METAZOA INSECTA. 479 



animals so far described, the social instinct reaches its 

 most complete development in the ants. This commu- 

 nistic life of the social Hymenoptera has been brought 

 about by the specialization of certain individuals which 

 started as solitary insects of either the male or female 

 sex. It has doubtless required a long period of time and 

 a vast number of generations to bring this social organiza- 

 tion to such a state of efficiency as we find it to-day. If 

 mental qualities were made the basis of our classification, 

 ants would not only be the most specialized of insects but 

 would come nearer to man than the ape (Lubbock). It 

 is obvious, however, that such a classification would not 

 represent the genealogical succession of animals upon our 

 earth ; such a succession, it must be borne in mind, can 

 be determined only by a knowledge of the structure and 

 the development of animals. 



The species of solitary wasps far outnumber the 

 social species in the United States. The fossorial or 

 digger wasps are solitary in their habits and are either 

 male or female. Like most insects, also, their life is so 

 short that as a rule they neither see nor care for their 

 young, although the preparation they make for them is 

 exceptional in its character. 



Pepsis caerulea Linn. (No. 1223), one of the Pompilidae, 

 is a robust insect with the thorax attached to the abdomen 

 by so short a peduncle that it is sometimes described as 

 having none. Its mandibles are large, strong organs, 

 and these together with the fore and hind pairs of legs 

 are adapted for digging burrows in the ground, in each 

 one of which an egg is placed. This habit is no new 

 feature of insect economy, but what is unique is the knowl- 

 edge the parent possesses of paralyzing insects without 

 killing them. She must provide sufficient animal food 

 for the entire life of her young ; if the spiders, caterpillars, 

 and the like were killed they would soon be unfit for food; 

 therefore she thrusts her powerful sting and poison into 

 the nerve centers of the particular animal her larva feeds 



