TRANSFORMATIONS OF BUFFALO GNATS. 21 



front at molting, and the remainder of the skin toward the caudal 

 extremity, the tracheae being withdrawn through the spiracular open- 

 ings. The number of molts has not been determined. 



MIGRATORY HABITS AND LOCOMOTION OF THE LARVAE. 



On being suddenly disturbed or on the diminishing of the current, 

 the larva? let themselves be carried down stream steadied by the silken 

 thread previously described, which they have attached -to some sta- 

 tionary object, and seek a more favorable situation. In many in- 

 stances the larvae have been observed almost to regain their former 

 position, by winding up the thread with the thoracic proleg and 

 mouth parts, but as a rule they are unable to work against the current. 

 The method of locomotion in still water is similar to that of geo- 

 metrid larvse. By looping the body, they bring the anal extremity 

 forward beside the proleg, which is then released on the caudal sucker 

 again obtaining a firm hold. 



A heavy rain causing a sudden swiftening of the current will 

 often entirely, change the distribution of the larva?. This fact is of 

 considerable economic importance, as it may account for the sudden 

 appearance of Simulium in localities usually exempt. This was well 

 illustrated in the following instance, observed by the writer at Spar- 

 tanburg, S. Cr A fair-sized stream had been free from larvse for 

 two months or more, when one of the tributaries near its source be- 

 came heavily infested with young and half-grown larvae. A heavy 

 rain and consequent washout carried practically every larva from 

 the tributary down the large stream for nearly a mile, where they be- 

 came successfully established, causing a subsequent heavy infestation. 



Young larvae seem to migrate more readily than full-grown larvae. 

 Several experiments were tried in regard to this. A vessel containing 

 larvae of all sizes was placed under a running faucet, causing the 

 vessel to overflow. The full-grown larvae as a rule remained in the 

 comparatively still water within the vessel, often spinning their 

 cocoons and successfully pupating. The young and half-grown 

 larvae seemed invariably to detach themselves and would be found 

 within two hours gathered on the surface over which the water was 

 flowing. 



At one time the attention of fish culturists was drawn to Simulium. 

 as the larvae were stated to cause the death of young trout by en- 

 tangling them in a silken web. This was disproved in the American 

 Entomologist and Botanist, in 1870, by Mrs. Sarah J. McBride, 1 who 

 clearly demonstrated that the " death web of young trout " was a 

 myth, and that the threads were not strong enough, or interwoven, 

 so that fish could be entangled in them. 



1 See Bibliography, p. 35. 



