LABORATORY METHODS IN MALARIA 369 



longer period it is advisable to give them a new feeding on a 

 guinea-pig or other laboratory animal. Some workers place a 

 piece of raisin or other fruit on the cage to supply food, but such 

 material seems to promote the growth of yeasts or bacteria in 

 the stomachs of the mosquitoes. If any food other than blood is 

 used, white sugar seems to be the most suitable. It may be con- 

 veniently supplied to the mosquitoes by placing a small amount 

 on a piece of moist filter paper which is then laid on top of the 

 cage. Mosquitoes are usually kept at room temperature while the 

 malarial parasites are growing. 



Uninfected mosquitoes kept in stock while awaiting use are 

 cared for in the same way as the infected batches. They may be 

 kept at lower temperatures in which they live longer and require 

 less food. 



Cages should be kept free from dead mosquitoes. Sometimes 

 many of the mosquitoes will die, whatever precautions are taken; 

 but it is usually possible to preserve a large proportion of them, 

 even when kept at room temperature in the tropics. Mortality is 

 apt to be higher during the first few days. Those which die after 

 the second day may be removed and dissected, since oocysts are 

 then usually large enough to be discernible, especially in batches 

 of mosquitoes which have been kept at temperatures of 70° to 

 85° F. Dead mosquitoes if not left at room temperature more than 

 half a day usually remain in good condition for dissection and ex- 

 amination. In no case should they be allowed to dry, as dissection 

 then becomes very difficult or impossible. A good routine is to 

 remove the dead mosquitoes morning and evening for dissection. 

 Those removed in the evening may be kept on ice and dissected 

 on the following day. 



If no positives appear among the dead mosquitoes, it is advis- 

 able to begin dissecting the living ones on the fourth or fifth day 

 after feeding on the carrier, a time when oocysts are large enough 

 to be very easily seen. If the whole batch should prove to be 

 negative, one is then spared the trouble of keeping them longer. 

 Should a positive appear, one may keep the remainder alive until 

 the later stages of parasites are developed. Dead mosquitoes may 

 be easily removed through the opening in the bottom of the cage 

 by means of small forceps. The living ones are removed by means 

 of the device described (Fig. 22), placed in a series of cotton- 

 plugged tubes, one or two per tube, and chloroformed as needed. 



