CHAPTER XIV 



RELATION BETWEEN DENSITY OF POPULATION 

 AND INSECT SURVIVAL 



We have already seen that the insect Drosophila reproduces less 

 rapidly the greater the population density (chap, vii), and that, 

 on the other hand, the confused flour beetle {Tribolium) during 

 initial stages in cultures of flour reproduces more rapidly if the pop- 

 ulation for given environments be larger than the minimum (chap, 

 x). In later chapters we shall find that crowding exerts certain 

 physiological effects upon aggregated insects which are expressed in 

 morphological changes. These facts concerning mass relations be- 

 tween non-social insects are the more interesting in view of the high 

 state of social organization to be found at times in this group. 

 Unless we can find a strong substratum of generalized co-operative 

 survival values among the subsocial insects, we shall have difliculty 

 in demonstrating a connection between the phenomena associated 

 with relatively slightly integrated animal aggregations and those 

 connected with the more closely knit social communities. 



For the present we desire to consider the relations between num- 

 bers of non-social insects and their survival in the face of unfavorable 

 conditions. Three lines of evidence exist upon this point: (i) the 

 data from laboratory and field experiments concerning the relation 

 between numbers of insects present and their survival in the pres- 

 ence of toxic reagents; (2) studies on the relation between population 

 density and longevity in Drosophila; and (3) the more usual type of 

 natural-history observations showing that the survival value of 

 crowding in the presence of predators holds for insects as well as for 

 the larger animals, for whom it has been more frequently reported. 



MASS PROTECTION FOR GRASSHOPPERS 



Two workers have independently reported mass-survival values 

 for insects exposed to poisonous substances. Deere, for the majority 



236 



