DENSITY OF POPULATION AND INSECT SURVIVAL 245 



egg production in fowls, Pearl and his associates regarded the under- 

 lying stimulus to lie in the psychological field, and it is possible to 

 regard the effects recorded here as expressions of some sort of stimu- 

 lus physiology, such as Goetsch has recorded in his experiments 

 upon the effect of crowding in retarding growth of rapidly swimming 

 animals such as young tadpoles. 



There are, however, other possibilities which must be considered. 

 In vials stoppered with cotton there may be an increased CO2 ten- 

 sion developed which may in part account for the observed effects. 

 Then, too, the reduction in length of life at suboptimal densities 

 may be an expression of the inability of the small populations pres- 

 ent to gain control of "wild" organisms other than the food yeasts 

 present in the cultures, while the supra-optimal density effects may 

 be related at least in part to food shortage and to excess of excretion 

 products. Obviously, the resolution of this situation into causal 

 factors is not easy; but it is equally obvious that we have here one 

 of the most suggestive of the phenomena yet presented, indicating 

 the wide application and fundamental importance of the physiologi- 

 cal effects of animal aggregations upon the aggregants. 



MASS PROTECTION IN NATURE 



A different aspect of the effects of numbers on insect survival is 

 illustrated by the observations of Haviland (1926) upon the protec- 

 tive value of feeding aggregations of certain chrysomelid beetle 

 larvae {Coelomera cayennensis) . Haviland says: 



"The larvae are thickly hairy and the last segment of the body is 

 expanded into a strongly chitinized, shovel-shaped flange. The lar- 

 vae feed in a compact mass on the upper surface of Cecropia leaves. 

 Their heads are all directed inwards while the caudal expansions 

 thresh to and fro on the outside of the circle. The chief enemy of 

 these larvae is a carnivorous Pentatomid bug {Phyllochinis) which 

 loiters on the outskirts of the throng awaiting the opportunity to 

 impale a larva with his proboscis and drag it from its fellows. As 

 long as the circle of shovels is unbroken, the bug stands httle chance, 

 for his stylets cannot penetrate their polished armor and he cannot 

 reach the soft bodies beyond. But as the larvae feed they move out- 



