u8 



ENTOMOLOGY 



Tracheal Gills. In many aquatic nymphs and larvae the spiracles 

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



Pic. 171. Diagrammatic cross-section of the thorax of an insect, a, alimentary canal; 

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

 branch; 3, ventral branch. 



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

 which contain tracheae and tracheoles and are commonly lateral or 



caudal in position. Lateral tracheal gills 

 are highly developed in ephemerid 

 nymphs (Fig. 172), in which a pair occurs 

 on some or all of the first seven segments 

 ot^the^ abdomen; a few genera have 

 cephalic or thoracic gills. Larvae of Tri- 

 choptera have paired abdominal gills 

 varying greatly in form and position, 

 and Perlidae often have paired thoracic 

 gills. Caudal tracheal gills are conspicu- 

 ous in nymphs of damsel flies (Fig. 173) 

 as three foliaceous appendages. A few 

 coleopterous larvae of aquatic habit, as 

 Gyrinus and Cnemidotus, possess tracheal 



.-,-, j i n P..I 



&&*> <*O also caterpillars of the gCHUS 



Parapouyx (Fig. 1 74) , which f eed on the 

 leaves of several kinds of water plants. 



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

 foliaceous or filamentous, presenting always an extensive respiratory 

 surface ; their integument is thin and the tracheae spread closely beneath 



FIG. 172. Lateral gill from abdo- 

 men of a May fly nymph, Hexagenia 



variabiiis. Enlarged. 



