TWO-WINGED INSECTS 



377 



see gnat.) When not in use, the proboscis is retracted to save it from 

 injury. (Compare with bee, butterfly, and bug.) 



Though its mouth parts are only adapted for suction (organs of 

 mastication being absent), we nevertheless see flies consuming sugar, 

 bread, and other solid sub- 



stances. These accordingly 

 must be previously dissolved. 

 For this purpose a saliva is 

 poured forth from the pro- 

 boscis. However, this saliva, 

 if it were dropped directly 

 upon the solid morsels, would 

 be absorbed by them, and so 

 lost to the fly; to prevent 

 this, the proboscis is dilated 

 at its end into two large pads 

 (the head of the pestle), which 

 are traversed by narrow canals. 

 The saliva flows along these 

 canals, and is discharged by 

 narrow clefts upon the surface 

 of the pad, where it forms a 

 layer of such thinness as not 

 to be absorbed by the food 

 morsels. On the other hand, 

 a small portion of the latter is 

 dissolved by the saliva, which 

 is then, together with its dis- 

 solved contents, drawn up 

 again through the proboscis. 

 Hence a fly cannot consume 

 food of an insoluble nature. 



/. 



U. 



mib.K4 



House Fly. 



I. 



Head and Anterior Part of Thorax ( x about fifteen 

 times) : At. , A spiracle on thorax ; F. , antennae ; 

 Kt., maxillary palpi; R., proboscis; A., com- 

 pound eye, at II. more strongly (about thirty 

 times) magnified. III. Proboscis (also magnified 

 about thirty times) : P. , Pad -like swellings of the 

 proboscis traversed by canals (K.) ; 0., opening 

 of proboscis. 



E. Development. 



(a) The fly deposits its 

 small white eggs upon all 

 sorts of decaying substances, 

 more especially horse - dung. 

 (This explains the abundance in which this insect occurs in stables and 

 farmhouses.) Only twelve hours later 



(b) The larvce escape from the eggs. They bore their way down into 



