294 Mr. J. A. Dell on the Structure 



body and wings are abundantly covered with grey hairs, 

 which give the fly at first sight the appearance of a small 

 moth, hence the name Psychoda, Round the edges of the 

 wings are a few black tufts, and the veins are thickly 

 covered with hairs. The hairy surface protects the body 

 from accidental wetting. A fresh-killed fly, when pressed 

 below the surface of the water, carries down with it so 

 large a quantity of air, that on being released, it shoots at 

 once to the top and remains floating. The flies may often 

 be observed resting on the windows of empty houses, and 

 are also abundant in privies, urinals, etc. At the Leeds 

 Sewage Works they cluster on the under-side of the pieces 

 of coke upon which the sewage is sprayed. They are 

 sluggish, and do not readily take to flight. In returning 

 from the sewage works to the University I have often 

 found that the walk of three or four miles was insufficient 

 to dislodge Psychoda flies which had clung to my dress. 

 They can, however, run fast, and they are sometimes 

 carried far by wind. The fly does not, so far as is known, 

 feed at all. 



In copulation the male runs alongside the female, 

 stroking her with his antenna?, while the wings, antennas 

 and halteres are thrown into spasmodic vibration. The 

 large forceps is then extended directly backwards, and the 

 abdomen of the male bent round so that the hinder ends 

 of the two bodies are brought into apposition. The 

 forceps then close upon the body of the female, unless 

 they miss, which not infrequently happens, as the male 

 fly is unable to see what he is doing. In such a case the 

 whole manoeuvre is repeated. The males are apparently 

 unable to distinguish which of their companions are 

 females, as they very frequently attempt to copulate with 

 one another. They have often been seen to die while 

 still attached to the female. 



As usual in Nemocera the eggs are deposited in an 

 elastic, jelly-like mass. They are irregularly arranged in 

 it, and unconnected with one another. The egg-mass 

 thus formed is indefinite in shape, and may be fixed to a 

 stone or other solid object. The total length of an egg- 

 mass may be 1-2 mm., and such an egg-mass commonly 

 contains 35-40 eggs. The eggs are small ('2 mm. to *3 

 mm. in length) and sausage-shaped ; they are opaque, and 

 loaded with yolk-granules. Hatching takes place 10-14 

 days after laying, but may possibly be more rapid in a 



