204 DISPERSAL OF PARASITIC EGGS 



distended with white milky juice recognizable as the juice of 

 the tape-worm. In such flies, too, several tape-worm eggs were 

 found in the intestine. It is evident, therefore, that house-flies, 

 although they possess no piercing or biting mouth-parts, are 

 able in course of time to penetrate the fairly tough external 

 covering of tape-worms and to extract the internal contents. 

 In this they are helped to a considerable extent by the fact 

 that tape-worms undergo a process of decomposition (autolysis) 

 independent of putrefaction. This is further hastened by the 

 action of putrefactive bacteria. Dead tape-worms will remain 

 soft and 'juicy' for 48 hours after exposure to the air. Later 

 their fluids evaporate and they begin to shrivel up and become 

 dark brown in colour. Living tape-worms or their segments 

 will remain alive on exposure to air for two or three days, and 

 in suitable media (saline solution, etc.), they may be kept alive 

 for over a week. There can be little doubt, therefore, that living 

 tape-worm segments when expelled from their host may remain 

 a source of attraction to flies for several days. 



" Similar observations were made in the case of round-worms 

 {Toxascaris limbata, Ascaris niegalocepJiahi). These appeared to 

 possess much less attraction for flies. Not infrequently the)' 

 were attacked with some readiness and in the same manner as 

 tape-worms. The extremely thick cuticular investment of round- 

 worms, however, is much more resistant than the covering of 

 tape-worms, and in no case were the flies able to penetrate this 

 even after the lapse of three or four days, by which time the 

 worms had become dry and shrivelled up. 



" It seems worthy of note that solid particles were rarely found 

 in the crops of flies dissected in the course of these experiments. 

 The intestine, however, except in flies which have been feeding 

 for several days on nothing but fluid food, invariably contains a 

 large number of particles. After feeding on faeces, for instance, 

 the intestine becomes filled with the debris of which the faeces 

 are composed, but none of this is found in the crop. The 

 particles met with are of various sizes and irregular shapes, but 

 they rarely exceed "04 mm. in diameter. It would thus appear 

 that when flies feed on fluids or soluble solids such as sugar, the 

 food is first sucked into the crop, and when this is full it passes 



