There has been very scanty findings about the natural infection of mosquitoes with Wuchereria bancrofti which are accountable for the role of the mosquitoes found in an area playing upon the dissemination of filariasis. To inquire into the above subject, the author dissected in search of filariae all the mosquitoes collected from 15 filarious villages by different methods described below. I. 14 catches were made early in the morning within the mosquito-net where at least one microfilaria carrier sleeped. II. 47 catches were made early in the morning inside of infested households, and one in a house which is just close to infested ones. III. 34 catches were made at night for about 30 minutes from 8-10 p.m. when the mosquitoes entered infested houses to bite the inmates. IV. 11 out of 16 catches were made at night by human-baited-trap being set up at the front-yard of infested households, while two catches in village No.26, each one in villages Nos. 21, 22 and 23 were made setting up the trap at school yard locating at 300-500m distance from infested households. V. 19 catches in cow-sheds. 2 in goat-sheds were made at night for about 20 minutes from 7-11 p.m.. These cattle-sheds were connected with or just close to the houses having carriers, excepting two cases in which the cow-sheds were at about 500m from an infested village. Another 3 catches were made by goat-baited-trap at night in the front-yard of infested households. VI. One catch (five females of C. pipiens pallens) was made inside of a cave which is apart by 150m from an infested households. The numbers of mosquitoes collected at these villages showing a variation of microfilarial incidence, with those harboring filariae are tabulated in Table 1. The total numbers of individuals of 15 species collected and dissected and the natural infection rates of them are given in Table 2, together with the findings on the suitability of them summarized from the data of Mochizuki (1913), Hu (1940), Yamada (1927) and Omori (1952, 53, 54). The full account of the natural infection of C. pipiens pallens collected by different methods are indicated in Table 3. In Table 4, the inferable developmental stages of filarial larvae in mosquitoes at the time of capture and the dates on supposition of having taken infective meal are given, which are taken from that whether the mosquitoes were engorged with blood or unfed when captured and from the advance in development during the elapsed days from capture to dissection supposing that the larvae have just reached the stage found at the time of dissection according to the temperature condition of the days. The feeding habits of mosquitoes captured in the areas are illustrated in Fig.1. The seasonal distribution of natural infection rate of C. pipiens pallens is illustrated in Fig.2. From these tables and figures the general results are summarized as follows. 1) This is a report dealing with the result of examination on the natural infection of mosquitoes collected by various methods during from 1951 to 1954 at 15 filarious villages in Western Kyushu, Japan. 2) Of 15 species of mosquitoes collected in human dwellings or by human-baited-trap and in cattle-sheds, or by goat-trap at or near infested households, two species, Culex pipiens pallens and Aedes togoi were only found infected with filariae (Table 1 & 2). 3) C. pipiens pallens is the most predominant and is fond of humans to feed (Fig.1) and moreover its natural infection rate is the highest, thus showing that this species is the most dangerous in these districts. A. togoi is common especially in villages located in rocky coastal region but the number of its comming into houses or cattle-sheds is rather less numerous than the next predominant several species and is no more attracted to humans than to cattles and is very low in the natural infection rate (an infected female was captured early in the morning at the inside of a house h

(View PDF for the rest of the abstract.)