NUTRITION AND METABOLISM 81 



by illumination, may persist for several weeks in complete darkness. 



In these same insects, if the factors or stimuli already enumerated 

 continue to act over long periods, the amounts of the different pig- 

 ments formed may be permanently changed ; and this may be effected, 

 also, by nutrition, particularly by the moisture of the food. This 

 permanent effect is termed 'morphological colour change' ; but when 

 brought about by optical stimuli, at any rate, it appears again to be 

 produced, like the temporary change, by way of the nerve centre and 

 the internal secretion. The co-ordination of these mechanisms may 

 lead to remarkable resemblances to their background in various 

 Orthoptera. 



As was shown by Poulton many years ago, larvae and pupae of 

 certain Lepidoptera also possess wonderful powers of acquiring the 

 tone or coloration of their background. This phenomenon has been 

 analysed most carefully in the case of Pieris pupae. In these the 

 quantity of pigment deposited in the cuticle appears to be determined 

 by the specific quality of the light received into the eyes of the pupa- 

 ting larva, acting through a centre situated in the head. 



Another very striking colour change is that in locusts, the solitary 

 phases of which are quite differently pigmented from the gregarious 

 phases. The gregarious type of coloration can be evoked by crowding 

 the young forms together; and here again the immediate cause of the 

 colour change is probably the hormone secretions, perhaps in the 

 corpus allatum or corpus cardiacum, controlled by the brain. 



Light production 



A certain number of insects are luminous; they liberate energy in the 

 form of light during their metabolism. This property probably arose 

 as an accidental accompaniment of the metabolism that occurs in 

 the fat body. In the most primitive luminous insects (Collembola) 

 the fat body throughout the insect is luminous; and in the most 

 highly evolved fire-flies (Coleoptera) the light-producing organ con- 

 sists of specialized fat-body cells lying beneath a transparent window 

 in the cuticle with other specialized cells loaded with uric acid 

 granules as a reflector beneath. But in the luminous Mycetophilid 

 larva Bolitophila it is the Malpighian tubes which form the luminous 

 organ. 



