106 HELIOTROPIC MECHANISM 



In the light, nomial Proctacanthus, and flies in general, hold the 

 body well from the surface when walking and leave only the tracks 

 of the feet on smoked paper, but after blackening the eyes, or in the 

 dark, some part of the body also leaves a trail. The whole experi- 

 mental picture of inactivity, muscular weakness, and incoordination, 

 when the eyes are darkened, points to a decreased neuromuscular 

 tonus which is normally maintained reflexly by the action of light on 

 the eyes.-" 



Effects of Blackening One Eye. — The fact that positively hehotropic 

 flies, with one eye removed, move in circles toward the good eye was 

 discovered by Loeb,- and Parker-^ found that when one eye of Vanessa 

 antiopa was blackened, the animal circled in the same way both when 

 flying and creeping and he pointed out the relation of this phenomenon 

 to the theory of heliotropism. Holmes showed the same behavior of 

 other forms, such as Hyalella, Orchestia, and Talorchestia, and also 

 that the postures of the legs of the two sides of Ranatra were difTer- 

 ent under these conditions.^"' ^^ Both the circus motions and the 

 postural changes due to unequal illumination of the twQ eyes, or pro- 

 duced by blackening one eye, are in reality shown, in some degree, by 

 practically all hehotropic insects. 



If the right eye of Proctacanthus ij blackened, the insect circles to 

 the left both when walking and flying. The reason for the abnormal 

 progressive movements is evident if one examines the resting posture 

 of the robber fly, which is more striking in this insect than in any 

 other examined (Fig. 2). On the left side, that of the uncovered eye, 

 the legs are all in a state of flexion, closer together than normally, and 

 drawn well under the body, while on the other side all the legs are 

 extended and spread out like the ribs of an open fan. The anterior 

 leg of the normal side is adducted to the right, i.e. to the side of the 

 blackened eye, and may even cross the corresponding leg of that side. 

 The condition is a sustained, tonic one, by virtue of which the only 



^^ We offer the suggestion that the restlessness of these insects in the light, and 

 their quietness in the dark, may be a true photokinesis, that in reality both photo- 

 kinesis and heliotropism have an identical physiological basis in the effects of light 

 which reflexly increases the neuromuscular tonus and excitability, and that it is 

 only the orienting effect of light which prevents the hehotropic forms from exhib- 

 iting photokinesis and thus coming to rest in the dark. 



21 Parker, G. H., Mark Anniversary Volume, 1903, 455. 



