CULEX AND STEGOMYIA: LARVAE 201 



In all culicid larvae other than Anopheles the openings of the tracheae 

 lies at the end of a tube of chitin of varying length, known as the syphon. 

 This has the form of a cylinder, arising from the dorsal 

 surface of the eighth segment, and usually narrowing 



a little towards the distal end. The apex is closed by 15 an( j \ 6 \ 

 a series of finger-like flaps of chitin, which can be opened 

 when the larva ascends to the surface to breathe. At the base there is 

 a thickened ring by which the syphon is attached to the eighth segment, 

 and it is at this point that the tube, as a rule, attains its greatest breadth. 



Christophers regards the shape of the syphon tube as a point of 

 considerable taxonomic value, and has used as a taxonomic character the 

 'syphonic index,' obtained by dividing the length of the tube by its 

 greatest breadth. The tube may be short and stout, or it may be of 

 extraordinary length and very narrow. It may be furnished with several 

 branched or simple hairs. On each side there is a row of peculiar spines, 

 the number of which is important ; these may be arranged in a regular 

 row, or the row may be interrupted. At the base of the syphon, on the 

 dorsal surface of the eighth segment, there is a row of spines represent- 

 ing the comb which surrounds the respiratory opening in Anopheles. 



Apart from the presence of the syphon tube, there are many minor 

 differences between the larvae of different genera. The head, for 

 instance, is larger in Culex than in Anopheles, while in Stegomyia 

 both head and thorax are relatively small. The antennae in Stegomyia 

 are small and degraded, and have no small recumbent hairs, while the 

 papilla is often well developed. In the genus Culex the antenna 

 may show great variation. Striking deviations from the type are 

 to be found in cannibal larvae, such as that of Culex. concolor. In 

 this species the syphon tube is short, and the larvae, therefore, either 

 float almost horizontally on the surface or with their heads a little 

 below it ; in this position they remain stationary, waiting for their 

 prey. The feeding brushes, as already noted, are transformed into 

 clasping organs, and the clypeus is concave instead of straight. 



The differences between the larvae of species even in well estab- 

 lished genera are so great that they cannot be profitably discussed 

 here. The character of each species with which the w r orker is con- 

 cerned should be determined by an actual examination of the living 

 larva between a slide and cover glass, and the specimens subsequently 

 bred out and identified, the larval skin being retained for reference. 

 The points which are of special importance are the presence or 

 absence of hairs or spines on the antennae, the shape and number 

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