I 9 4 OUR HOUSEHOLD INSECTS 



manner just round the mouth. When the labellse are 

 closed, or only partially open, the teeth are concealed 

 between their folds, and cannot therefore be brought 

 into use. But when the labellse are opened to the fullest 

 extent, they are used for rasping the surface of the food, 

 thus aiding in exposing new surfaces to the action of 

 the saliva. It follows, therefore, that repeated attacks 

 of the teeth and the tips of the chitinous rings of the 

 pseudo-tracheae upon any delicate surface, must produce 

 numberless little scratches and asperities, which will be 

 more or.less detrimental to the aspect of that surface. 

 This, quite apart from fly-spots, i.e., dabs of excrement, 

 is a source of damage to pictures, the covers of books, 

 or other objects the exposed surfaces of which are 

 delicate enough to receive impressions from such en- 

 graving tools as the fly's proboscis carries. The extreme 

 flexibility and power of movement the labellae possess 

 can easily be observed by holding the fly between finger 

 and thumb, when they will be set in incessant motion, 

 their outline continually changing as first one part and 

 then another is bent, now up, now down. Thus the tip 

 of the proboscis can be closely applied to any kind of 

 surface, into the little irregularities of which the flexi- 

 bility of the labellse easily enables it to fit. Flies, Musca 

 domestica in particular, seem to have the power of ex- 

 tracting nutriment from the most unpromising materials, 

 witness the perseverance with which, on a hot summer's 

 day, they keep dabbing their proboscis down upon one's 

 coat as they course hither and thither over its surface ; 

 for it can hardly be supposed that they would so per- 

 sistently keep up the practice if they found that they 

 derived no benefit from it. 



