66 THE STRUCTURE AND FUNCTION OF THE PROBOSCIS 



A large number of experiments were carried out in order to 

 ascertain the size of the largest bodies which could be swallowed. 

 For this purpose flies were made to feed on drops of various 

 fluids containing in suspension bodies of definite size and shape 

 such as spores of moulds, pollen, etc. Immediately after feeding 

 they were killed and dissected and the crop and intestinal 

 contents examined for the presence of the suspended particles. 

 It was found that the spores of Noseina apis and of various 

 moulds measuring up to '006 mm., in fact all bodies measuring 

 in their smallest axis less than the diameter of the interbifid 

 space, could be readily swallowed. 



The case is however different with larger bodies such as 

 pollen grains. Many feeding experiments were carried out with 

 emulsions of the contents of bees' colons, containing many easily 

 recognizable bodies of various sizes, including pollen grains in 

 various stages of digestion. In most cases pollen grains, except 

 of very small size, could not be detected in the crop or intestinal 

 ■contents of the flies. On rare occasions however numerous 

 pollen grains were found, both in the crop and in the intestine. 

 On closer examination many of these were found to be empty 

 flattened shells, readily distorted to a slight degree, but afterwards 

 regaining their shape. Such bodies could be easily sucked 

 through the interbifid spaces. Still more rarely one or two 

 apparently undigested pollen grains were found. Also in some 

 experiments with recently gathered pollen from the pollen 

 baskets of the healthy bees the grains ('02 x '04 mm.) could be 

 detected in the crop or intestinal contents of a small proportion 

 of the experimental flies. No object measuring more than 

 '02 mm. in its smallest diameter was ever swallowed. 



It was found that objects of comparatively large size were 

 more frequently ingested when suspended in viscid fluids, such 

 as honey, or when flies were endeavouring to extract the fluid 

 from semi-solid masses composed of large particles. 



In experiments on the part played by flies on the dispersal 

 of parasitic eggs Nicoll (iQii, p. i8) found that flies could 

 ingest such large objects as the eggs of tape- worms, measuring 

 up to "045 mm., which cannot pass into the pseudo-tracheaj^ 



^ I'or an account of these experiments see Chapter x\. 



