40 BULLETIN 66, UNITED STATES NATIONAL MUSEUM. 



our knowledge of the elenchid and halictophagid parasites of the 

 Homoptera is constantly on the increase, thanks to the zeal of Mr. 

 Koebele and Mr. Perkins. The Homoptera may very truly be said to 

 be gregarious, for, to quote Osborn and Ball (1897), there are "numer- 

 ous species of Jassidse which swarm, often by millions to the acre, upon 

 various species of grass." Under such conditions the minute tri- 

 ungulinids swarming over the grass should have little difficulty in 

 reaching a host, and males should have but a slight distance to travel 

 in order to find a female. 



Solitary insects. — Under this grouping fall all of the Sphecoidea, the 

 Eumenida?, and all of the Apoidea except Halictus, which are listed as 

 hosts. The Sphecoidea include ground dwellers and mud daubers, 

 among which the bembecids often dwell in large communities. The 

 writer has observed only two such communities at Lincoln, Nebraska; 

 but Hartman (1905) writes: 



Microbembex and Bembex are both common in the sandy woods, where they often 

 form large mixed colonies, building their nests side by side in great numbers. When 

 a novice first comes upon one of these populous colonies on mid-day, when business is 

 at its height, he is bewildered by the great number of wasps engaged in the general hub- 

 bub around the many holes that riddle the ground. 



The Sphecidae are not communal, being somewhat averse to socia- 

 bility. Hartman (1905) recalls a fierce combat of two AmmopTiila 

 which happened to dig their nests near each oilier. The nests of 

 Sphex {Ammopliila), ( ■Morion (Sphex) and Priononyx are dug in the 

 ground, the parents being very particular as to the location. But one 

 egg is laid in a hole. Scelipliron (Pelopseus) is a mud dauber and 

 builds several cells adjacent to each other. 



The Eumedidse are solitary workers, constructing a series of loose 

 cells at the base of grass clumps, constructing tubes in holes in wood, 

 in steins of plants, or cementing neat little pots on twigs. 



Among the Apoidea a great diversity of nesting habit occurs. As 

 far as the writer knows, Panurginus, though dwelling in the ground, 

 (Pierce, 1904) is not communal in habit. Halictoides , Andrena, and 

 Halictus are ground dwellers. Mr. Crawford has found large numbers 

 of Andrena pulchella and Halictoides marginatus dwelling in one com- 

 munity at Lincoln, Nebraska. Halictus is also found often in com- 

 munities. Prosopis nests in galls and constructs galleries in briars. 



Among these solitary insects may be noted two conditions — those 

 that are strictly solitary and those that are semisocial, and live in 

 communities. It may be surmised without much doubt that under 

 the latter condition it is easier for male parasites to find females, 

 since the hosts are generally in swarms. Likewise triungulinids 

 from feeble hosts can more readily reach new hosts in a community 

 than where the hosts are widely separated. Halictus is really semi- 

 social, as several females use the same hole. 



