No. 567] 



BIOLOGY OF THE THY SAN OPT ERA 



173 



always lives in concealed situations as due to a demand 

 for protection, we may assume that it is due to the strong 

 positive stereotropism of this species— aided in some 

 cases by positive geotropism, where the flower inhabited 

 is upright, but notwithstanding positive geotropism 

 where the flower is inverted. The rapid escape by crawl- 

 ing or flight when disturbed is not due to the fact that 

 this is the best way of avoiding danger, but to the posi- 

 tive reaction to light. Other species avoid danger by 

 going deeper into crevices, because they are negatively 

 responsive to light. 



Anaphothrips striatus lives on grasses doubtless be- 

 cause it can not live on any other food, or because the 

 reproductive processes are not stimulated by. any other 

 host plant. But their distribution and behavior on the 

 grasses may be explained largely in terms of their reac- 

 tions to the three agents tested in the experiments. The 

 males usually live in concealed situations on the plants 

 (curled-up leaves) because they are mostly negatively 

 phototropic, and crawl down the leaves until they reach 

 these concealed situations. Females may live either in 

 exposed or in concealed places, for some of them are 

 negative to light, others indifferent. The larvae are either 

 exposed or concealed, because they are indifferent to 

 light. The eggs from which they hatch are probably laid 

 by negatively phototropic females in the young curled 

 leaves, and the leaves unfold as the larvae develop; this 

 explains why the exposed larvae are much larger, on the 

 average, than are those concealed in the young leaves. 

 Perhaps the relation of cause and effect as here stated is 

 reversed, at least for some cases. Concealment— caused 

 in one way or another — may lead to negative phototro- 

 pism, as in the larva which was made temporarily nega- 

 tively phototropic by being kept in the dark. 



The adults are lodged between the ridges on the upper 

 side of the leaves of the grass Spartina, not for the sake 

 of protection, it seems to me, but because they are posi- 

 tively stereotropic. Doubtless between the ridges is the 



