102 



KINETIC HORMONES — I 



LLH is therefore probably in the organ equivalent to the sinus 

 gland, since isopods lack eyestalks and the gland lies within the 

 head capsule, in the vicinity of the optic centres, where it is 

 associated with a blood sinus (Hanstrom, 1939). The source of 

 the darkening hormone is unknown, but must be, in part at least, 

 behind the head. 



Table 13. Illumination of different areas of the eyes of ligia 



Blue light is most effective in eliciting these responses in Ligia. The index values 

 are averages (from H. G. Smith, 1938). 



* illuminated from below. 



Insecta. The black pigment cells of the phantom midge larva 

 of Chaohorus ( = Corethra) differ from those of Carausiiis in being 

 mesodermal. They normally cover the surface of the two pairs of 

 air sacs and show a background response of the usual protective 

 type; but concentration of pigment on a white background is 

 brought about by an amoeboid change in shape of the cells which 

 become spherical instead of elongated. The cells also tend to 

 aggregate in small groups instead of being evenly spread out to 

 show the maximum amount of colour, as they do on a dark back- 

 ground. Extracts of the brain cause pigment dispersion, and 

 therefore darkening, as in Carausiiis (Dupont-Raabe, 1956). 



Vertebrata. Melanophore control varies in different fish; 

 among Teleostei it is only in some species that it is wholly under 

 hormone control or even partially under hormones and partially 

 under nerves. Amphibia, where the control is purely hormonal, 

 will be considered first. 



