86 HABITS 



(i) The habits of flies after feeding on fluids. 



" A number of observations were made on the habits of flies 

 after feeding on various fluids. After gorging themselves the 

 insects usually climb up the sides of the cage and move from 

 place to place, frequently stopping to rub one leg against another 

 or to clean their heads and wings by passing their legs over 

 them. At intervals, however, they sit still and large drops 

 of fluid, coloured red or blue, if the food has consisted of 

 carmine or nigrosin syrup, or opaque and white if it has 

 consisted of milk, exude from the tips of their proboscides. 

 These drops gradually enlarge until they are about equal in 

 size to the insect's head. After a longer or shorter period the 

 drop is slowly withdrawn or deposited on the glass. Flies are 

 frequently observed to exude and withdraw such drops several 

 times. If disturbed they either deposit them or withdraw them 



/ 



Fig. 23. House-fly in the act of regurgitating liquid food. Such a drop when 

 deposited forms a 'vomit' spot. (From Hewitt, 1912, p. 30.) 



with great rapidity. When deposited on the glass, as frequently 

 happens, the drops gradually dry and each gives rise to a round 

 stain with an opaque centre, surrounded by a clearer zone 

 bounded by a distinct thin, more opaque marginal ring (see 

 Plate XVIII, fig. 2). On watching these flies the impression 

 conveyed is that the insects have distended their crops to an 

 uncomfortable degree and that some of the food is regurgitated 

 in order to relieve the distension. 



" Flies have often been seen to suck up the drops deposited 

 by their companions. 



" Whatever may be the cause of the procedure the habit is 

 very common after feeding on all kinds of fluids, such as milk, 

 syrup and sputum, and the stains or ' spots ' left by these drops 

 can be recognised on all surfaces on which flies naturally settle. 



