In the previous paper of this series on the ecology of trombiculid mites, some accounts of the collecting method using bakelite plates were reported. The technique made it possible to survey the unengorged stage of the larvae of chigger in natural conditions on the fields. The unengorged larvae are the very stage which attack human or other hosts and are of epidemiological importance. The present study was designed to obtain the informations about the surface distribution of Trombicula scutellaris Nagayo et al, 1921, which is suspected to be the vector of Shichito Fever, the scrub typhus in Izu Seven Islands of Tokyo. The experiments were conducted at the Hachijo Island, the most southern one in the Archipelago, in December 1953, during the season of the endemic. Small bakelite plates (5×5cm^2) were placed upon the surface of the ground and the number of chigger mites that crawled up to the plate after 5 minuites was observed. To survey all over the surface of 1 square meter ground, two hundred and twenty two sheets of small plate were used. They were sufficient to cover nearly all of the surface of 1m^2 area. The temperature and relative humidity (2 cm height over the surface), the illuminating intensity, pH of soil at the surface and the soil temperature (10cm beneath the surface) were also observed at 25 points in the area. The results of the experiments indicate that the unengorged larvae of Tr. scutellaris are not scattered at random on the ground surface, but they form a certain type of congregation or patch. The findings that support this view are as follows : 1. The frequency distribution of the observed number of mites per plate did not fit to that of Poisson's distribution. 2. The divergence coefficient (F_0=V/X^^-) was obviously larger than 1. 3. The gamma type compound of Poisson distribution fitted to our case. 4. Un-uniformity of numbers of scattered mites was also shown by the analysis of variance. 5. The spacial correlation was examined according to the method introduced by Student (1907). 6. It was pointed out that both the structural correlation coefficient (r) and ratio (η) were not zero, and r>0, η>0 were confirmed. Thus the surface distribution of this chigger mite was shown not to be that of "by chance, " and the structure of spacial distribution apparently consisted of patches. The correlations between the observed number of chiggers and the micro-meteorological observations were also examined. The results showed that the temperature had positive correlation with the number of chiggers and the humidity negative one. The soil temperature, illuminating intensity and soil pH were found not to have any correlation.

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1. Simulium aokii Takahasi, 1941 (possibly the synonym of S. oitana) is one of the commonest blackflies in Japan which seriously attack humans. In the present paper, the distribution and behavior of this species observed by the author and his colleagues are reported. 2. Eggs, larvae and pupae of this species were found abundantly in lowland streams such as irrigation ditches. They were occasionally collected in mountain streams of Honshu and Kyushu. 3. The oviposition of the females were made in the evening on wet substrates on or near the stream surface. The author observed, for example, that in 70 minutes 16 females deposited their eggs side by side in a large mass, finally attained to about 0.7cm wide and about 4cm long, on the surface of a wet blade of the aquatic plant (Sparganium ramosum), one of their most typical ovipositing substrates. While a few females laying eggs on their favorite substrates, the others were flying around to wait for their turn. An egg mass which was oviposited by a female consisted of 150-300 eggs. The eggs required about 10 days to hatch in the laboratory in March, under the room temperature of 8-16℃ at noon. 4. The larvae were found most abundantly in clear lowland streams of 1 to 0.3m wide and under the flowing velocity of 10 to 30m per minute. 5. In nature, the larvae prefer to attach on the tip of the lower surface of long tapering leaves less than 10cm below the water surface. However, the main facter in the choice of their habitat is not the kind of substrates, but their environmental conditions stated above. 6. The larvae being sedentary in its habitat on the favorable conditions, but they frequently change their position by looping-movements and by hanging with a fine silk thread which is secreted from the salivary slit. 7. The flight range of the females were observed in biting experiments on humans set in positions of every 100 meters from their closest breeding streams. The maximum number was seen right close the stream, but a few were collected up to the most distant site of 1, 000m. 8. Their seasonal occurrence was observed by collections at least once a month all through the year. The biting activities were found over the year, but were most serious from May to July. 9. The host preference for human, horse, cattle and goat was experimented. This species was found to be mainly attracted to human. Only a few specimens have so far been collected from cattle or horse.

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This report contains the results of investigations on the increase in effectiveness of DDT by adding piperonyl cyclonene for the DDT-resistant insects. In the present experiments DDT-resistant or nonresistant strains of human body louse (Pediculus humanus corporis), common housefly (Musca domestica vicina), and Drosophila melanogaster were used. These insects were exposed on the residues of aceton solutions of pp'-DDT containing various amounts of piperonyl cyclonene applied to filter paper in a petri dish. It revealed that piperonyl cyclonene, added the same or a halp amount of the DDT, markedly increased the toxicity of DDT-resistant insects, but showed less marked effect upon the susceptible or nonresistant strains.

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