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. S. aokiiは本州, 九州に広く分布し, 平地から山脚地帯にかけての小流に発生する極く普通の種類である.屡々山間の溪流にもみられる.又, S. venustumと共に本邦に於て激しく人を襲う代表的な種類である.2.著者の観察によれば, 産卵は夕刻に多く, 水中の植物, 岸の上から垂れた草の葉或はコンクリート壁を問わず産卵場所は水表上の常に水沫で濡れる状態の場所であつた. 1〜数10匹で集団的に或限られた小面積の好適な場所に短時間に行われた.そして1〜数10個の卵塊は集合的にうみつけられた.1匹の雌による1回の産卵数は150〜300と考えられた.3月の室温で約10日で孵化を始めた.3.幼虫は川幅約1〜0.3m, 流速10〜30m/分, 水質は水底が見える程度の清流に最も多数みられた.4.幼虫が水中で吸着している基物は, 基物自体に対する選択よりも, 寧ろ基物のおかれている環境条件によつて選択される.自然界では水面下約10cm以内の植物に大部分吸着している.特に細長い葉面の裏面先端部に多い.5.幼虫は水中で, 好条件下では数日間同一場所に固着しているが, 又或程度尺取虫状匐匍運動或は分泌した糸に懸垂して移動を行つている.6.蛹の発生水域から1, 000mの距離迄100mおきに実験者が位置し, 刺咬する成虫を採集して活動範囲調査を行つた.発生水域附近が最も刺咬数は多く, 1, 000mの地点でも若干採集された.7.成虫の刺咬活動は周年みられるが, 特に3月から7月にかけて最盛活動を示した.8.雌成虫は最も人を好んで吸血するものと考えられた.牛, 馬及び山羊を用いて襲来成虫を採集したが, 馬で9匹, 牛で1匹採集されたに過ぎなかつた.

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(1) DDT抵抗性昆虫の駆除に関する基礎研究の一部として, Pyrethrinの協力剤の一つであるPiperonyl cycloneneをDDTに添加した場合の, 抵抗性昆虫に対するDDTの殺虫力の増大について, コロモジラミ成虫, イエバエ成虫, 及びキイロショウジョウバエ成虫を供試して実験をおこなつた.(2)抵抗性のコロモジラミ, イエバエ, 及びキイロショウジョウバエに対し, DDTアセトン液(pp'-DDT 3%)に, Piperonyl cycloneneを3%, 或いは1.5%混入すると, DDT単独の実験区に比較して, 極めて優れた効果が認められた.(3)抵抗性の割合弱い系統に対しては, Piperonyl cycloneneの添加により, 抵抗性の強い系統に対するほどの効果の増大は認められなかつた.(4)Piperonyl cycloneneの添加をDDTの5〜10倍の多量にした場合は, DDTと等量, 乃至は半量添加の場合よりも, 却つて殺虫力は減退する結果を得た.

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