134 ENTOMOLOGY 
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: (1) The primary, open, or 
holopneustic type described 
Fic. 160: 
above, in which the spiracles 
are functional; (2) the sec- 
ondary, closed, or apneustic 
type, in which the spiracles 
are either functionless or ab- 
sent. This type 1s illustrated 
in Collembola and such aquatic 
nymphs and larve as breathe 
either directly through the skin 
or else by means of gills. 
The two types, however, are 
connected by all sorts of inter- 
Lateral gill from abdomen of a May mediate stages. 
fly nymph, Hexagenia variabilis. En- Tracheal Gills.—In many 
larged. 
aquatic nymphs and larve 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 trache:e, 
and are commonly lateral or caudal in 
position. Lateral tracheal gills are 
highly developed in ephemerid nymphs 
(Pic. 169), in which a pair Gecurs on Caudal gills of an 
agrionid nymph, en- 
some or all of the first seven segments 7.04) 
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 Perlidze often have paired thoracic gills. Caudal 
tracheal gills are conspicuous in nymphs of Agrionidz (Fig. 
170) as three foliaceous appendages. A few coleopterous 
