68 MEDICAL ENTOMOLOGY 



ill-defined bundles of muscles running between the internal surface of 

 the neck and the prothorax. 



The structures which pass through the neck are numerous, and some 

 knowledge of the anatomy of this region is of importance in dissection. 

 When the head is separated from the body by traction these structures 

 are exposed, and with care they can be separated from one another, and 

 those not required cut off. In the first place, there are always two compara- 

 tively large tracheae, one on each side, passing from the thoracic spiracles 

 to supply the head with air. On the inner side of these there is a pair of 

 salivary ducts or the commencement of the glands. In the middle line 

 there are, from the dorsal to the ventral side in order, a minute blood 

 vessel, a prolongation of the dorsal heart, so small as to be only seen in 

 the larger flies ; the duct of the crop or oesophageal diverticulum,* and be- 

 low it the oesophagus ; and below this the nerve cord which connects the 

 brain with the thoracic ganglia. A little experience with the larger flies 

 enables one to distinguish between these by their size and colour, and in 

 dissection it is always sound to cut away boldly all that is not required. 



THE THORAX 



The principles of the structure of the walls of the thorax have already 

 been explained in the paragraph on the segmentation of the insect body. 



Briefly, the typical thorax should consist of three parts, 

 Segmentation 'f 



each representing a segment, and termed the prothorax, 

 mesothorax, and metathorax, respectively. The wall of each segment 

 should be composed of a tergite or dorsal plate, a sternite or ventral 

 plate which may be divided in the middle line, and two lateral or 

 pleural plates, the episternum in contact with the sternite and the 

 epimeron in contact with the tergite. But in such a highly-specialized 

 group as the Diptera one would hardly expect to find a primitive 

 arrangement persisting, and the relations of the parts are moreover 

 profoundly altered by the great development of one pair of wings, 

 especially since the majority of the Diptera are powerful flyers. Indeed, 

 this factor, with the consequent necessity for adequate surface for muscle 

 origin and insertion, has introduced so much change that it is a matter of 

 the utmost difficulty to allocate the various sclerites to their original 

 positions. The wings which have persisted are the mesothoracic pair, 

 and in consequence the mesothorax has become greatly increased at 

 the expense of the segments in front and behind it, until it now occu- 

 pies by far the greater portion of the external wall. The prothorax 



* In Musca and its allies the duct of the crop is ventral to the oesophagus. 



