420 MEDICAL ENTOMOLOGY 



edge of a small drop of saline. It is best to have the specimen on its 

 back, but this is not essential, and it is sometimes difficult to retain it in 

 this position, as in the case of hump-backed flies or those with a long bent 

 proboscis. Place a pointed needle on the right edge of the penultimate 

 segment, holding it with the left hand, but without making any attempt 

 to cut. The pressure of the needle will push the contents of the 

 abdomen to the opposite side, leaving a small piece of flattened 

 integument in the region of the point. Now take a sharp lancet-shaped 

 needle in the right hand, and cut the integument outwards from the 

 point of the first needle, 'severing either the chitin or the inter-segmental 

 membrane. Repeat the manoeuvre on the opposite side, thus free- 

 ing the terminal portion of the abdominal wall from the rest, or else so 

 weakening it that it will tear easily, and then draw the insect further into 

 the drop of saline. Unless the salivary glands of the same specimen are 

 required, the head should be cut off' at this stage with a sharp lancet- 

 shaped needle ; it should never be pulled off. This severs the attachment 

 of the gut in front, so that its only connection with the thorax and ab- 

 domen is through small tracheae and lobules of fat body. Now place one 

 needle on the thorax, holding it flat against the most posterior part, but 

 without exercising any pressure, place the other on the last segment, 

 and make a series of slow steady pulls towards you, holding the thorax 

 steady at each pull. The alimentary tract and the reproductive system, 

 which are attached at the hind end of the body but now free in front, 

 will emerge from the abdominal segments. As the organs come out the 

 lateral tracheae will show up as silvery white tubes on each side, and 

 should be cut through to release the tension. After a few pulls the 

 whole alimentary tract up to the oesophagus and the organs of reproduction 

 will become free from the abdomen, and will retract. The oviduct, or 

 the vasae, are now cut through, and any tracheae uniting the two sys- 

 tems severed by the needle. The two parts are separated, and the one 

 which is required removed to a clean slide for examination. 



Unless there is some special reason for doing so, the mid-gut should 

 not be dissected until the contents have been at least partially digested. 

 If one attempts to get it out while distended it may rupture, and in any 

 case the pressure at the narrow end of the abdomen, through which it 

 has to be drawn, will force the contents of the posterior part forwards, 

 and thus possibly introduce a fallacy into subsequent observations. If it 

 is necessary to dissect it while full of fluid blood one of the lateral walls 

 of the abdomen should be cut up with a sharp needle to enlarge the 

 aperture, and the process of drawing it out should be carried out very 



