6 5 
may be done. It seems that G. palpalis flexes its proboscis in two 
distinct ways. The first, the more usual, is a sharp bending of the 
labella. Flies who had just fed were often seen, while preening 
themselves, to sharply flex their labella so that they stood out, 
almost at right angles, from the palps. Fhe second movement was 
only seen in flies from whom the palps had been removed preparatory 
to dissection. In these the whole proboscis bowed until it described 
a well-marked curve directed ventrally. The second movement is 
probably due to the contraction of the powerful “ tendons " lying in 
the labium * 
G. palpalis does not fill with blood so quickly as is usually 
imagined. The length of time between the insertion of the proboscis 
at the commencement of feeding and its withdrawal was noted in 
flies carefully fed on an experimental animal. The greatest care was 
taken to avoid disturbing them. The longest fed for eleven minutes, 
the shortest for minutes, while the average was slightly over three 
minutes. 
The extraordinary way in which tsetses distend themselves with 
blood is well known. 
A female G. fusca was watched steadily filling itself with blood for six minutes 
It was stopped feeding then lest it should burst itself. Its abdomen was so distended 
that there was a space of almost a millimetre between each of the dark coloured 
chi tin plates on the dorsal surface of its abdomen. 
In about a minute after feeding commences a drop of yellowish or 
brownish, opaque, but liquid faeces is extruded and a minute later 
while still feeding— clear serum rolls, drop by drop, from the anus. 
Sometimes tsetses who were plainly in need of food absolutely 
refused to attempt to feed. Others frantically probed animal after 
animal and failed to get blood. It has occasionally been thought that 
in captivity male flies did not seem to be so eager for blood as were 
the females. 
In the cool of the early morning or evening G. palpalis is not 
nearly so voracious as at mid-day. 
One is, however, occasionally surprised, as at Lubefu on the Kasai, in finding 
them active and biting freely early on a cold damp morning, or as at Kasongo and 
I.ukolela in seeing them feed until dusk and fly actively about after dark 
G. palpalis has twice been caught about the lamps at night. 
I hese flies sometimes seem to prefer natives to Europeans, and dark to light 
coloured clothes. This first point is frequently commented on by European-, 
travelling in canoes. They see that the native paddlers are worried by the flies while 
they are themselves often comparatively unmolested. 
* Memoir XVIII of the Liverpool School of Tropical Medicine, page 53. 
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