THE IMMATURE STAGES 33 
Two pairs of muscles each side operate the labium; one pair, pro- 
jector and retractor, lying in the submentum, operates the middle 
hinge; another smaller pair, extensor and flexor lying in the mentum, 
swings the lateral lobe. 
The jaws are of a distinctly flesh-eating pattern. The mandibles 
with their Z-shaped sharp chitinous ridges cut the victim to pieces, - 
while the maxillae, shaped like meat forks, turn it conveniently for 
cutting. So it is made up into pellets and swallowed. These pellets 
stop for a time in the proventriculus or crop whose walls are lined with 
chitinous teeth and prickles. This is a chewing stomach where further 
comminution of the food takes place before it is passed on into the 
true stomach for digestion. 
The structures so far mentioned all nymphs of Odonata have in 
common, but in the breathing organs we come upon structures in 
which dragonflies differ markedly from damselflies. Both breathe by 
means of tracheal gills, but in the dragonflies the gills are numerous 
and are located inside the body, and in damselflies they are but three 
vertical gill plates attached at the tip of the abdomen like tails. 
The dragonfly gill chamber is a modified portion of the hinder 
(rectal) part of the alimentary canal. It is set off by a constriction from 
the part in front that has to do with nutrition, and is enlarged into 
an oval chamber that half fills the abdomen; the gills hang from the 
inner walls of this chamber in longitudinal rows. They are minute, 
thin walled, and very numerous. They are filled with fine air tubes 
(tracheoles) that are connected with four big air trunks running length- 
wise of the body. The walls of the gill chamber are provided with 
muscles for changing its shape. When it expands, water is drawn 
in from the rear, bringing fresh oxygen. When it contracts, the water 
is expelled. The posterior (anal) opening is guarded by the cluster 
of five spinous caudal appendages which terminate the abdomen. 
Between these are three little valves that guard the immediate open- 
ing. They serve as strainers, partly closing the opening while water 
is flowing in; they fly open like shutters when the water is squirted 
out. 
The regular expanding and contracting of the abdomen is readily 
seen in a living dragonfly nymph. The water currents may be watched 
also if a bit of colored fluid be placed at the end of the abdomen.* 
The gills of the damselflies are three flat plates placed edge upward 
* As by holding a copying ‘‘indelible”’ pencil there a moment, until the color 
dissolves. See Needham’s Guide to the Study of Freshwater Biology for directions 
for the dissection of the dragonfly gill chamber. 
