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nal tracheae being probably withdrawn through the spiracular open- 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. 
