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AN INTRODUCTION TO ENTOMOLOGY 



are fastened heavy ballast stones, usually two on eacn side. 

 Brachycentrus nigrisdma. — The larva of this species builds a 

 case of the remarkable form shown in Figure 705. "It is constructed 

 of minute twigs, root-fibers, and fragments of wood cut to the 

 proper length to give even and straight edges, gradually diverging 

 toward the anterior end. In cross-section the outer surface of the case 

 is square; the interior is lined with a cylindrical tube of tough silk." 

 "During the first six weeks of their lives the larvae are active, crawling 

 about in quiet eddies along the banks of streams in search of food. 

 After this period they move to the center of the stream and live 

 sedentary lives, with one edge of the larger 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 assume the 

 position shown in Figure 705, protruding their heads slightly and 

 extending their prothoracic legs straight fon\^ard. 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 

 larv£e or bits of vegetation that float within their grasp." (Lloyd.) 



