DENSITY OF POPULATION AND INSECT SURVIVAL 239 



bottom without movement; others might show shght movements 

 of the feet, particularly if these came into contact with a solid ob- 

 ject, even when there was no other visible movement. The recovery- 

 period from HCN was also shorter than from exposure to the other 

 gases used; but otherwise the results were similar, particularly in 

 that the group showed greater survival value than isolated indi- 

 viduals. 



Such results from different numbers of insects exposed to the 

 same volume of the same concentration of toxic gases are according 

 to expectation from the experience previously recorded with aquatic 

 organisms exposed to various toxic conditions. The apparent ex- 

 planation is that, when the toxic gas is distributed among a number 

 of organisms, no one individual is as likely to receive a dose sufficient 

 to cause death as if it were exposed singly to the same quantity of 

 the same gas. The explanation of the findings when groups and 

 singles are exposed to volumes in proportion to the numbers present, 

 indicating that there is group protection in proportion to the num- 

 bers present even in the face of similar conditions for each indi- 

 vidual, is not clear, and further work is needed at this point. 



It seems probable that under such conditions of crowding, even 

 with equal space and an equal amount of the toxic gas present per 

 individual, the group may cause a greater concentration of carbon 

 dioxide in its neighborhood, or by some other effect may very well 

 cause these animals to have a lowered rate of metabolism. Under 

 these conditions, and with the concentration of poisonous substance 

 at the right strength, one would expect the members of the depressed 

 group to survive longer than would the more active single individ- 

 uals. This suggestion is in line with the susceptibility work of Child 

 (191 5) and the resistance of Daphnia to various chemical reagents, 

 as developed by Fowler and discussed in a previous chapter. 



Experimental results similar to those of Deere have been obtained 

 by Bliss (personal communication) under natural conditions. In his 

 field studies Bliss found that the camphor scale insect {Pseudoanidia) 

 has a greater natural death-rate the greater the density of the popu- 

 lation on a given twig. When exposed to adverse conditions, such as 

 are represented by spraying with an oil spray, the opposite condi- 



