MEDICAL ENTOMOLOGY 



The legs are well developed and consist of six segments resembling 



those of ticks ; the number of segments may, however, be considerably 



reduced in some of the parasitic forms. The tarsus in 



Laelaps has peculiar spines at its apex, which, together 



with the short or long spines on the segments of the legs, are of value in 



distinguishing the species. 



The alimentary tract is very similar to that of ticks, and when fully 



developed, as in a Laelaps, consists of the pharynx or 



Internal Anatomy of p um pj n g organ, the oesophagus, the mid-intestine with 



its diverticula, the hind-intestine, with the rectum and 



Malpighian tubes. 



The pharynx or sucking pump is situated on the ventral surface of 

 the capitulum, and consists of a straight or slightly bent tube formed of 



chitinous plates united to each other by a membrane. 



The Alimentary Tract . * . iL j ' i j i * i 



A series of muscles arise from the dorsal and lateral 



walls of the capitulum and are inserted into the walls of the pharynx 

 above and at the sides ; on contracting, these pull them apart and thus 

 produce a negative pressure. In addition there are small circular 

 fibres which surround the pharynx and interlace with each other; these 

 muscles constrict the pharynx and thus aid in propelling the blood into 

 the oesophagus. 



At the posterior end the pharynx becomes continuous with the oeso- 

 phagus, which is a slender tube lined with a delicate chitinous intima 

 and surrounded by small bundles of circular muscular fibres. After a 

 short course the oesophagus passes downwards and then upwards through 

 the brain to enter the mid-intestine on its posterior surface. The mid- 

 intestine, as in the ticks, lies in the middle line and is a short thin-walled 

 sac. On each side it gives off a short diverticulum which divides into an 

 anterior and posterior branch ; the anterior branch passes forwards and 

 ends in the neighbourhood of the mouth parts ; the posterior branch 

 meets its fellow of the opposite side and ends in the neighbourhood of 

 the rectum. The mid-intestine and its diverticula are lined by a single 

 layer of large columnar epithelial cells which rest on a basement 

 membrane; external to this there is a series of fine parallel muscular 

 fibres. During digestion the cells exhibit appearances similar to those 

 which have already been described in the case of ticks. 



Posteriorly the mid-intestine becomes continuous with the hind-intes- 

 tine, which passes downwards and backwards to end in the rectum, and is 

 not patent throughout its entire length. The rectum is a large sac with 

 thin walls lying in the middle line ; it is usually filled with a whitish 



