26 BULLETIN 329, U. S. DEPARTMENT OF AGRICULTURE. 



shedding of the larval skin. From the mesothoracic spiracular cham- 

 ber extends a broad tracheal trunk leading to the base of the main 

 trunk of the respiratory filaments, which do not contain tracheae, but 

 are hollow. According to Taylor, 1 the air is not taken directly into 

 the gill base from this hollow space, but is absorbed through the 

 external chitinous fibrillae and thence into the tracheal extension 

 through a membrane. 



EMERGENCE FROM THE PUPAL STAGE. 



Four or five hours before emergence there is a very noticeable in- 

 termittent movement of the adult within the pupal skin, which is 

 gradually distended with air toward the anal extremity, the abdomi- 

 nal tracheae being probably withdrawn through the spiracular open- 

 ings. Coincident with this the anal portion of the adult is with- 

 drawn from the pupal skin and a threadlike membrane, seemingly 

 the lining of the hind intestine, may often be seen extending from 

 the anus of the adult to the pupal skin, to which it remains attached 

 after emergence. As the pupal skin is locked by the strong chitinous 

 hooks to the pupal case, the adult exerts a strong pressure toward the 

 cephalic end and the pupal skin splits along the dorsal portion of the 

 thorax and head, forming a T-shaped aperture. The adult at once 

 rises to the surface of the water surrounded by a bubble of air which 

 has been collected in the distended pupal skin, and running along the 

 surface of the water at once takes flight. The curious way in which 

 the adult rises to the surface surrounded by a bubble was commented 

 on by the earliest writers on the group, and in situations where the 

 pupae are found in thousands, as in parts of Hungary, and along the 

 Illinois and Mississippi Rivers in America, the water seems almost to 

 boil as they emerge. 



LIFE CYCLE AND NUMBER OF GENERATIONS. 



The number of generations varies according to the species and 

 the latitude. In the Southern States the species seem to breed con- 

 tinuously from about the middle of March until the approach of 

 severe cold weather, generally about the end of November. The life 

 cycle of one generation during the summer takes approximately four 

 weeks ; 7 days in the egg stage, 17 days as larvae, and 4 days as pupae. 

 The time from the egg to the adult stage varies according to the rise 

 and fall of the temperature. There are probably from five to six 

 generations annually in South Carolina of the species here dealt 

 with, except S. pictipes, which normally has three generations. In 

 Illinois there are only three or four generations of S. venustum 

 annually. 



1 Loc. cit. 



