142 INJURIOUS AND USEFUL INSECTS 



and mounting in balsam according to the instructions previ- 

 ously given (p. 13). 



The proboscis and its joints will be more easily understood 

 if you suppose that your arm above the elbow stands for 

 the rostrum, and the fore-arm for the labium. The hand, 

 with the palm turned downwards, will represent the palette. 

 The maxillary palps must be imagined to project just above 

 the elbow. Each labella is supported by a chitinous frame- 

 work, formed of the radiating terminations of the salivary 

 ducts, on which appear many transverse lines. The surface 

 is roughened, and acts as a file; it is also moistened by 

 the saliva. When a fly alights on a lump of sugar, it dis- 

 charges a small quantity of saliva, spreads it about with the 

 palette, rubs the moistened surface, and then sucks up the 

 dissolved sugar. The friction of the palette of a house-fly 

 upon a sensitive part of the body, such as the back of the 

 hand, is described by some people as " biting," but the 

 skin is never pierced. There is, however, a Dipterous insect, 

 the stable-fly (Stomoxys), very like a small house-fly, which 

 enters houses, and is able to pierce the skin with its 

 labrum. 



The thorax of the blow-fly, as seen from above, consists 

 of three parts, one behind another. There is (r) a short, 

 transverse plate, (2) a large squarish plate, to which the 

 wings are attached, and (3) a small triangular plate, pointing 

 backwards. It would seem natural at first sight to identify 

 these plates with the three thoracic terga ; but the arrange- 

 ment of the internal organs, and especially of the muscles, 

 shows that they are all parts of the mesothorax. The true 

 prothorax and metathorax are small, and can only be dis- 

 covered by a close scrutiny. The great activity of the fore 

 wings, and the practical suppression of the hind wings, have 

 apparently led to an enormous development of the meso- 

 thoracic muscles, and consequently of the mesothoracic 

 segment, while the prothorax and metathorax have been 

 reduced in a corresponding degree. 



On the under surface of the thorax, between the fore and 

 mid legs, the large mesothoracic sternum can easily be seen. 

 The prothoracic and metathoracic sterna are much less evident. 

 On each side, and just dorsal to the base of the fore leg, 

 is an oval (mesothoracic) spiracle; another (metathoracic) 



