LARVA OF ANOPHELES 91 



extends a rather thick layer of plasma with scattered nuclei. Each tube contains 

 a tracheal trunk which sends off numerous branches to the sides ; the tube is 

 hollow and represents a blood sinus. The tracheal gills are connected with 

 muscles and thereby serve as closing apparatus of the colon. While ordinarily 

 they are disposed wreath-like around the anus, they can, by muscular exertion, 

 fold together into the form of a wedge.'' 



MODIFICATIONS OF LARVAL STRUCTURE. 



Other mosquito larvas present essentially the same structures as the larva of 

 Culex, although variously modified. With a good knowledge of the Culex larva 

 there should be no difficulty in understanding the structures of other larvse. 

 Many of these differences are brought out in the systematic part of this work, 

 others may be briefly noted here. 



The most striking difference is presented by the larva of Anopheles in the 

 absence of a respiratory tube and its adaptation to the surface-feeding habit. 

 Aside from its adaptation to habits the larva ofAnopheles is primitive, as is in- 

 dicated by many of its structures. The larva when at the surface of the water 

 is still submerged ; it is suspended from the surface-film by means of the struc- 

 tures surrounding the spiracular openings on the eighth segment and by the 

 " stellate tufts,'' situated on the dorsal surface of the body. These stellate tufts 

 are little rosettes of leaf -like hairs with a common stem which spread out upon 

 the surface of the water. The stellate tufts are situated dorsally in pairs on 

 certain of the abdominal segments, and in some Old World species also upon 

 the thorax. 



The head of the Anopheles larva is narrower and more elongate than in other 

 mosquito larvse. The elongate appearance is due to the fact that the dorsal 

 region of the head is well produced between the antennge. The "mouth- 

 brushes " are more compact and closer together than in the Culex larva and 

 their mechanism is simpler. The primitive character of the larva is indicated by 

 the comparatively simple mandibles, the large, square maxillge and the very 

 large, nearly independent, maxillary palpi. These latter are not inserted in a 

 plane with the maxillae but hinge farther back upon a part of the ridge of the 

 hypocranium, which is interrupted at this point. 



The tracheae open separately through spiracles on the eighth segment. An- 

 other primitive character of the Anopheles larva is the presence of chitinous 

 dorsal plates on the abdominal segments. These are small and transverse, situ- 

 ated at the anterior margin on all but the ninth segment. This last has a large 

 saddle-shaped dorsal plate. 



The " lateral comb " of scales of the eighth segment of the Culex larva is 

 represented by two large chitinous plates, situated laterally near the posterior 

 extremity of the segment. These are attached to the sides of the segment by 

 their upper margins and project downward and slightly outward; the lower 

 margin is produced into a series of long irregular teeth, which point backward. 

 The two plates are connected posteriorly by a chitinous strip in continuation of 

 the thickened upper margins. These structures serve as a support to the 

 mechanism surrounding the spiracles. In the first stage larva of Anopheles 



