92 HOUSEHOLD INSECTS 



It is safe to say that an application of oil should be 

 made at least twice a month to be surely effective. Pos- 

 sibly in certain cases oftener, especially in drains where 

 the current is fairly strong. 



Is the pouring of oil into water-closets effective in 

 sewer ditches ? — It has been asked of the writer so many 

 times whether or not the same thing could be accomplished 

 by pouring oil into the closets and allowing it to run down 

 the sewer pipes, as by spraying, that it seemed worth 

 while to give this point considerable attention. Ac- 

 cordingly, I poured two quarts of illuminating oil on the 

 surface of the water in a main sewage ditch near the 

 mouth of the tile to watch its effect below. A fairly quiet 

 pool about two rods long and about fifteen rods below 

 the outlet was selected as the first place of observation. 

 Both sides of the pool were lined with multitudes of larvae 

 and pupae lying in the shallows and in the miniature bays 

 hollowed out of the sides of the bank. By the time the 

 oil reached the pool it was well distributed. The result 

 was, however, that the current was too strong to allow 

 it time to spread into the quieter parts and bays of the 

 pool. It was carried for the most part straight by. 

 Many of the larvae and pupae, however, lying next to the 

 current were so greatly disturbed that they blundered 

 into the middle of the stream and were drowned beneath 

 the film of oil. In a second and similar pool, about ten 

 rods farther down, the effect was noted again. The oil 

 had spread out even more by this time, but the effect was 

 about the same. The majority of larvae and pupae escaped 

 because the current gave the oil insufficient time to spread 

 over them. Nevertheless, it is thought that several 

 applications of oil, say one or two hours apart, would 



