RETINAL IMAGE TO ANIMAL REACTIONS. 113 



complex honey bee as studied by AUnnich (1919). And Garry 

 (1918) has recently demonstrated in a most striking way the tropic 

 nature of the pose and locomotion in certain flies. Thus the adult 

 insect, though subject to the most diverse movements in an illumi- 

 nated field, has underlying its whole system of response a basis of 

 simple phototropism. This relation is nowhere better illustrated than 

 in the blowflies. The maggots of these flies are strongly phototropic 

 in a negative sense and exhibit those balanced reactions to opposing 

 lights that are characteristic of the purest form of phototropism. 

 They possess eyes, but these eyes are little more than direction eyes. 

 When they emerge as adults, they have well-developed compound 

 eyes. Under laboratory tests they are said to be positively photo- 

 tropic, but in the field they exhibit such a variety and complication 

 of photic response as to recall the state of the mourning-cloak butter- 

 fly. Bees are without doubt positively phototropic, but their daily 

 life in the illuminated field in which they live is as complex in many 

 respects as that of a human being. As von Frisch (1914) has re- 

 cently shown, they can be taught to associate color with food supply, 

 and it is impossible to explain their homing instincts without assuming 

 memories, visual and otherwise, of an order fairly comparable with 

 those found in the vertebrates. Thus many insects, though funda- 

 mentally tropic in their underlying nervous organization, have built 

 upon this organization an immense superstructure of reaction types 

 mostly of an instinctive kind that obscures and hides the original 

 simple tropic scheme. This overgrowth in phototropism is dependent 

 upon, first, the development of an eidoscopic eye whose image is rich 

 in detail, and, secondly, upon the development of central nervous 

 organs capable of caring for such detail. In this respect the insects 

 offer remarkable transition forms between the purely phototropic 

 simpler organisms and those in which phototropism seems to have 

 vanished completely. 



It is a fair question to ask whether vertebrates exhibit any tropic 

 responses whatever. Most students of this subject would answer 

 this question, I imagine, in the negative. Yet it is very difficult to 

 explain, for instance, the feeding habits of the dogfish without assum- 

 ing a tropic basis. When hungry dogfishes are liberated in a pool 



