HOUSE FLIES AND BLUEBOTTLES 193 



the appearance of breathing tubes or tracheae ; they differ, 

 however, in being open on their ventral faces, and in 

 having the strengthening rings, not merely incomplete 

 and unconnected with one another, but also much stouter 

 and farther apart than would be the case with tracheal 

 tubes. On account of their superficial resemblance to 

 the latter, however, they are called pseudo-tracheae. The 

 photographs in Plate IT. show their microscopical appear- 

 ance. They are, of course, not breathing organs, but 

 serve as a sucking and straining apparatus, for conveying 

 to the mouth the juices on which the fly feeds. 



They communicate, either directly or indirectly, 

 through the two inner marginal tubes, with the mouth, 

 whence they can receive a supply of saliva, to be used 

 in moistening the object which is to yield nutriment 

 and dissolve whatever soluble matter it contains, the 

 resulting liquid being then conveyed along the channels 

 to the mouth. For example, when a house fly brings 

 its proboscis down on a lump of sugar, saliva is poured 

 out and dissolves a little sugar, and the syrup is then 

 sucked along the pseudo-tracheae into the mouth, which 

 is situated at the junction of the two labellae. Similarly 

 the juices of flesh, whether cooked or uncooked, can be 

 extracted by the bluebottle. The supply of saliva is 

 very copious, and the glands which secrete it are corre- 

 spondingly large, extending right down into the abdomen. 

 If a fly be held between the fingers so that the move- 

 ments of the proboscis can be seen, there will soon be 

 observed a clear drop of saliva collecting over the inner 

 surface of the labellse, and retained there partly by the 

 aid of numerous hairs round the edges. 



But there is at the same time another set of organs 

 used in the taking of food, consisting of some fifty or 

 sixty forked rods called " teeth," placed in a radiating 



N 



