84 The Larva of Chirouomus 



tlie water. The larva lias another mode of charging 

 its blood with oxygen. It frequently comes up to the 

 surface by night, and though it does not actually reach 

 the air, it bathes its body in well-aerated water. The 

 blood-gills no doubt effect an exchange of gases, giving 

 off carbon dioxide and taking in oxygen. The only 

 visible action which can be detected by the microscope is 

 the in-and-out pulsation of the blood, driven to the gills 

 by the heart. 



Bionci-<?ins Blood-gills are not usual even in aquatic Nemoceran 



of other larvae. The ventral tubules described above seem to be 



larvae?^'^'^^^ peculiar to Chijonomus. The larvae of Culex, Anopheles, 



Corethra, Mochlonyx, Tanypus, and others have four anal 



gills ; the Simulium-larva has only three, which are 



retractile into the rectum. In the Dicranota-larva the 



anal gills are articulated and traversed by tracheae. The 



larvae of Eristalis and Helophilus, which are not Nemo- 



ceran, have about twenty long retractile anal gills, which 



are retractile into the rectum and traversed by tracheae. 



Such examples render it probable that the blood-gills of 



one larva may be replaced by tracheal gills in another 



larva ^ 



Tracheal The Cliironomus-larva has no tracheal gills, but they 



gills (.t ^j.g common in other aquatic Nemoceran larvae. They 



larvae. occur in a variety of positions ; thus they may be ventral, 



like the two pairs of articulated appendages on the last 



segment of the Dicranota-larva; caudal (i.e. terminating 



the body), like the last pair of the same larva - ; lastly, 



they may be segmental (i.e. segmentall3^ repeated), as in 



the larvae of Phalacrocera and Blepharocera '■''. Other 



variations probably exist, but full and exact descriptions 



are not always to be met with. 



The tracheal gills of Nemoceran larvae seem to be 

 derived in some cases, but not in all, from a set of circum- 

 sjJtracular papillae, which surround the s|)iracles, and 



^ The larva of Pleetrocnemia (Trichoptera) has five retractile anal gills 

 (T. H. Taylor in Miall, 1895, p. 266). 



^ The tracheal gills of the larvae of Ptychoptera and Bittacomorpha 

 seem to be similarly placed, though the long, retractile, respiratory tube 

 is continued beyond them. 



'' Cf. the larvae of Paraponyx, Sialis, and Trichoj^tera. 



