262 MR. F, BALFOUR-BROWNE ON THE [ Mar. 2, 
“‘ mandibulate ” type, a pair of maxille, and a labium of very special 
form usually known as “the mask.” It is in shape more or less 
an isosceles triangle, the apex being turned posteriorly and lying, 
when in a state of rest, underneath the first thoracic segment. 
The base of the triangle is presented forwards and at each of the 
anterior corners is attached an appendage representing a labial 
palpus, which again is rather triangular in shape, being flattened 
above and below and having three sides, one of which is anterior. 
At its outer anterior extremity each palp bears a movable claw, 
and along its anterior border each palp is beset with four or five 
teeth, the innermost being much the largest and slightly curved 
inwards. ‘The inner border of each palp is serrated, the teeth 
being small and curved slightly backward. The anterior border 
of the labium itself is also slightly serrated *, and when the mask 
is not in use the palpi lie against this border in such a way that 
the anterior edges of the two interlock, the movable claws over- 
lapping. On the upper side of each palpus, attached near its 
outer border and not far behind the movable claw, is a single 
long incurved hair or bristle. 
The thoracic segments bear each a pair of legs, but at this stage 
there is no trace of the wings nor are the pleural ridges present. 
Except for the possession of the legs there is nothing in out- 
ward appearance to distinguish the thoracic from the abdominal 
segments. These latter are also very similar to one another, 
except that the body tapers slightly towards the posterior end. 
The tenth segment bears the three caudal lamelle, one median 
situated above the anus and two laterals, one on either side of 
the anus. The two main trachee, which pass along each side of 
the thorax and abdomen, curve inward and upward in the tenth 
segment and run out side by side in the middle lamella, At the 
points where they curve upward a single branch continues as a 
fine tube into the lateral lamella of its own side. The lamelle at 
this stage are tubular, perhaps slightly triangular in transverse 
section, tapering to their apices and ending in a fine long hair ; 
they bear along two or three lines a series of small hairs and in 
Agrion pulchellum they contain two patches of pigment, one near 
the apex and one about halfway from the base. 
The transparency of the nymph at this stage enables one to see 
fairly well the internal organs. The gut consists of a pharynx 
leading to an esophagus which opens out into a large thin-walled 
sac—which is apparently contractile—occupying the thoracic 
segments. At the posterior end of this sac there is a constriction, 
and immediately behind this is another large thin-walled sac 
which occupies most of the first six abdominal segments. This 
sac—the mid gut—is constricted posteriorly and opens into a small 
globular contractile portion of the intestine, in the anterior end 
of which open two Malpighian tubules, one on either side and 
* The serration does not appear on either the palpi or the labium until a later 
stage. Only faint traces of it are visible at this stage. 
