422 MEDICAL ENTOMOLOGY 



required intact the best procedure is to open the thorax down the 

 middle dorsal line, and to evert the two halves, thus separating the 

 muscle bundles between which the glands lie, and rupturing the small 

 tracheae which hold them in position. The arch of chitin at the 

 thoracic inlet is then broken through very cautiously, and the head 

 pulled away from the thorax as in the dissection of the smaller flies. 

 As soon as the expanded portion of the proventriculus comes into view, 

 the oesophagus anterior to it is cut across, and the organ allowed to 

 retract. The large cervical tracheae and the nerve cord are also cut, 

 so that the only structure connecting the head with the thorax is the 

 common duct of the salivary glands. When this has been isolated and 

 seen, thrust a needle through the middle of the head and turn it so that 

 the proboscis points forwards, thus keeping the duct in a straight line. 

 After a few gentle tugs the glands emerge from the thorax. 



When dealing with large insects, such as T. albimediiis or striatus, 

 especially when the mid-gut is full of blood, it is best to nip up a piece 

 of the integument at one side with a pair of fine forceps, and then to slip 

 one blade of a fine pair of scissors underneath, and to cut down as 

 far as possible. When this has been done the dorsal wall can be reflected 

 to the opposite side and cut off entire. In dealing with Tabanidae great 

 care should be taken to prevent the duct of the crop becoming entangled 

 with muscle fibres, if it or the proventriculus is required for examination. 



The dissection of the Muscid flies is a little more difficult on account of 

 the greater length of the alimentary canal and the extension of the 



salivary glands into the abdomen. The alimentary tract 

 Dissection of Muscid Q female flies may be extracted by the f o n owing 



method. Having pulled off the legs and wings, grasp 

 the fly by the head in the left hand, holding it firmly. This results in 

 the extension of the ovipositor, owing to the displacement of air from the 

 head to the hinder part of the body. As soon as the ovipositor is extend- 

 ed lay the fly, still grasped in the left hand, on a slide, with the ovipositor 

 in a drop of saline ; take a pair of fine forceps in the right hand, and 

 pinch up the membrane at the base of the ovipositor, between the fifth 

 and sixth segments. Make a steady pull, and the body wall will rupture 

 at this point, the ovipositor, with the posterior attachment of the 

 gut, being drawn out. It brings with it the whole of the internal 

 organs, the alimentary tract rupturing at the junction between the 

 oesophagus and the proventriculus. The coils of the gut are very 

 complex, and great care should be taken to distinguish between the part 

 in front ef the Malpighian tubes and that behind. 



