556 MEDICAL ENTOMOLOGY 



plan which offers the best chance of success is to first free the host from 

 its parasites, and then to place a definite number of known history on 

 the ears, which are then enclosed in bags of muslin. If necessary pre- 

 cautions can be taken to prevent the host from scratching off the bag. 



INTERNAL ANATOMY 



The alimentary canal of the louse is a simple one, and resembles 

 generally that of Cimex, from which it is distinguished, however, by the 

 high position of the Malpighian tubes, the presence of 



' vesti g es of rectal P a P illae > and the absence of a rectal 

 ampulla. When dissected out it is about twice the 

 length of the body of the insect. 



The oesophagus commences at the posterior end of the pharynx in the 

 head, and passes through the brain in the ordinary manner, lying dorsal 

 to the large nerve cord in the neck. It enters the thorax between the 

 salivary ducts, and joins the mid-gut between its two lateral lobes. The 

 wall is composed of a rather thick and crinkled chitinous intima, usually 

 thrown into many irregular folds, and having only a scanty layer of 

 hypodermal cells. The oesophagus will stand a good deal of stretching 

 without rupture. 



The mid-gut is a large cylindrical chamber, commencing in the thorax 

 and occupying almost the whole of the abdominal cavity. As it receives 



the blood directly from the oesophagus in the act of 

 The Mid-gut ui r ,, IT j- * 



feeding, it is capable of considerable distension, and 



may become so large after a full meal as to produce a visible swelling 

 of the body of the insect, the red blood showing through the transparent 

 integument. During the time digestion is going on its shape is con- 

 stantly changing on account of the peristaltic contractions of the muscles 

 of its wall, so that it sometimes appears to be constricted in the middle 

 or at the ends, or to have narrower parts. None of these breaks in the 

 contour of the organ is permanent, and in the resting condition the 

 shape conforms generally to that of the cavity in which it lies. When 

 not filled up with fresh blood the chamber contains a variable amount 

 of brownish black granules, the remains of digested blood. 



At the anterior end (Plate LXX, fig. 1) the mid-gut is divided into two 

 lobes, one on each side of the oesophagus. These project into the cavity 

 of the thorax, and as they contain the same black granules as are found 

 in the rest, of the gut their position is easily recognized in the living 

 insect. They resemble the posterior portions of the organ in structure, 



