328 ANIMAL AGGREGATIONS 



color between migratoid and danicoid phases. They attained the 

 fifth nymphal stage by June 1 1 , when they had become quite green 

 with the exception of 6 larvae, which were dark gray, almost black. 

 In the original cage all the control larvae retained their typical mi- 

 gratoid coloration. The same experiment was repeated with dani- 

 coid nymphs, and only green forms appeared in the ground cage. 



Plotnikov performed 15 experiments with larvae of the second 

 brood of L. danica reared under conditions of crowding. When the 

 larvae were kept in small cages or in glass jars, with 30 — 50 nymphs 

 to 450-675 cc. space, a typical migratoid coloration was invariably 

 obtained, but no dark specimens were to be found. When the 

 nymphs were kept singly in the glass jars, they began turning green 

 as early as the second stage. They were quite green by the fourth 

 or fifth instar. When groups of 4 were put into 100 cc. of space in 

 glass jars the fifth-instar nymphs were a mixture of migratoid and 

 green, with some transitional forms. 



In these experiments, in which the developing nymphs retain 

 migratoid coloration only when they are crowded, Plotnikov reports 

 that the nymphs having the migratoid color also lack the keeled 

 pronotum typical of the L. danica nymphs, so that their structure 

 as well as their color is affected by crowding. 



In another series of experiments with non-migratoid nymphs, 86 

 second and third instar nymphs were placed in a 2,000 cc. cage for 

 12 days, at the end of which time they were in the fourth and fifth 

 instars. Forty-five of these had definite migratoid coloration, while 

 9 more showed tendencies in that direction. Again, when 44 non- 

 migratoid nymphs were put into a 1,000 cc. cage, a few typical mi- 

 gratoid animals were obtained. One regrets that there is no record 

 of controls for these experiments reared under conditions such that 

 crowding would be impossible. 



Plotnikov considered the question of factors causing the migratoid 

 color to appear in crowded cages. He eliminated cannibalism as a 

 causal factor by feeding the isolated migratoid nymphs with killed 

 hoppers. Under these conditions the isolated animals lost their typi- 

 cal migratoid coloration. He considers, too, that he has eliminated 

 the humidity factor, since broods in highly humid jars gave the same 



