346 PHYLUM ARTHROPODA. 
expiratory muscles. We see their action when we watch a 
drone-fly panting on a flower. Inspiration is passive, as in 
birds, and depends on the elasticity of the skin and of the 
tracheal walls ; expiration is active, and depends upon these 
muscles. They are chiefly situated in the abdomen, but in 
some beetles (at least) they are also present in the metathorax. 
The tracheze seem to arise as tubular ingrowths of skin, 
and, primitively, each segment probably contained a distinct 
pair ; but their number has been reduced, and they are often 
in part connected into a system. With the doubtful excep- 
tion of one of the primitive Collembola, and the certain 
exception of caterpillars, no insects have any tracheal 
openings in the head region. There are rarely more than 
two pairs in the thorax; there are often six to eight pairs in 
the abdomen ; the maximum total is ten pairs. Each trachea 
is kept tense throughout the greater part of its course by 
internal chitinous thickenings, which apparently have a 
spirai course. The branches of the trachez penetrate 
into all the organs of the body, carrying oxygen to every 
part. The very efficient respiration of insects must be kept 
in mind in an appreciation of the general activity of their life. 
As the conditions of larval life are often different from those of the 
adult insects, the mode of respiration may also differ in details. 
In insects without marked metamorphosis, and even in some beetles 
in which the metamorphosis is complete, the young insect and the adult 
both breathe by tracheze with open stigmata. Both are said to be 
‘“holopneustic.” 
When the larvee live in water, the tracheal system is closed, other- 
wise the creatures would drown. This closed condition is termed 
‘“apneustic.” These larvee (of dragon-flies, may-flies, and some others) 
breathe by ‘‘ tracheal gills” (see Fig. 183)—little wing-like outgrowths 
from the sides of the abdomen, rich in trachese—or by tracheal folds 
within the rectum, in and out of which water flows. In either case, 
an interchange of gases between the tracheze and the water takes place. 
In adult aerial life the trachez of the body acquire stigmata, and the 
insect becomes ‘‘holopneustic.” 
In most insects with complete metamorphosis, the larva (e.g. cater- 
pillar or grub) has closed stigmata on the last two segments of the 
thorax (those which will bear wings), but there is a pair of open 
stigmata on the prothorax. In the adult the reverse is the case. 
There are some other modifications—for instance, what obtains in the 
parasitic larvee of some flies, e.g. gadflies. In these the stigmata are 
open only at the end of the body. In all cases, however, the stigmata 
of the adult are already present as rudiments in the larva, though they 
may not open till adolescence is over. 
