io8 



ENTOMOLOGY 



Tracheal Gills. In many aquatic nymphs and larvae the spiracles 

 are suppressed (though they become functional in the imago) and res- 



FIG. 169. Diagrammatic cross-section of the thorax of an insect, a, alimentary canal; 

 d, dorsal vessel; g, ganglion; s, spiracle; w, wing; i, dorsal tracheal branch; 2, visceral 

 branch; j, ventral branch. 



piration is effected by means of gills; these are cuticular outgrowths 



which usually, though not invariably, contain tracheae, and are commonly 



lateral or caudal in position. Lateral 

 tracheal gills are highly developed in 

 ephemerid nymphs (Fig. 170), in which 

 a pair occurs on some or all of the first 

 seven segments of the abdomen ; a few gen- 

 era, however, have cephalic or thoracic 

 gills. Larvae of Trichoptera have paired 

 abdominal gills varying greatly in form 

 and position, and Perlidae often have 

 paired thoracic gills. Caudal tracheal gills 

 are conspicuous in nymphs of Agrionidae 

 (Fig. 171) as three foliaceous appendages. 

 A few coleopterous larvae of aquatic habit , 

 as Gyrinus and Cnemidotus, possess tracheal 

 gills, as do also caterpillars of the genus 

 Paraponyx (Fig. 172), which feed on the 



leaves of several kinds of water plants. 



Though manifold in form, tracheal gills are generally more or less 



foliaceous or filamentous, presenting always an extensive respiratory 



FIG. 170. Lateral gill from ab- 

 domen of a May fly nymph, Hexa- 

 genia variabilis. Enlarged. 



