320 ANIMAL AGGREGATIONS 



grasses; but if food is scarce, they will eat almost anything. They 

 readily become cannibalistic, eating injured members of the swarm. 

 Mating does not begin until a day or two after the insects become 

 winged; it continues at intervals thereafter until death. Nymphal 

 aggregations are evidently not due to sex attraction; nor is the 

 aggregation of the newly emerged winged adults, either with each 

 other or with the nymphs which have not yet molted. Mating takes 

 place immediately following egg-laying during the daytime; hence 

 overnight aggregations, even of the mature adults, are not primarily 

 due to mating reactions. 



Uvarov cites cases which demonstrate that the migration of the 

 adult swarms is not related to flood supply, since they will leave 

 dense stands of vegetation upon which they normally feed and mi- 

 grate out into arid regions. Neither is it due to a search for suitable 

 nesting sites, for they will leave the regular nesting grounds and 

 deposit their eggs wherever the physiological urge becomes sufh- 

 ciently strong, regardless of the fact that the place may be entirely 

 unsuitable for the development of the eggs. Further, he does not 

 believe that the migration is a negative reaction to high parasi- 

 tization, since the heavily parasitized individuals do not migrate, 

 and since the others carry along with them their destructive red- 

 mite and fly-larva parasites. Swarms have been known to stop 

 and deposit their eggs on a barren hillside, where their eggs will 

 develop poorly, if at all, and within sight of dense growths of one 

 of their principal food plants growing in the type of habitat where 

 their eggs would develop well. Uvarov believes that the emigra- 

 tion flight is both induced and regulated mainly by internal physio- 

 logical factors. 



The non-gregarious grasshoppers are solitary, not in the sense 

 that there is but one or, at most, a few in a considerable area, but 

 in the sense that for some unknown reason these Acrididae lack the 

 tendencies which lead to mass movements. Their individual re- 

 actions to different environmental stimuli seem approximately like 

 those of the individuals from the gregarious locusts, with the excep- 

 tion that their reactions are not so closely dependent upon those 



