166 ANATOMY. 



thorax (PI III. f. 4). The majority of the larvae of the genera most 

 closely allied to this gnat possess no such air tube, hut true branchiae 

 or gills, yet the larvae of Citironomus* have likewise two conical air 

 tubes upon the anal segment (PI. III. f. 5) ; besides which they are 

 easily distinguished by a more elongate vermiform shape t, as well as 

 by their blood red colour, from the true larvae of the Culicidcc. A 

 similar structure is found in the larvae of Stratiomys ; in them the 

 entire last segment of the abdomen is elongated into a tube, and at the 

 aperture of the tube it is provided with a wreath of plumose hairs 

 placed in the form of a star. This coronet, which is much larger than 

 that of the larva of Culex, likewise supports the much larger creature 

 upon the surface of the water when it goes thither for fresh air ; and it 

 likewise takes air bubbles, which are inclosed by the setae, down with it 

 to the bottom of the stagnant pools which it inhabits, as a provision 

 for its next inspiration j. The larvae of the genus Eristalis display 

 a considerably longer anal air tube ; in these also the last joint is 

 extended into a membranous tube, in which a second narrower and 

 corneous one is contained, which at its open end is provided with a 

 similar crown of hair. It is into this tube that the two branches of the 

 tracheae pass after having united into one. The thick, white, cylin- 

 drical larva which lives in the mud of pools, in sewers, and in excre- 

 ment, directs this tube to the surface of the water, which hangs there 

 by means of the above-mentioned setae, while it itself lies tranquilly at 

 the bottom, or else continues feeding. If the water should rise, for 

 example, after rain, it lengthens this tail by pushing the inner tube as 

 far out as is requisite. This elongation can be extended to several 

 inches, whereby the length of the tail exceeds several times that of the 

 body. For the expiration of the air thus received two other very short 

 air tubes are placed upon the first segment of the body, directly behind 

 the head ; the anterior ends of the above described main stem of the 

 tracheae pass into these after having previously, as well as the posterior 

 ends, become united by means of a transverse branch. 



We also observe anal air tubes in the genera Nepa and Ranatra, but 

 which are distinguished from those above described in the first place by 



* The larvae have gills (branchiae), as I have recently observed (Author, MS. Note). 



f These larva were formerly considered as a genus of annelides, and were called 

 Branchiurus. See Oken's Zoologie, 1 band. s. 383. Taf. 9., and Viviani Phosphor. Maris, 



:;. is, n. 



* See Swammerdammj Biblia Natuiw, I'l. XXXIX. f. 1 ;',. 



