Of sprays he recommends as the least objectionable and at the same time one of the most effective, formalin. "The dark portions and angles of sleeping apartments should be sprayed with a one per cent. solution of this substance every day during the season in which the flies are prevalent. A fine spraying apparatus is necessary for its application and an excessive amount must not be applied. It is considered an excellent plan also to spray the mosquito curtains regularly every day towards sunset; nets thus treated are claimed to repel the attacks of these insects." This effectiveness of formalin is very surprising for, as we have seen, it is almost wholly ineffective against bed-bugs, mosquitoes, house flies and other insects, where it has been tried. A measure which promises to be very effective, where it can be adopted, is the use of electric fans so placed as to produce a current of air in the direction of the windows of sleeping apartments. On account of the inability of the Phlebotomus flies to withstand even slight breezes, it seems very probable that they would be unable to enter a room so protected. Culicidæ or Mosquitoes From the medical viewpoint, probably the most interesting and important of the blood-sucking insects are the mosquitoes. Certainly this is true of temperate zones, such as those of the United States. The result is that no other group of insects has aroused such widespread interest, or has been subjected to more detailed study than have the mosquitoes, since their rôle as carriers of disease was made known. There is an enormous literature dealing with the group, but fortunately for the general student, this has been well summarized by a number of workers. The most important and helpful of the general works are those of Howard (1901), Smith (1904), Blanchard (1905), Mitchell (1907), and especially of Howard, Dyar, and Knab, whose magnificent monograph is still in course of publication.