(1) The medical importance of trombiculid mites is indicated and reference is made to their geographical distribution.
2. Methods used by investigators for assessing the distribution of the mites in the Far East are discussed as well as methods used in the study of Trombicula autumnalis.
3. A detailed account is given of a light-trap and the method of using it; the reaction of the mites to the trap is discussed.
4. The results of a detailed survey are presented to show the significance of the method in relation to the distribution and bionomics of trombiculid mites.
The main results of this survey are as follows:
(a) The mites are characteristically distributed in pockets of infection.
(b) A study of the ecoclimatic conditions favourable to mites shows that humidity is a limiting factor.
(c) The free-living larval stage disappeared from the soil in October, and nymphs were found in the soil in September. These facts, together with the results of laboratory breeding experiments at room temperatures, which showed that it was normal for nymphs to appear in October, indicate that the nymphal and adult stages over-winter.
(d) Mites are not associated with fruit bushes if the physical conditions of the soil are unfavourable. A tentative explanation is given to account for the frequent association of fruit bushes with miteirritation.
(e) The gregarious habit of clustering is primarily due to a tactile response.
5. It is concluded that the light-trap method offers a new approach to the problem of surveying an area in order to diagnose trombiculid mite infection. It is also of value in obtaining abundant material for laboratory investigations.