76 



KINETIC HORMONES — I 



this diurnal rhythm for a time under constant conditions. They 

 show a Umited background response, becoming darker on an 

 illuminated dark background than they are on a white or yellow 

 background. In constant light, the melanin disperses in response 

 to moisture, under the control of what is probably the same 

 darkening hormone, since its secretion is also stimulated through 

 the nervous system (Giersberg, 1928). This was established by 

 putting the insect into a humid box, with its head projecting 

 through a diaphragm into the dry air outside (Fig. 3-11); this 

 induces darkening of the whole animal from head to tail in about 

 half an hour. If a ligature is put round the body to prevent the 

 circulation of the blood and hormone to the tail end, the darkening 

 only affects the part in front of the ligature. If the ventral nerve 

 cord is cut at a level just outside the humid box, no darkening takes 



Fig. 3-11. The stick-insect, Carausius, with the hinder part of the 

 body in a moist chamber and the head and thorax projecting 

 through a membrane. The pigment dispersion, caused by the 

 moisture, is transmitted by a darkening hormone from the 

 suboesophageal ganglion. This acts only upon the head and pro- 

 thorax because of the ligature just behind them (from Giersberg 



1928). 



place, because the stimulus from the damp skin is not conducted 

 to the brain. But if the same animal, with the nerve cord cut but 

 the body unligatured, is then reversed, with its head in the box 

 and the tail left out, the whole body darkens, because the stimulus 

 from the skin of the head can reach the brain, which therefore 

 stimulates the secretion of the hormone. This then circulates freely 

 to all parts of the body. Secretion of the darkening hormone has 

 been located histologically in the suboesophageal ganglia and 

 confirmed experimentally by injection of extracts into animals from 

 which the brain had been removed. Headless animals, lacking the 

 source of the darkening hormone, have their pigment fully con- 

 centrated, as in the normal light-adapted animals ; they therefore 



