194 BULLETIN OF THE BUREAU OF FISHERIES. 
Mask.—The most noticeable thing about a nymph is the so-called mask, which is 
folded back beneath the head and which may or may not cover the lower part of the 
face (fig. 6), This mask is really the lower lip or labium, whose outer end terminates 
in three lobes, one median and two lateral; the latter may take the form of stout claws 
(4ischnidz) or of spoon-shaped lamellz (Libellulide). The mask is hinged near the 
center and when not in use is folded at the hinge; the lateral lobes are turned inward 
across the front end of the median lobe, and the whole apparatus is folded back beneath 
the head. Figure 7 is a side view of the head of L. /uctuosa, showing the mask folded 
back, while the lateral lobes at its tip cover the lower half of the face. This is the con- 
dition found in the nymphs of all the Libellulide. Figure 9 is a side view of the head 
of Anax junius; here the lateral lobes do not cover the face at all, but extend straight 
forward as stout claws beneath the chin. This condition is found in the nymphs of 
the Aischnide, with the exception of the genus Cordulegaster, and as an accompanying 
character the head is depressed or flattened. The 
length of the labium varies considerably in different 
dragonflies, but is usually longer in the A’schnide 
than in the Libellulide and reaches back, when 
folded, between the bases of the second legs. 
With the mask folded the nymph either conceals 
itself in the mud or trash on the bottom or steals 
up on its prey and when within striking -distance 
shoots the mask forward in front of the head and 
grasps the victim between the lateral lobes. Figures 
8 and 10 show the same two specimens of Juctuosa 
and junius with the mask thus extended. 
The distance which they can reach, of course, 
varies with the size of the nymph and the length 
_ _. of the mask; some of the large Anax nymphs can 
peg ae -i>- aw sas Eohemis sim? cover 15 to 25 mm. ‘This, in addition to the 
covering the lower part of the faceuptothe lurching forward of the body, enables them to 
epbesistee grees catch insects like Corixa, much more agile than 
themselves, and even to capture small fishes. 
To assist in holding their prey, the lateral lobes of the mask are toothed along their 
inner margins in the A’schnide (fig. 11). In the Libellulide they are armed with a 
long, slender spine at the tip and a row or raptorial sete behind this along the outer 
margin, varying in number in the different species. There is a crescent of similar 
sete, also varying in number, along the body of the mask, called the mentum, on either 
side of the median line. And there are more or less regularly arranged spines and 
hairs along the remaining margins of all three lobes. Figures 12 to 14 illustrate some 
differences between the species. 
Maxill@.—Once grasped between the lobes of the mask, the prey is drawn quickly 
to the mouth, where there are two pairs of organs ready to dispose of it. The first of 
these, the outside pair, are the maxille, which are very much alike in all nymphs; 
each maxilla has two fingerlike branches or rami, the outer (ventral) of which is 
