262 



Illinois Natural History Survey Bulletin 



Vol. 23, Art. 1 



sand grains, circular in cross section and 

 tapering fairly evenly. At pupation part of 

 the narrow end is cut off, and the case then 

 appears nearly cylindrical. 



Adults. — Length 6-7 mm. Color uni- 

 form dark brown, appearing almost black in 

 life. Male genitalia, fig. 893: cerci some- 

 what triangular, well separated to base; 



Fig. 892. — Micrasema rusticu?n larva. 



tenth tergite forming a pair of lateral scle- 

 rotized processes enlarged at apex and bear- 

 ing a group of short setae ; at base of these 

 processes is a pair of low tubercles bearing 

 a seta; claspers long and straight, the pos- 

 terior margin of the apex forming a slight 

 tooth. Female genitalia simple, similar in 

 general structure to those for Brachy- 

 centrus. 



This species is very similar to charonis 

 and wataga, both of which occur in the 

 eastern states; the diagnostic differences in 

 the genitalia are illustrated in figs. 894, 895 ; 

 charonis is characterized by the bent apex 

 of the clasper, wataga by the long style and 

 heavily sclerotized lateral plates of the tenth 

 tergite, rusticum by the short style, weaker 

 tenth tergite and almost straight clasper. 



In Illinois we have taken this species only 

 in and along the Kankakee River at Mo- 

 mence and Kankakee. At Momence the 

 cases were found under stones in the river, 

 and mature pupae were obtained, thus link- 

 ing the larval and adult stages. In this 

 locality we have taken this species only in 

 spring, indicating a single-brooded condition 

 here. The species seems to follow the two 

 Brachycentrus species in seasonal succession 

 and has been taken in abundant swarms 

 during the middle of May. 



The species is widely distributed through 

 the eastern states, extending southwestward 

 into Oklahoma. It appears to be fairly 

 scattered in distribution, since we do not 

 have a large number of locality records, 

 but is frequently taken in swarms. Records 

 are available for Georgia, Illinois, Indiana, 

 Michigan, New York, Oklahoma, Saskatch- 

 ewan, Virginia and Wisconsin. 



Illinois Records. — Kankakee: May 31, 

 1938, Mohr & Burks, 1 $ ; Kankakee River, 

 May 21, 1940, Mohr & Burks, 8^, 69. 

 Momence: Kankakee River, May 26, 1936, 

 H. H. Ross, 1 5 ; May 17, 1937, Ross & 



893 RUSTICUM 



894 CHARONIS 



MAVILLARY 

 PALPUS 



^r' *^^sw4^ 



895 wataga 



Figs. 893-895. — Micrasema, male genitalia. 

 J, clasper; B, spine of tenth tergite; C and D 

 tenth tergite, respectively dorsal and lateral 

 aspects. E, male head. 



