300 MEDICAL ENTOMOLOGY 



The pharynx is an elongate chitinous chamber of the usual type, and 



is easily seen in the living larva. It leads posteriorly to the oesophagus, 



which is wider and more muscular than that of the 



The Alimentary adult . it ig ugual to find t j lat the oesop h a gus is wider 



Plate I, fig. 1 an d more m.uscular in insects whose food is solid or 

 semi-solid than in those which take only fluid food. 

 The oesophagus opens into a short and cylindrical proventriculus, which 

 is also a highly muscular structure, and is sharply distinguished from 

 the succeeding part of the gut by its clear translucent appearance in 

 the fresh condition. The mid-gut extends from the proventriculus to 

 the hind end of the body, and is thrown into one or two simple coils, 

 not constant in their position. It is separated from the proventriculus 

 by a short constriction, and is again constricted at the posterior end, 

 just anterior to the opening of the Malpighian tubes. Between these 

 points the lumen is wide, and is thrown into numerous sacculations by 

 the contractions of the muscle fibres in the wall. The mid-gut is of a 

 very striking orange red colour in the fresh condition, and is filled with a 

 semi-solid mass of a light chocolate colour, which oozes out if the wall 

 is punctured in dissection. The hind-gut is short and simple, and is 

 coiled up in the posterior end of the abdomen. The total length of the 

 gut is about twice the length of the body of the larva. 



To dissect out the alimentary tract : Pin the larva down in a dissect- 

 ing trough, extending it as much as possible. Then take a pair of 

 fine forceps and pinch up a portion of the integument near the anterior 

 end, and cut it with a pair of fine scissors. Once the initial incision 

 has been made it is easy to cut a strip of the integument away from the 

 rest, carrying the incision to the opposite end of the body. The large 

 lateral tracheae should now be torn off at the posterior end and removed. 

 The whole of the alimentary tract is then exposed, and is easily sepa- 

 rated from the integument, to which it is only attached by a few tracheal 

 twigs. 



The salivary glands are simple and tubular, and bear a remarkable 

 resemblance to those of the adult insect. 



The respiratory system resembles that of the mosquito larva. There 

 are two large lateral tracheae which run the whole length of the body, 



on each side of and slightly dorsal to the alimentary 

 The Respiratory , _. . , . , 



System canal. These communicate with the external air 



through an opening, which can be closed, on a small 

 eminence on the dorsal surface of the penultimate segment. As they 

 pass forward they give off branches for the supply of the tissues in the 



