174 
J. L. HARRISON 
Table i 
Times at which Chiggers Detach 
For Trombicula akamushi and T. deliensis on Rattus rattus jalorensis and R. r. argentiventer 
only, is shown the total numbers of chiggers found at the 8 a.m., 12 noon, and 4 p.m. counts. 
For comparison the numbers in italics are that part of the total proportional to the time interval 
(i.e. the “ expected ” number if the mites detached at random). The observed and expected 
numbers are compared by a chi-squared test, giving the probability that the difference is due 
to chance. 
Mite 
Rat No. of 
Gross number of mites at: 
chi 2 
Probability- 
rats 
8 a.m. 
12 noon 
4 p.m. 
T. aka. 
... R.r.a. ... 13 obs. ... 
1,916 
... 291 
199 
exp. ... 
1,604 
... 401 
401 
.. 190 
less than 0.001 
T. aka. 
... R.r.j. ... 10 obs. ... 
348 
... 108 
16 
exp. ... 
314 
79 ... 
79 . 
.. 64 
less than 0.001 
T. del. 
... R.r.a. ... 9 obs. ... 
62 
10 
2 
exp. ... 
49 
... 12 ... 
12 . 
.. 12 
0.002 
T. del. 
... R.r.j. ... 16 obs. ... 
55 i 
... 54 ... 
40 
exp. ... 
431 
... 107 ... 
107 . 
.. 100 
less than 0.001 
If we accept these mites as typical, therefore, we must consider the feeding times not as a 
continuous distribution, but as tending towards a discrete series at one day intervals. 
Fig 1 shows the sort of result obtained. It shows the total numbers of Trombicula deliensis 
recovered from thirteen rats for each day after trapping. Forty-two of the forty-three feeding 
times are closely grouped about the mean of 3.56 (standard deviation 4.2, standard error of the 
mean 0.65) with a modal group of 2 days, and a maximum of 9 days. The forty third chigger 
was recorded as feeding for 27 days, a deviation which, were the distribution normal, would be 
1. Recoveries of Trombicula deliensis from 13 rats exposed for one day. Heights of 
columns =numbers of chiggers recovered each day after retrapping the rat. 
Broken line = arithmetic mean. 
STUD. INST. MED. RES. 
