J. L. HARRISON 
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longer than the actual period of attachment and feeding, but the discrepancy is likely to be less 
than the error of estimation, and is, therefore, of little practical importance. 
Some efforts were made by the Scrub Typhus Research Laboratory at Imphal, India, to 
estimate the feeding-times of the local chiggers and thus to gain some idea of their relative 
turnovers (Audy, 1947). The only interval observed was the time taken for the detachment 
of the bulk of chiggers attached to freshly caught wild rats, on which the mites had already 
been feeding for unknown periods; but in the case of the vector Trombicula deliensis , 
experimentally infested rats were observed. Rough estimates were obtained and presented 
with a note that the data required checking by more careful and continued observations. The 
present paper describes experiments involving such observations on species prevalent in Malaya, 
while also presenting a statistical method of dealing with data obtained from wild-caught hosts. 
Materials and Methods 
A number of methods of estimating the feeding time are available, but in the present paper 
only those methods are considered which involve timing the mites feeding on captive animals, 
and the hosts used have mostly been rats. 
The host from which chiggers are to be recovered is kept in a small cage over an enamelled 
iron tray filled with water. The cage is supported on legs which stand in the water, and chiggers 
which detach fall through the mesh of the floor onto the water surface where they float. Faeces 
and other debris fall into the water and sink. The trays are examined from time to time, all 
floating chiggers are picked off the surface, and the trays are cleaned. Chiggers are either 
mounted immediately for identification or they are kept in watch-glasses of water, where they 
remain on the surface until they moult, when the cast skin can be mounted and identified as 
readily as the larva itself. The nymph may be used for taxonomic studies, or for breeding. 
In this way feeding times have been measured on chiggers from three sources : 
1. from the natural infestation of trapped wild rats and other animals. 
2. from the natural infestation of rats during mark-and-recapture experiments. 
3. from the feeding of laboratory-bred mites on immobilised white mice. 
Wild rats trapped alive during the ordinary routine of collecting normally bear chiggers. 
Such rats are often kept until the chiggers detach themselves, thus providing both a measure 
of apparent feeding time and engorged chiggers for breeding. Since there is no way of knowing 
how long the chiggers have been on the rat before it was trapped, the interpretation of these 
figures presents special difficulties which are discussed below. 
In the mark-recapture experiments on rats, the rats are trapped, marked, and released 
again at the point of capture, to be recaptured again from time to time. At each capture the 
rat is lightly anaesthetised and any chiggers present are removed under a binocular dissecting 
microscope. The standard of efficiency of the removal is sufficient to assure that practically 
all mites are so removed. Sometimes a rat is recaptured within 24 hours of release, and often 
such rats bear mites which must have climbed onto the rat during its short period of freedom. 
Such rats are kept until the mites detach naturally so that their feeding time may be measured 
directly. 
The recapture of an infested rat within 24 hours is, however, comparatively rare, and 
therefore, to increase the number of available observations, rats recaptured within 48 hours of 
release and found infested are treated in the same way. 
Chiggers bred in the laboratory are fed by dropping them onto the head of an anaesthetised 
white mouse, which is then imprisoned in a small cage over a tray of water in such a way that 
it cannot scratch its ears (where the chiggers attach). In this way the feeding time can be 
measured fairly accurately, but the drawbacks are that the host and the conditions of attachment 
STUD. INST. MED. RES . 
