XVI] BIONOMICS 263 



bodies for this species is very marked and has been noticed by 

 many observers. 



Thus Simpson, while travelling in a river-launch up an 

 infested river in Nigeria, noticed that as long as the boat 

 remained stationary at the landing stage, no tsetse-flies at- 

 tempted to come on board, although swarms of them were 

 flying about near the banks of the stream. But as soon as 

 the launch moved away, the flies rose in swarms and flew after 

 the boat, many settling on the bows and deck and being carried 

 up the stream for very considerable distances. 



The fly is not seen until after sunrise, the insects usually 

 appearing about 7 to 7.30 a.m. on bright still mornings. The 

 time of appearance, however, varies with the locality and in 

 densely shaded regions may be considerably later. In the 

 afternoon the numbers diminish about 4 to 4.30 p.m. and on the 

 approach of sunset most of them disappear ; only very excep- 

 tionally have they been known to feed at night. In cloudy or 

 rainy weather very few flies are to be seen, and wind at once 

 drives all of them into shelter. Although the presence of the 

 sun seems necessary for the fly's activity, it prefers the shade, 

 and indeed, if exposed to the direct rays of the sun dies in a 

 very short time. 



It has been noticed by all observers that the tsetse-fly 

 prefers a dark skin to a white, and dark clothes to light ones. 

 Europeans when accompanied by natives are rarely molested 

 to any great degree. Ensor states that white clothing confers 

 the greatest degree of immunity from the attacks of these 

 insects, and this statement is supported by all travellers in 

 fly regions. Button and Todd write as follows : " One day 

 while coming down the Kasai the Captain put on a blue cloth 

 jacket. Many more tsetses were found to settle on his coat 

 than on the white duck suits of the European passengers and 

 of the negro steersman." As a rule palpalis does not bite 

 through clothes but it is easily able to pierce socks and ordinary 

 white drill suits. 



When feeding the fly spreads out its front pair of legs, and 

 then lowers its proboscis into the skin (Fig. 68). It then begins 

 to gorge and its abdomen swells visibly and the insect may 



