34 
J. R. AUDY 
Effects of seasonal changes 
Seasonal effects are not actually revealed by the tabular summary though they have been 
found by almost all the workers who have made intensive collections. In the Imphal area 
(1945-1946) larvae of the vector T. deliensis had a pronounced wet-season incidence. They 
were replaced by Euschongastia lanius and the related E. kohlsi during the cold dry season 
(Audy 1947, : 949 j Audy et al. 1953). Collections made in the same patch of scrub a few months 
apart gave completely different pictures of mite incidence. 
Two batches of chiggers from an endemic area, Stonecutter’s Island, Hong Kong, were 
sent to us by Mr. J. D. Romer. T. deliensis made up the bulk of the first batch while the second 
batch contained no T. deliensis but a number of chiggers of three species of Euschongastia in the 
lanius-gvowp (new subgenus Audy 1953, this Study , p. 152). This was so like the findings 
in Imphal that we were able to infer that the first collection was made during a wet hot period 
and the second during a cooler dry season (Table 2). 
Table 2. 
Infestation of Rattus rattus sub-species in Manipur (Imphal), and 
Hong Kong (Stonecutter’s Island). 
Mites per rat 
(approximately) 
Rain- 
fall 
Temperature 
(means in °F) 
No. of 
Place. 
Trombicula 
deliensis. 
Euschongastia 
(/amws-group). 
spp. 
p.m. 
ins. 
Max. 
Min. 
rats. 
Period. 
Imphal 
50 
nil 
12 
85 
72 
over 200 
July, 1945 
Hong Kong ... 
20 
nil 
15.6 
87 
81 
8 
Aug.-Sept., 
1949 
Imphal 
0-5 
30 
nil 
79 
72 
120 
Jan., 1945 
Hong Kong .., 
nil 
10 
1-4 
76 
70 
9 
Jan.-Feb., 
1950 
Mohr (1947) carried out some very interesting studies in a transect across forest edge in 
New Guinea during dry weather. He found more T. deliensis on rats from the forest than on 
those from open scrub. In the light of our experience in Imphal and Malaya we would inter- 
pret this as a differential seasonal effect. The open scrub rapidly dries out while the forest 
floor remains damp, especially in ravines and gullies. The same effect was found in Imphal 
where 70-80 per cent, of rats caught in both open grassland and adjacent gallery forest during 
the height of the rains were infested by T. deliensis , while during the dry season the infestation 
dropped to below 0.5 per cent, in the scrub but to over 10 per cent, in the forest. 
Distribution of the major groups of trombiculids. 
Table 3 shows consolidated data for the countries summarised on p. 36ff., supplemented 
by data for countries outside this area. It is difficult to draw conclusions from such overall 
data but several general impressions emerge. The following tentative observations may be 
added to those of Wharton, aided by Fuller (1952). The number of Leeuwenhoekiine species 
recorded is often a matter of special host selection, but Africa, the Americas, and Australia 
appear to have the most species. In Australia (and to a much lesser extent in New Guinea) 
we have a wealth of species of Acomatacarus on mammals and Australasia certainly appears 
to have been an active centre of evolution of this group. The African representatives include 
a number of species of Acomatacarus from reptiles and from a few mammals. 
STUD. INST. MED. RES. 
