§ 3.222 EFFECTORS WITH MOVABLE PIGMENT GRANULES 77 



form good test subjects for extracts containing the darkening 

 hormone, and show that extracts from other parts of the brain are 

 also active, but those from the corpora allata and cardiaca are 

 inactive (Dupont-Raabe, 1954). 



A Hghtening hormone has not yet been located, partly because 

 of the lack of a suitable preparation for testing extracts. It is not 

 clear why damp-adapted insects with dispersed pigment are 

 not used. One source, at least, of the concentrating hormone 

 must be situated in the body, since it is present in headless animals 

 from which the corpora cardiaca are almost certainly absent 

 (Dupont-Raabe, 1956). Otherwise, it might perhaps have been 

 supposed that the corpora cardiaca would yield a Carausius- 

 lightening hormone, since they yield a neurosecretory substance 

 (Cameron, 1953) which concentrates the melanophores of Crago 

 (Knowles, Carlisle and Dupont-Raabe, 1955, § 3.223), and some, 

 at least, of the erythrophores of Leander. It seems significant that 

 the concentration of both these crustacean chromatophores are 

 light-adaptations, and it is a light-adaptation hormone that appears 

 to be missing from Carausius. 



3.222 Pigment movement in retinal cells 



Crustacea. The compound eyes of most Crustacea have three 

 groups of pigment-containing cells round each ommatidium; 

 distal and proximal retinal cells that surround the cone (Fig. 3-12), 

 and reflecting cells that extend below the basement membrane but 

 are not shown in the figure. These last contain white pigment, 

 which behaves like the pigment in white chromatophores (§ 3.223) 

 and is probably under similar hormone control. The proximal 

 retinal cells contain dark pigment granules, which may be fixed 

 in appositional eyes, such as those of Eupagurus, but which in 

 many species can be dispersed outwards to isolate the sensitive 

 rhabdomes and convert superpositional eyes into a temporarily 

 appositional type for accurate vision in bright light (Bruin and 

 Crisp, 1957). The movement of the granules in these cells is 

 sometimes a direct effect of light, as in some chromatophores 

 (§ 3.223); but in others it is controlled by hormones, that appear 

 to be the same as those controlling the distal cells (Knowles and 

 Carlisle, 1956). 



