404 The American Naturalist. [May, 



The young however offer unexplained phenomena, in not reacting to 

 light but having definite positions upon the stem and under sides of 

 the leaves of plants that do not appear due to an}- of the above causes. 



Ants however are not heliotropic in the winged state until the 

 period of sexual excitement, when they swarm out for the nuptial 

 flight. Such ants gave the same results in the laboratory experiments 

 as did the caterpillars under the same circumstances. 



The two sexes differ, however, in that the males continue to go 

 towards the light when its intensity has become too slight to affect the 

 female. The others seem not to be heliotropic at all, from the author's 



The ninth chapter is devoted to the phenomena exhibited by flies. 

 The larvse of Musca vomitoria were experimented upon, either in test 

 tubes or directly upon the table, and yielded the same result regarding 

 the directive influence of light, white and colored, with the important 

 exception that the animals move away from and along the direction 

 of the light, being in fact negatively heliotropic. 



That this is true of the youngest larva) at time of hatching is 

 demonstrated by allowing eggs to hatch out upon plates blackened 

 with soot: the young then leave traces of their lines of progress MM 

 these are away from the light. If two windows at right angles supply 

 light, the direction of march is in the diagonal between the two lintf 

 of stimulation. When, however, the eggs hatch in darkness, the young 

 crawl in all directions. Certain complications, however, arise in the 

 course of young larvae as they seem to have a peculiar tendency to 

 arrange themselves with the ventral side turned toward the hght, 

 provided this is strong sunlight. Heat appears to exercise no definite 

 directive influence, though the presence of food causes them to move 

 towards it and there is also a strong contact stimulus evinced in t e 

 tendency the larvse exhibit to crawl under foreign bod 

 thus under plates of transparent glass where negative heliotropism 

 cannot be cited as the cause of the movement. . 



Though the larva? are thus negatively heliotropic the adult fly * 

 positively heliotropic as can be seen by repeating the experimen 

 applied to the plant lice. Here however the problem is often made 

 more complex by the numerous other stimuli which may act upon 

 the fly and more or less nullify the results of heliotropism. 



Negative heliotropism is shown also in the larvse ol 

 Tenebrio molitor and Melolnulha . vuh/urix. combined in the lira 

 with the peculiar contact sensitiveness, stereotropisni, that leads 

 larvae to collect in the concave angles of a dark box. 



