DISPERSAL OF PARASITIC EGGS 205 



into the intestine directly, as noted by Graham-Smith. Insoluble 

 solid food, however, is probably taken directly into the intestine. 

 This suggests that the fly is able to exercise some voluntary 

 control over the passage of food into the crop or the intestine 

 respectively. 



" The feeding of fly larvae was studied specially in regard to 

 their behaviour towards round-worms. The voracity with which 

 larval flies feed is well known, and the increase in their size from 

 day to day is remarkable. They appear to be as omnivorous as 

 adult flies. When fresh round-worms were offered to the larvae, 

 they were at once attacked. . The larvse swarmed over the worms, 

 nibbling at them with great vigour. They seemed, however, 

 quite unable to penetrate the tough cuticle unless there were a 

 crack or a small tear, and, if other food were not provided, the 

 larvffi died. On the other hand, when the worms were cut or 

 broken before being introduced, the larva; devoured the internal 

 parts with extraordinary rapidity. Starting at one end of a 

 broken piece, they would eat their way right through to the 

 other end leaving nothing but a tube of cuticle. In this way 

 half a dozen larvae would devour, within two or three days, a 

 large worm 20 or 30 times their own bulk. On examining 

 larvae which had fed on female egg-bearing worms, large 

 numbers of eggs were found surrounding the larvai but not 

 actually sticking to them. On examining the intestine of the 

 larvae no intact eggs were ever found, but numerous fragments 

 of shells were always recognizable. No embryonic worms in 

 any stage were seen. From these experiments I am convinced 

 that even full grown larvae are unable to swallow unruptured 

 eggs as large as those of the worms used ("O/ mm.). 



Methods. 



" The common house-fly, Miisca doniestica, was used almost 

 exclusively. Only a few experiments were made with the lesser 

 house-fly, Fannia canicnlaris, and the blow-fly, Cnlliphora ery- 

 throccphala. In most cases the flies were obtained from the 

 surrounding locality, but artificially-reared flies were used on 

 several occasions. 



