GENERAL FACTORS CONDITIONING AGGREGATIONS 75 



and speculated much concerning this gregarious condition of a soli- 

 tary wasp. The Raus (191 6) found three related species sleeping in 

 such assemblies, from which it would seem probable that Fabre was 

 observing a slumber aggregation. With Chalyhion caerulum both 

 males and females may be found aggregated at night in about equal 

 proportions. As many as a thousand have been found in one colony. 

 Marked individuals will return to the same sleeping place for at least 

 2 weeks. No one knows how the male of the species passes the day; 

 the female labors about the nest. 



The solitary Sphcx wasps appear to choose their sleeping quarters 

 independently; but since they select the same sort of place, they 

 tend to form spaced aggregations. Prionyx sleeps sometimes singly; 

 sometimes gregariously crowded close together on the top of a weed, 

 with equal numbers of males and females present but without ob- 

 served copulation. The males and females of the horse fly Tabanus 

 sulcijrons are reported also to collect in favorable places to sleep 

 (Hine, 1906). Similar observations are on record for various other 

 insects. 



There is no evident protection from enemies in such assemblies. 

 The sleep may be sound, and may extend so late that early birds 

 could pick off the sleeping insects in numbers, as beetles are reported 

 to kill off sleeping butterflies (Floerscheims, 1906). 



Schrittky (1922) observed in Paraguay an aggregation of from 20 

 to 27 butterflies (genus Heliconus) that gathered nightly during 

 August and September. The butterflies could be hancUed in the early 

 mornings without waking them. The temperature then ranged 

 around 5° C. The butterflies were quite restless in the evening long 

 after dark, when the temperature was higher than in the morning. 

 He also observed males of the genus Tetrapedia in aggregations at 

 night; females are found in temporary aggregations until the time 

 of fertilization, after which they separate. 



Banks (1902) gives observations on males of the solitary digger 

 bees of the genus Melissodes. He saw these bees at dusk in his back 

 yard clinging with mandibles and feet to grass blades. He records 

 three or four returning for several nights. He cites the record of 

 Schwarz (1896) to show that Melissodes pygmeus clasp twigs with 



