§ 3.221 EFFECTORS WITH MOVABLE PIGMENT GRANULES 75 



black melanin, which has been more fully observed, moves almost 

 at right angles to it, to cause darkening in appearance (Fig. 3-10). 

 The upward, or dispersing, movement of the melanin depends on 

 both moisture and light, the effects of which are transmitted by 

 the nervous system to the brain, and thence, either by nervous 

 stimulation or by neurosecretion through the circumoesophageal 

 connectives, to the suboesophageal ganglion (§2.111). This 

 releases a Car^w^w^-DARKENING hormone which passes in the 

 blood to disperse the melanin. The ganglion cells only release the 

 hormone if the connectives from the brain are intact (Dupont- 

 Raabe, 1956). 



If moisture is maintained constant, the melanin granules con- 

 centrate in the light and disperse in the dark, and tend to maintain 



mm 



?v m^^KA^sh* 



^W^^^'^^^^ 





Fig. 3-10. Two diagrammatic sections through the skin of the 

 stick-insect, Carausius, to show the movement of pigment granules 

 in epidermal cells. The clear space above indicates the position of 

 the cuticle. The pigment in the upper section is concentrated m 

 the light-adapted position, and in the lower dispersed, in the dark- 

 adapted position. The green pigment (1) remains stationary; the 

 red pigment (2) disperses laterally above the nuclei; the dark, 

 melanin-like pigment (3) disperses mainly outwards (from Giers- 



berg, 1928). 



