114 HARRY BEAL TORREY AND GRACE P. HAYS 



A female with a full brood pouch was placed in a Petri disk, 

 round and round which she proceeded to move in the light of 

 a 25 w. tungsten bulb. Many times when she was facing the 

 light, the latter was turned off. Invariably she came at once 

 to a dead stop. Only occasionally when the light was turned 

 off while she was facing away from it would she react similarly; 

 being obviously less responsive in such cases. Sudden increases 

 of intensity, (i.e., when the light was turned on) always pro- 

 duced definite inhibition of locomotion. 



One of the brood of this female responded but rarely to sud- 

 den increases of intensity when facing the light, not at all to 

 sudden decreases and never while going away from the light. 



The fact that young are more readily directed in locomotion 

 by light while they appear to be at least no more sensitive than 

 adults to sudden changes in intensity of light, suggests the 

 possibility of two mechanisms governing the two types of reac- 

 tion. There is a wide variation in the responsiveness of adults 

 to sudden changes in intensity, however. The problem pre- 

 sented here will be investigated further. 



5 



Though the eyes of Porcellio are sensitive to light, their power 

 of forming images is approximately very small. Totally blind 

 individuals avoid obstacles with the ease of normal individuals. 

 When the second antennae of either are removed, however, 

 they often bump squarely into obstructions, avoiding them only 

 after contact through legs or body. The importance of the 

 second antennae is emphasized by their constant activity dur- 

 ing locomotion, when, by a rapid succession of tappings on the 

 substratum, and wavings in the air, they explore the region 

 immediately to the front. The usual random movements that 

 are made by the anterior end of the earthworm and blow fly 

 larvae are in Porcellio restricted to these mobile antennae. 

 Since the head segment of Porcellio does not move perceptibly 

 from side to side, it is only necessary to amputate the second 

 antennae to eliminate what correspond to the usual random or 

 trial movements in earthworm and blow fly larvae. 



Such an operation was made in several cases. It was soon 

 found, however, that, with or without the second antennae, 

 Porcellio responded to photic stimulation under the conditions 



