I 9 4 INSECT TRANSFORMATION 



grubs are thus able to construct a labyrinth of intersecting lines 

 about their resting-places, so that they can move rapidly from 

 spot to spot without risk of being washed away by the strong 

 currents which suit the requirements of their lives. Clinging 

 to the thread by means of the fine hooks on their prolegs, they 

 move along the line with ease and safety in the direction of 

 the water's flow, while the anchorage thus afforded enables 

 them to make way against the current surely if slowly. 



The pupa (Fig. 101 c d) of Simulium has, like that of 

 Chironomus, paired filamentous gills on the prothorax. It is 

 protected by a flask-shaped cocoon (Fig. 101 e) of closely- 

 interlaced silken threads spun by the larva before its final 

 moult. The abdominal segments are provided with rows of 

 hooks which anchor it firmly while the head and the waving 

 thread-like gills project from the front end. It may seem 

 difficult to imagine how from this submerged pupa the midge 

 can make its way into the upper air. Nearly a century ago, 

 however, it was discovered 1 that, through the action of the 

 gills, air accumulates beneath the pupal cuticle so that when 

 the latter slits, a small bubble which necessarily surrounds 

 the fly rises to the surface, where bursting, it liberates the 

 winged insect. This can run upon the surface of the water 

 and climb up the aerial part of some aquatic plant ; thence, 

 as soon as its wings are fully expanded and dried, it takes its 

 short flight. 



The air-bubble that surrounds the emerging Simulium, and 

 the running of the little midge over the surface of the water 

 may serve to introduce a subject of great interest in the sur- 

 roundings of aquatic insect larvae : their relation to the surface- 

 film. Many adult insects, such as pond-skaters and various 

 flies run over the surface of water which they depress without 

 breaking, so that they never get wetted. Most inquiring 

 people know that by careful manipulation, a steel needle may 

 be supported by the surface-film of water in a glass, though 

 the breaking of the film will cause the needle immediately to 

 sink. This tensile property of a water-surface, useful to the 

 adult insects just mentioned that live above it, is also most 

 valuable to many insect larvae that live beneath it, because 



1 G. J. Verdat : " Memoire pour servir a 1'Histoire des Simulies " 

 Naturwissens. Anz. d. allgemein. Schweiz. Gesellsch. 1822. 



