134 



ENTOMOLOGY 



FIG 



were formerly and erroneously supposed to diminish the 

 weight of the insect, but are now regarded as simply air- 

 reservoirs. 



Types of Tracheation. Two types of tracheal system are 

 distinguished for convenience: (i) The primary, open, or 



holopneustic type described 

 above, in which the spiracles 

 are functional; (2) the sec- 

 ondary, closed, or apncustic 

 type, in which the spiracles 

 are either functionless or ab- 

 sent. This type is illustrated 

 in Collembola and such aquatic 

 nymphs and larvae as breathe 

 either directly through the skin 

 or else by means of gills. 

 The two types, however, are 

 connected by all sorts of inter- 

 mediate stages. 



Tracheal Gills. In many 

 aquatic nymphs and larvae the 

 spiracles are suppressed (though they become functional in 

 the imago) and respiration 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. 169), in which a pair occurs on 

 some or all of the first seven segments 

 of the abdomen; a few .genera, how- 

 ever, have cephalic or thoracic gills. Larvae of Trichoptera 

 have paired abdominal gills varying greatly in form and posi- 

 tion, and Perlidse often have paired thoracic gills. Caudal 

 tracheal gills are conspicuous in nymphs of Agrionidae (Fig. 

 170) as three foliaceous appendages. A few coleopterous 



Lateral gill from abdomen of a May 

 fly nymph, Hexagenia variabilis. En- 

 larged. 



FIG. 170. 



Caudal 

 agrionid 

 larged. 



gills of 

 nymph, 



