THE MOSQUITOES OF THE SOUTHEASTERN STATES 33 



country appears to be comparatively low, probably much lower than 

 is generally supposed. From an examination of 9,340 specimens col- 

 lected on plantations in the vicinity of Mound, La., in 1922 (71), 

 only 10 were found to contain the sporozoite form of the parasite in 

 the salivary glands and therefore to be capable of transmitting the 

 infection at the time of capture. This gave a sporozoite rate of 0.107 

 percent, or approximately 1 infective specimen per 1,000. The annual 

 malarial rate in humans on the same plantations during that year was 

 about 45 cases per 100. At Edenton, N. C, (17) a gland-infection 

 rate of 0.33 percent was obtained from dissections of 1,486 mosquitoes 

 over a period of 3 years. 



The larvae of Anopheles feed almost entirely at the water surface, 

 and since they seem to make no selection of material provided it is 

 small enough to be ingested, the food consists of the general variety 

 of small organisms that are found at the water surface. From a 

 large series of observations at Mound, where quadrimaculatus was 

 the predominant anopheline, Bradley (27) reported that flagellates, 

 diatoms, and the green algae made up a large proportion of the 

 plankton content of the surface layer in the natural waters of that 

 area. The approximate average numbers of organisms per cubic 

 centimeter in breeding places having more than one larva per dip 

 were as follows for the breeding seasons of 1928 and 1929, respec- 

 tively: Total plankton, 8,600 and 6,300; flagellates, 5,400 and 4,200; 

 diatoms, 1,800 and 500; green algae (other than flagellate forms), 

 700 and 900. Present in smaller numbers were the ciliates, blue-green 

 algae, and amoeboid protozoa, although the first two were fairly 

 abundant at times. The four principal genera among the flagellate 

 forms were Euglena, Chlamydomonas, Trachelomonas, and Phacus, 

 which composed about 75 percent of the total organisms in this class 

 in the places of high larval densities. The report of studies made 

 by Boyd and Foot (20) shows a similar plankton content in the sur- 

 face waters at Edenton. 



In rearing the larvae in the laboratory various foods may be uti- 

 lized, both those from the natural breeding places and artificial foods 

 such as yeast. Barber (2) found that Anopheles quadrimaculatus 

 could be reared to maturity on cultures of single species of organ- 

 isms, including algae, bacteria, or infusoria, and that dead organic 

 material was less suitable than living organisms. Komp (85) reared 

 Anopheles successfully in a food culture made from Spirogyra killed 

 by heating. Boyd, Cain, and Mulrennan (19) report that the in- 

 fusoria developing in a ripened hay infusion, when supplemented 

 by yeast, forms an almost ideal food for the larvae, as it invariably 

 produced large and healthy individuals. The ripening process re- 

 quires at least 30 days in the summer, as the infusions must pass 

 through a period of acid fermentation before they are satisfactory 

 as a food supply. These authors found that the largest proportion 

 of the larvae reached maturity at about 70° F. At this temperature 

 development from egg to imago required about 21 days (18). At 

 high summer temperatures, with an ample food supply, larval devel- 

 opment under natural conditions may be completed in about 1 week, 

 although some larvae develop more slowly than others under the 

 same conditions. With a minimum of 3 or 4 days for the other 

 aquatic stages (egg and pupa) and about 4 days for the preoviposition 



109619—39 3 



