29 
only where the current is swift or where its movement is so inter¬ 
rupted by shallows or by objects lying or growing or suspended 
in the stream as to produce at least a surface whirl or ripple. 
The larvae are rather peculiar creatures, with slender, cylindric¬ 
al, maggot-like bodies, thickened and club-shaped at the hinder 
end, by which they adhere to some submerged object, and with 
a pair of fanlike clusters of filaments near the mouth. They are 
commonly grouped in colonies, often thickly covering the object 
to which they are attached. They spin from their mouths silken 
threads, with which they form a loose network covering the sur¬ 
faces they occupy, and by means of which they can recover their 
position if swept away by the current. They move mainly like a 
measuring-worm, with the aid of a sucker near each end of the 
body. They pupate in a case or nest composed of web spun from 
the mouth (Fig. 21, p. 46) and the pupa breathes by a pair of 
tufted gills extending forward from the open mouth of the case. 
In the two species whose life histories have been fairly well 
followed, namely pictipcs and venustum, about two months elapse 
in summer between the laying of the egg and the appearance of 
the winged fly, the egg stage lasting about one week, the larval, 
four weeks, and the pupal, three. In colder weather the develop¬ 
ment proceeds more slowly. As these species hatch from the egg 
in New York in the first part of May, there is time, at this rate, 
for three successive generations, the last of which hibernates in 
the larval stage, pupating in April of the following year. We have 
sufficient data concerning the times of occurrence of the winged 
black-flies in Illinois to bring all but three of our species under 
this category. The single-brooded species appear in the winged 
stage in central Illinois in April and May, the date of maximum 
abundance here in two successive years having been about April 
2 5. The farther south one goes, and the earlier the spring, the 
earlier is the swarming time of the gnats. Indeed, there is one 
report from Louisiana of the appearance of winged buffalo-gnats 
during every month of an unusually mild winter, and a consequent 
failure of the usual spring rush in February and March.* Altho 
six of our Illinois species send out summer generations, these are so 
scanty and scattering that it is difficult to find winged specimens 
in the field, even by careful, expert search, at any time except in 
spring. 
Breeding Situations oe the Black-elies 
The number of our Illinois species and the fact of their dis¬ 
tribution in all parts of the state make it practically certain that 
Mack-flies may be found, sooner or later, wherever and whenever 
*Tn?:ect Life, Vol 4, p. 143. 
