MORPHOLOGICAL EFFECTS OF CROWD LNG 317 



to that on cool days and on warm sunny days. The young hoppers 

 spend the latter in groups basking in the sun, while at night or on 

 cool days they crawl under stones or other shelter, or climb some 

 plant. In either case they may be found in dense collections. Anal- 

 yses to date do not show the relative importance of heat and of light 

 in these reactions; but both, and particularly the former, are known 

 to be of decided importance. Movement stops at night when the 

 body temperature falls below the threshold for torpor, and begins in 

 the morning when it rises above that level. A further rise in tempera- 

 ture will send the hoppers into greater activity, which results in such 

 a scattering that the surface covered by a band in full daytime activ- 

 ity is three times that occupied by the same numbers at night. 

 Comparison with the observations of Boyer and Buchsbaum, which 

 have been summarized in chapter iv, in connection with slumber 

 aggregations of insects, suggests that the temperature threshold of 

 activity will be found to be lower on sunny than on dull days. On 

 cool days the hoppers have been observed to keep their nighttime 

 aggregation throughout the day. Uvarov (1928), from whose book 

 much of the present account is summarized, cites observations which 

 indicate that the temperature threshold for activity is higher in 

 older nymphs. 



With still further increase in temperature the bands start on their 

 irregular and apparently aimless wandering. Some become restless 

 and make small irregular jumps. These seem to initiate jumping 

 on the part of others which at first is aimless, but which at length 

 settles into a definite direction. Uvarov considers favorably the 

 suggestion by Loeb (191 8), based upon the work of Lyon (1904) 

 and others, that there is a tendency of an animal to move so as to 

 stop the movement of images of surrounding objects on the retina. 

 One hopper jumping thus starts others seeing it to jump in such a 

 way that there will be no movement across the retina. In a band 

 this is taken up continually and passes along as a sort of automatic 

 restimulation. Further, Grasse (1923) has shown that even non- 

 gregarious grasshoppers give greater activity when several indi- 

 viduals are experimented upon together than when one is taken 

 singly. 



If some such explanation holds, we have the common direction of 



