176 MEDICAL ENTOMOLOGY 



forming a broad transverse ridge, at the end of which the raised antennal 

 sheaths can be distinguished. The thorax is gently convex, and has 

 attached to its lateral edges a tuft of from six to eight or more long 

 filamentous hairs, or thick bifid processes, the thoracic gills. Each fila- 

 ment projects freely from the open end of the pouch. The structure 

 and number of the gills are useful taxonomic characters. The abdomen 

 may or may not be furnished with spines, and usually has at the tip two 

 bent or recurved hooks, by which it is anchored to the bottom of the 

 pouch. At first the pupa is of a light mahogany colour, but it becomes 

 slowly darker, until just before the imago emerges it is almost black. 



Bubbles of gas now begin to collect under the pupal skin, and, w r hen it 

 splits horizontally along the middle line, the fly rises to the surface in a 

 bubble. It crawls on the water or is washed down for a short distance, 

 but soon clings to some support, and then flies away and settles on some 

 neighbouring twig or blade of grass. 



The essential factors governing the well-being of the larvae of Simulium 

 are water in rapid motion, and a support on which to fix themselves. 



These conditions are always available in streams 

 Breeding Technique . J 



coming trom some elevated place. A sudden bend, 



a declivity, or any obstruction will cause an acceleration in the motion of 

 the water. In such places blades of grass, dead leaves, sticks, and stones 

 accumulate, and it is here that the larvae are found in large numbers. 

 It is quite futile to attempt to breed out the flies by keeping the larvae 

 and pupae in any kind of aquarium, for they are sure to die. Newstead 

 states that he has bred out the imagines by placing the leaves, stones, 

 etc., to which the pupae were attached in a glass jar covered with fine 

 muslin netting, merely keeping them moist ; in some cases the insects 

 hatched out of pupae which were quite dry. 



In attempting to breed Simulium, it is best to place some simple 

 apparatus around the larvae in the water, and not to remove them to 

 trays or dishes. A tin frame, similar to the top of the fly jars (see Chapter 

 4 and Plate XLIV, fig. 5), should be fixed over the place where the 

 larvae are attached, either by burying it in the sand, or tying it with 

 string to some supports at the sides. When the larvae have pupated, a 

 fine meshed net can be fitted over the frame, and in this way the water 

 will still flow past the pupae, and the flies, which will hatch out under 

 natural conditions, will settle on the net after rising to the surface. 

 As the males are not blood-suckers they can seldom be obtained except 

 by breeding them out. 



Lutz suggests another simple way of breeding Simulium. A large 



