202 MEDICAL ENTOMOLOGY 



of the teeth on the mental plate, the syphonic index, the number of spines 

 on the syphon tube, and in the comb at its base. It is particularly 

 important to note whether the larva is carnivorous or not ; if it is 

 it must be bred with a known species with which it cannot be mis- 

 taken. The worker should be careful to compare only larvae of 

 approximately the same size and age, as the later stages differ from 

 the early one in some particulars. The hairs are generally more 

 numerous and more branched in the full grown larva than in young 

 specimens. 



The alimentary tract of the larva (Plate XXXIV, fig. 3) is a simple 

 one, and when dissected out is only a little longer than the body. 



The mouth leads into an oval or roughly quadrilateral 

 ... . pharynx, actuated by a complex musculature, which 



is situated in the middle of the head, and can be 

 seen through the integument in living larvae. The oesophagus is a 

 short and narrow tube passing through the neck, with a large 

 amount of muscular tissue in its walls, mainly arranged in circular 

 fibres. In the thorax it becomes continuous with the first part of the 

 mesenteron. The junction between the two is effected in very much the 

 same way as in the adult Philaematomyia already described, the end of 

 the oesophagus being invaginated into the commencement of the mesent- 

 eron so as to produce a valve. The space between the two walls of 

 the invaginated portion is a considerable one, and is probably, according 

 to Imms, a blood sinus. Immediately below the valve there are eight 

 coeca; these are short but broad sacs, constricted in the middle, the 

 set being arranged in a regular row around the entrance to the mid-gut. 

 The cells of the coeca secrete a granular material which is excreted 

 into the lumen ; they stain very deeply with haematoxylin. The 

 mid-gut extends as far as the eighth segment, and is the same shape 

 as the cavity in which it lies, and is not convoluted. Its wall is 

 lined with a secreting epithelium similar to that in the adult insect. 

 There is in the larva, however, a well developed peritrophic mem- 

 brane. There are five rather short and stout Malpighian tubes, both 

 in the larva of Culex and Anopheles. The hind-gut consists of a 

 short and narrow ileum, well supplied with circular muscle fibres, 

 and a wider pear-shaped portion, the colon, with which is included 

 the rectum. There are no rectal papillae, these being probably re- 

 presented by the external tracheal gills. The wall of the hind-gut has 

 a well marked chitinous intima. 



Many mosquitoes are naturally infected with flagellates of the genera 



