HOUSE FLIES AND BLUEBOTTLES 181 



came from them." In the case of the bluebottle, at 

 least, and also probably in that of the other fly as well, 

 the sound-producing organ is connected with the thoracic 

 spiracles or breathing apertures, and the harsh and stri- 

 dulating character of the sound suggests what appears to 

 be really the case, that it is caused by the vibrations of 

 hard solid bodies. A hemispherical cavity intervenes 

 between the spiracle and the main tracheal trunk, and 

 in this are situated some hard chitinous processes, by 

 the vibrations of which it is believed that the sound is 

 produced. 



Closely connected with the remarkable power of the 

 wings is the peculiar development of the thorax, which is 

 so characteristic of the Diptera. Roughly we speak, with 

 reference to insects in general, of the second apparent 

 division of the body as the thorax ; but it by no means 

 follows that the part occupying that position and most 

 distinctly visible when viewed from above is hornologically 

 the same in all cases. The complete thorax is composed 

 of three segments, called respectively pro-, rneso-, and 

 meta-thorax ; but in the more highly specialised groups 

 of insects, these three are not equally developed, and 

 sometimes it is one, sometimes another of the three 

 parts, the development of which on the upper surface 

 preponderates over that of the rest. Thus in the Cole- 

 optera (beetles) and Hemiptera (bugs), what is frequently 

 called the thorax on the dorsal view, is in reality only 

 the pro-thorax ; in the Hymenoptera (bees, &c.) it consists 

 mainly of both meso- and meta-thorax, and in the Diptera 

 almost entirely of the meso-thorax. This is the division 

 to which the fore-wings of all insects are attached ; and 

 as in the Diptera this is the only pair of wings that is 

 developed as such, the peculiar conformation of the thorax 

 of a fly finds herein an explanation. The chief thoracic 



