4 



DIPTERA 



fixed pupal case. Professor Comstock seems to have been the first to watch the process carefully, and he 

 describes it as follows : 



« Each midge on emerging forced its wa}- out through a transverse rent between the thorax and 

 abdomen. It then worked its body out slowly, and in spite of the swift current held it vertical. The 

 water covering the patch of pupae varied from one-fourth to one-half inch in depth. In the shallower 

 parts the adult had trouble in working its way to the surface, still clinging to the pupa-skin by its very- 

 long hind legs. W hile still anchored b}' its legs, the midge rests on the surface of the water for one or 

 two seconds and unfolds its wings: then freeing its legs it takes flight. The adults emerging from the 

 deeper water were swept away by the current before they had a chance to take wing. The time 

 required for a midge to work its way out of the pupa-skin varied from three to five minutes. » 



As is obvious, the whole process of emergence and escape into free air must be a quick one. 

 Usualty with insects it takes some time for the proper expansion of the wings, which are, in the pupa, 

 neither wider nor longer than the pupal wing-cases, but attain their full size only after withdrawal from 

 these cases. But in the Blepharocerid there is no time for that; the slender legs cannot hold long 

 against the beating of the swift water, and so the remarkable condition of a full development and 

 expansion of the wings in the pupa obtains in this family. The fully developed wings lie in the pupal 

 case folded both longitudinalh' and transverse!}-, and only need to unfold to be read}' to carr}- the fly 

 into the safe air. It is this folding which produces the secondary veining of the wings characteristic of 

 the family, this veining being simply the persisting creases and lines of the folding. 



The writer has often watched the emergence of adults, and has been struck by the great loss 

 (apparently) of life in the process. So many flies are swept away by the swift water before the wings 

 can be unfolded or before the legs can be loosened from the pupal sheath, that it seems no wonder that 

 the family is a disappearing one. It is a case of the dangers of an extreme specialization. If the fixed 

 pupae lie in water too deep (easily occasioned by a sudden rise in the stream at the time for emergence), 

 or on the other hand, become wholly bereft of the hfe-giving water by a falling of the stream, there is 

 no hope for the fly. The first contingency seems indeed to be somewhat provided for by the apparent 

 power of the insect of postponing for some time, if necessary, its emergence. Thus, in the event of a 

 heavv rain and consequent rise of the stream, the too deeply submerged pupa may lie unchanged 

 until the water has run off (a matter which happens speedily in swift streams) to a safe shallowness. 



The fully developed flies have been found numerously in the case of but few species. The flies 

 of Blepharocera teniiipes are found abundantl}- along certain small streams near Ithaca, N. Y., U. S. A. 

 The flies at rest cling by their long legs to the under sides of leaves on the bank from the water's edge 

 to eight or ten feet away. Of the hundreds of flies which were seen here in two or three visits all were 

 females; and they were engaged busily in feeding. This was accomplished by capturing on the wing 

 small Chironomid midges, and then returning to a leaf, where the uniortunate prey was lacerated by 

 the long, strong, saw-like mandibles, and the blood and body -juices drunk. The empty torn skin of the 

 prey was then dropped. As the males do not have mandibles, they must have a wholly different food 

 habit (probably non-predatory) and this may account for the absence of males from this feeding-ground. 

 The flight is rather slow and weak, a sort of timid fluttering. The flies of BibiocepJiala elegantidus were 

 found by Kellogg to be numerous along the Big Thompson river, in the Rocky Mountains of Colorado, 

 U. S. A. The flies spent most of their time at rest on the vertical sides of the boulders from a few 

 inches to two feet above the water surface, but always where the rock face was frequently wetted by 

 the spray of the dashing water. The flies rested with legs and wings outstretched and body close to the 

 rock. The wings touched the rock face and, indeed, the attitude seemed to be adapted to bring as much 

 of the body as possible into contact with the wet, smooth face of the rock, as if to resist, by increased 

 friction, the tendency of the fly to slide down the vertical surface. None of these flies was seen feeding, 



