200 MEDICAL ENTOMOLOGY 



they may be present on all the abdominal segments, as well as on 

 the thorax, or they may be confined to the more posterior segments. 

 Each palmate hair consists of a number of delicate leaflets arising from 

 a common stalk, and arranged in a radiating manner like the ribs of a 

 half-opened umbrella. The basal portion of each leaflet is broad, and 

 often appears concave, while the distal portion is usually narrowed and 

 pointed. The edges of the leaflets may be notched in a characteristic 

 manner. In rossii there is a small notch on one border. The number 

 nineteen in this species and shape of the leaflet, are of specific 

 importance. 



The eighth segment bears the respiratory openings or stigmata, the 

 terminations of the large lateral tracheae which traverse the whole 



length of the larva. These lie in the anterior part 

 Respiratory opening 



ot an irregularly quadrilateral area on the dorsal 



surface. This area is raised a little above the level of the preceding 

 segment, and is supported by a chitinous arch, the lateral arms of 

 which are provided with a series of spine-like teeth, corresponding to the 

 ' comb ' of the larva of Ciilex. The anterior and lateral margins of 

 the space in which the stigmata lie can be turned inwards so as to 

 partly arch over the area, or again retracted when the larva is breathing. 

 The raised portion is pushed through the surface film during respiration, 

 so that the tracheae are in free communication with the air through the 

 openings of the stigmata. When the larva sinks to the bottom, the 

 openings are closed by the arching over of the sides of the area. 



The ninth segment is narrower and rounder than the rest, and is a 

 little elongated. Its dorsal border is furnished with two pairs of long 

 feathered hairs, which are directed backwards as a tail. The ventral sur- 

 face bears two rows of feathered hairs, arising in the middle line from an 

 elongated and raised area of thicker chitin. The two rows are set very close 

 together, so that when examined in side view they appear as one, and 

 hang down at a right angle to the long axis of the body as a sort of fin ; 

 each hair is articulated into a little round pit in the chitinized area. 

 Between the dorsal and ventral sets of hairs there are four similar and 

 symmetrically arranged papillae. These are delicate transparent leaf-like 

 structures, possessing a considerable amount of retractility. They are 

 small in Anopheles, as it is a surface feeder, but become of considerable 

 size in those larvae, such as some species of Stegomyia, which feed 

 almost exclusively at the bottom of the water. These organs are 

 invaginations of the body wall, and, as they contain blood, function as 

 tracheal gills. 



