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TEXT-BOOK OF ENTOMOLOGY 



gills ; those of the former situated at the base of the coxae, and 

 brush-like, but containing no tracheae, though tilled with blood, 

 while those of Cnemidotus are very long, bristle-like, jointed, and 

 arising from the dorsal side of the thoracic and abdominal seg- 

 ments. The stigmata are wanting. (Schiodte.) 



The larva of the dipterous genus Tanypus respires by two caudal 

 papilliform processes > in each of which a trachea ramifies. 



Certain larvae with both stigmata and tracheal gills are enabled 

 either to live in or out of water or on the surface, as in the case of 

 certain beetles (Cyphonidse, Elmidse, Hydrophilidae, Fig. 452), or the 

 larval mosquito and Psychodes (Fig. 455) ; also the nymphs of 



dragon-flies. 



The larvae of the Cypho- 

 nidae (Helodes, Cyphon, 

 Hydrocyphon) possess but 

 a single pair of stigmata, 

 situated in the penultimate 

 abdominal segment, while at 

 the end of the abdomen are 

 delicate tracheal gills. The 

 two main tracheal trunks are 

 much swollen. When on the 

 surface of the water the larva 

 breathes through the stig- 

 mata situated near the end 

 of the abdomen ; when float- 

 ing in the water, the larva, 

 like that of Gyrinus, carries 

 along at the end of its body 

 a bubble of air. The gills, 

 are only of use, as Rolph 

 thinks, when the insect is compelled to remain a long time under water. 

 The larva of our native Prionocyphon discoideus (Say) is described 

 by Walsh as " vibrating vigorously up and down a pencil of hairs 

 proceeding from a horizontal slit in the tail " ; this pencil is com- 

 posed " of three pairs of filaments, each beautifully bipectinate. I 

 presume it is used to extract air from the water." When the larva 

 is at the surface the pencil of hairs touches the surface of the water, 

 and occasionally a bubble of air is discharged from the tail. '' The 

 general habit is to crawl on decayed wood beneath the surface, 

 occasionally swimming to the surface, probably for a fresh supply of 

 air." (Proc. Ent. Soc. Phil., i, p. 117.) 



FIG. 452. Freshly hatched larva of Hydrobius : 

 t, enlarged tracheae, the heart between them ; g l -g 7 , the 

 seven pairs of gills. A, end of body, enlarged, showing 

 the two terminal stigmata. Emerton del. 



