230 Annals Entomological Society of America [Vol. IX, 



One morning we took about fifty of these wasps from the 

 ceiHng and marked them with paint, to see whether the same 

 individuals returned night after night, and for how long a period. 

 During the next three nights from six to nine marked insects 

 could be distinguished in the clusters. Even so small a propor- 

 tion as this demonstrates that the same wasps returned to the 

 appointed spot night after night. Doubtless some of the wasps 

 succeeded in cleaning off the marks, and others were probably 

 mortally injured by the forceps or the paint. It was interesting 

 to see that on the first and second night thereafter most of the 

 marked insects took their position on the ceiling to the rear of 

 the joist, a position never taken before (the sacred spot was just 

 in front of this joist), but on the third night even these had 

 become fearless again and joined the main body. Two weeks 

 later, upon leaving the farm, I picked up 38 of the insects to 

 take with me, (19 males and 19 females) and to my great 

 surprise two of them still retained markings of sufficient dis- 

 tinctness to identify them, thereby proving that at least a part 

 of the insects faithfully returned to the roosting place for two 

 weeks or more. 



What explanation can there be for this diversity between the 

 habits of C. caeruleum and of Pelopceus caementarium, a wasp 

 very similar, who builds nests which are indistinguishable from 

 those of this species? The one is gregarious at night, and the 

 other, after a day's hard labor of exactly the same kind, seeks 

 solitary shelter under some friendly leaf. 



This discovery that the blue wasps congregate at night solves 

 the problem of the group under the rock at Meramec Highlands. 

 When the sky became clouded and darkness fell over the earth 

 at mid-day, this afforded sufficient stimulus to prompt the wasps 

 to hie them to bed, in much the same way as darkness affects 

 poultry. 



This work suggests many interesting problems: Is this 

 gregarious habit at night an incipient stage to higher develop- 

 ment and socialization, or is this condition a vestige from the 

 time when they or their ancestors were social insects ? 



Since the female alone works hard at nest-building and 

 provisioning, what becomes of the male during the day, where 

 does he go, and how does he spend his time? 



Since it seems that mating does not occur during the night 

 and since the female is busy at her work during the day, 'when 



