40 THE ROYAL SOCIETY OF CANADA 



Period of Digestion. 



After feeding the fly seeks a quiet comer and settles down, usually 

 on a vertical surface with its body vertical and head upwards, its 

 body being flattened against the surface upon which it is resting. 

 In so short a time as half an hour after feeding the abdomen may 

 almost have regained its normal size, though sometimes it remains 

 swollen longer depending, no doubt, upon the length of time since 

 the previous meal; a hungry fly will reduce the size of the crop or 

 sucking stomach more rapidly than a fly whose digestive tract still 

 contains food. In about two hours the red contents of the crop can 

 no longer be seen externally. 



The expulsion of clear fluid drops from the anus during the act 

 of blood -sucking has already been described. This continues after 

 the fly has ceased feeding. Upwards of a hundred drops may be 

 expelled. In a few hours, however, the drops are no longer fluid and 

 clear to opaque, but become viscid and clear in character. The 

 number of such viscid faecal deposits expelled in the case of three 

 flies was 34, 34 and 32. One fly expelled altogether 106 clear fluid 

 and clear viscid spots within six hours of feeding. 



The first appearance of digested food in the form of brown, 

 faecal spots occurred, in the flies under observation, 3| hours after 

 feeding. In the case of one fly undigested blood v (human) was voided 

 from the anus ten minutes after the fly had fed eight and one-half 

 minutes, but this was evidently abnormal as the fly died shortly after- 

 wards. The average interval, approximately, between the feeding 

 and the first appearance in the faeces of digested blood was about six 

 and one-quarter hours, in nine observations. The longest period 

 observed was nine hours and fifty minutes. 



The length of time occupied in the digestion of the whole meal 

 varies, depending upon the quantity of blood taken. The willingness, 

 of the flies to feed before the complete digestion of the previous meal 

 rendered it not always possible to make exact observations in this 

 point. The digestion of the meal before another feeding was per- 

 mitted in ten instances and the time required for the digestion of a 

 full meal varied from 49 hours 45 minutes to 95 hours; the average 

 length of time was 72| hours. The completion of digestion was in- 

 dicated by the cessation of the brown faecal spots and the voiding 

 of clear faecal spots. 



The rate of defaecation gradually decreases as the meal is digested. 

 The accompanying Table shows the average number of brown faecal 

 deposits and clear post-digestion deposits passed during each twelve 

 hour period. The fly No. 8g. illustrates what I think may be taken 

 as a typical case. It will be noticed that the greatest number of faecal 



