90
Experiments on the Transmission of Rinderpest
only one was left which refused to feed and died the following night, indicating
thereby that those flies which refused to feed on the first day had possibly a
meal of blood before they were captured. The one that fed on the first day was
again tried on the fourth day for about an hour when it showed no inclination
to feed and was found dead the next morning.
A second experiment with a further batch of 60 flies was carried out
and 40 out of them were seen to feed, out of which 35 were probably newly-
emerged, as judged by the colouration of their eyes. The duration of feed
in these cases ranged from 4 to 10 minutes. Thirty out of these 40 (the
rest being dead) were again tried on the fourth day when only one of them
was noticed to feed for about a minute and a half. These 30, along with the
remainder of the first day's unfed lot, were afterwards tried every day for the
next 9 days till all of them died. During this period not one of them was
seen to feed.
The foregoing experiments were repeated during May and June of the
following year and in these a total of nearly 80 flies were used. Of this number,
only 6 could be induced to take two complete feeds, the interval between
these feeds being about 72 hours in each case.
In another experiment the interrupted method of feeding was employed,
using a batch of 17 flies. Eleven started feeding and were allowed to continue
the feed for about half a minute. These were then separated and the rest dis-
carded. Next morning 2 were dull and refused to feed, while the other nine
commenced feeding again but were interrupted after about a minute. The
dull ones died during the night. On the third day, 7 out of the 9 were
living and 6 of these were observed to feed once again and were allowed to
do so for another half a minute, while the seventh was injured and subsequently
died. On the fourth day, 4 out of 6 fed readily and were interrupted after
a minute, while the other two refused to feed and died, one in the afternoon and
the other the following morning. On the fifth day, only 3 survived. Two
of these started feeding and were disturbed after half a minute, while the
third was allowed to complete its feed and took 3 minutes to do so. This
fly was kept separately. On the sixth day, the incompletely fed ones again
commenced feeding but were interrupted after half a minute. On the other
hand, the completely fed one refused to feed again although allowed half an
hour to do so, once in the morning and again in the afternoon. On the
seventh day, only one of the interrupted feeders and the one that had been
completely engorged were found living. The former could be induced to
feed again only for about a minute and never fed again during the following
five days of its life in spite of repeated trials, while the latter refused to feed
altogether and was found dead the next morning.
From the observations recorded above, it would appear that, while the
fly feeds readily a number of times by the interrupted method, it rarely takes
more than one complete feed in captivity. This would seem somewhat
remarkable in view of what is known concerning the feeding habits of other
species of Tabanus. Thus, according to Patton and Cragg [1913], Tabanids
feed every third day, while Cross and Patel [1921] observed that Tabanus
nemocallosus sucked blood four times within 5 days and ten times within 16
days.