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ENTOMOLOGY FOR MEDICAL OFFICERS 



species breathing seems to be carried on, very much as it 

 is in certain dragon-fly larvae, by delicate rectal gills (Fig. 32), 

 which are alternately pushed out and retracted. 



When the larva is about to pupate, it spins, from its 

 salivary glands, a silky cocoon, which may be found attached 

 to water-weeds, etc. The cocoon is often shaped something 

 like a slipper (Fig. 33), or it may be merely a widely open 

 cone. The pupa occupies it loosely, holding on by some 

 belts of small spinules on the posterior abdominal segments, 

 and from the open end of the cocoon the pair of thoracic 

 gill-tufts, by which the pupa breathes, floats out. Towards 

 the end of the pupal stage gas accumulates beneath the skin 



^reafhing 

 filament 



sucker 



Fia. 32.— Extruded Hectal Gills 

 of Simidium Larva. 



Fig. 33. — Pupa and Cocoon of 

 Simulium, 



of the pupa, so that when the skin splits to give exit to the 

 fly, the gas escapes in a bubble which carries the fly safely 

 to the surface of the water. The larval stage is said to 

 occupy about a month in the summer months, and the pupal 

 stage about a week. 



Since Siinulium is in many places such a formidable pest, 

 the question of destruction of larvae has to be considered. 

 Among natural enemies trout and fish of similar habitat 

 come first. Miall mentions caddis-worms, and my pupil. 

 Miss Sophia Summers, confirms this from observation in the 

 laboratory. Since the larvae require running water, it has 

 been recommended that parts of streams where they are 

 numerous should be dammed, to their ultimate destruction. 



