NORTH AMERICAN CADDIS-FLY LARVE. 
BRACHYCENTRUS NIGRISOMA. 
Hasirs.—During the first six weeks of their lives the larve 
are active, crawling about in quiet eddies along the banks of 
the stream in search of food. After this period they move to 
the center of the stream and live sedentary lives, with one edge 
of the large end of their cases firmly cemented to submerged rocks 
or sticks. Always they inhabit positions on the exposed surface 
of their support and always they face the unbroken current. While 
waiting for prey they assume the position shown in figures 142 
and 144, protruding their heads slightly and extending their pro- 
thoracic legs straight forward. ‘The mesothoracic legs are held 
upward while the metathoracic legs are extended to the sides. 
From this position they eagerly seize and quickly devour small 
larve or bits of vegetation that float within their grasp. 
In the unnatural conditions of still-water aquaria in the lab- 
oratory they attach their cases and assume their characteristic 
attitude of outstretched arms. If a particle of food material be 
moved within their grasp it is eagerly seized and devoured, but if, 
however, the food be placed in front of the larva, even in contact 
with its limbs or jaws, it is ignored or pushed aside. Apparently 
the larva does not recognize food that is not moved into its grasp. 
From time to time larve confined in aquaria detached their cases 
and moved from place to place. It seems probable that the larve 
in the streams, also, at times move about in search of building 
material, for it is unlikely that chance would place enough suitable 
case-building material within their reach. 
Larve which were put in the water after they had been ex- 
posed to the air until the moisture had dried from their cases, 
which accordingly floated, swam with their legs motionless and 
in the attitude assumed while waiting for prey. Their motion 
through the water was caused by the respiratory current being 
forced through the small openings at the caudal ends of their 
cases, somewhat after the manner of locomotion of certain dragon- 
fly nymphs. This was probably an unnatural mode of locomotion, 
never practiced in nature, at least not by this species. 
In preparation for pupation the larva spins a silken sheet, 
figure 148 C, across the front of the case. This sheet is perforated 
in the center by a number of small holes arranged in a circle. Sur- 
rounding the perforations there is an area which is free from 
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