1 8 BULLETIN OF THE BUREAU OF FISHERIES. 



VARIATIONS IN TUBES. 



The tubes, as explained above, are composed of debris fastened securely together 

 with silk. Taylor (1905) and Lauterborn (1905) have described other closely related 

 larva?, living in similar situations, that spin tubes of nearly pure silk. In texture the 

 tubes of Tanytarsus pusio Meigen and T. exiguus Johannsen are intermediate between 

 those of pure silk, such as are spun by Chironomus lobiferus when living within a stem, 

 and those composed of a mass of debris only loosely fastened together with silk, such as 

 are characteristic of the group that is represented by C. cayugce. 



The tubes figured by Mundy for Tanytarsus pusio are rather different in structure 

 and proportions from those of T. exiguus. The substance of the tubes gives them a gray, 

 slatey appearance that closely resembles the general color of the bottom. The arms 

 are proportionally stouter and are represented on the sides of the burrow by elevated 

 ridges. The tubes are as often fastened flat down to the surface upon which they 

 rest as elevated at the end, apparently depending upon the convexity of such surfaces. 



The arrangement of the tubes is not to any great extent dependent upon the 

 direction of the current. Small stones having from 8 to 10 tubes on their undersides 

 usually showed such a variety in the arrangement of those tubes that it would seem 

 that free space was of more importance than the direction of the. current. 



Johannsen (1905) says of the tubes of Tanytarsus exiguus: 



During the early summer most of the cases will be found attached by the stems alone, but later in 

 the season most of them lie flat on the rocks and are attached on one side like Simulium pupal cases. 



It seems evident that this species varies considerably in the type of tube which 

 it builds. The author's observations on this species in nature are confined to small 

 streams which were not very rapid, and in these localities the food supply has been 

 fairly abundant, as shown by the number and variety of the population. In such 

 habitats the predominance of the attached type of tube would seem to indicate that the 

 strength of the current and perhaps the food supply are the governing factors. Since 

 the writer's observations were made both in the fall and in the spring, the effect of 

 seasonal changes should be eliminated unless these tubes were able to persist throughout 

 the winter, which seems improbable in most cases considering the erosion to which 

 such small streams are subjected. 



THE NET. 



The arms, as before mentioned, are connected by webs so as to form a net to retain all passing 

 objects; but even with a high-power lens I have been unable to detect single threads. The network 

 seems only to be made up of irregular bands of slime or mucus passing between the neighboring arms, so 

 probably it issues from the creature's mouth in this form. [Mundy, 1909.] 



NET MAKING. 



To build its net the larva proceeds as follows: Running up one of the arms for some distance it 

 swings across to the next arm, carrying with it a thread of silk, then quickly back again, at the same 

 time retreating somewhat into its case. This zigzag movement is repeated two or three times until the 

 base of the arms is reached, when the whole process may be repeated over again until a sufficient num- 

 ber of threads have been stretched across to make a rude network which, whatever its workmanship 

 compared with that of a spider, is at any rate good enough for its purpose and effectually stops all objects 



