo-regulatory. In all species in which 

 these colour changes occur the controlling factors are hormones 

 differing in nature from the retinal pigment hormones but, like them, 

 elaborated in Invertebrates by the neuro -secretory system and in 

 Vertebrates by the neurohypophysis both of which show an endo- 

 genous rhythm. This question will be discussed in a subsequent 

 chajDter.^ 



The seasonal changes in colour of the coats of many Birds and 

 Mammals are analogous phenomena which are also to some extent 

 determined by photoperiod. It is well known that the majority of 

 common birds undergo a post-nuptial moult immediately after the 

 breeding season and a second pre-nuptial moult in spring when they 

 assume their wedding robes. The times at which birds assume their 

 nuptial and winter plumages are governed by a number of factors, the 

 most potent of which is a pituitary hormone with an inherent cyclic 

 activity depending in part on the length of the daily light periods 

 (Witschi, 1935 ; Bro\\Ti and Rollo, 1940 ; Lesher and Kendeigh, 1941 ; 

 Kobayashi and Okubo, 1955). A similar control operates the seasonal 

 moulting of many northern Birds and Mammals the colours of which 

 change from a sinnmer brown to a winter white. 



Among Birds, the ptarmigan of the northern tundra or the high mountains 

 (Host, 1942), and among Mammals the varying hare (Lyman, 1943) and the 

 ermine (Bissonnette and Bailey, 1944) are good examples of this ; these 

 phenomena of moulting and change of colour can be induced out of season by 

 artificially varying the diurnal periods of ilkimination. It is interesting that in 

 the hare the eyes seem to be the normal receptors of this stimulus since if these 

 are masked the changes do not occur. The pituitary seems to be the only 

 endocrine gland involved since castration and thyroidectomy in the hare are 

 without effect (Lyman, 1943), while hypophysectomy abolishes the cyclic 

 moulting of ferrets (Bissonnette, 1935-38). 



PHOTOPERiODiciTY IN BiOLUMiNESCENCE. The ability to produce light 

 occurs widely but sporadically among bacteria, fungi, and most types of animals 



1 Abramowitz (1937-38), Browii and Webb (1947-49), Brown and Sandeen (1948), 

 Webb (1950). 



2 p. 547. 



