A NYMPH OF THE GENUS CANIS. 57 

They are small and narrow, somewhat lance-shaped, and are 
fringed with a few long filaments, each of which receives a small 
branch of the tracheal vessel that runs through the length of the 
plate. A slight constriction near the middle of each plate appears 
to divide it into two parts, but it can scarcely be called an articula- 
tion. These plates are held nearly at right angles to the sides of 
the segment on which they are placed, and they do not vibrate 
during respiration. The second abdominal segment is without 
branchial appendages. The second pair of plates arise from the 
junction of the second and third abdominal segments, fitting under- 
neath the dorsal projection of the second segment, against which 
the plates are raised and kept rigid during respiration. They are 
very large and thick, obtusely oval, somewhat conical anteriorly, 
ciliated on the margin, and in the full-grown nymph they cover the 
remaining posterior gills during repose, the left plate slightly over- 
lapping the right. In the specimen represented in the drawing the 
length of these plates was about 5 of an inch. On the under sur- 
face of each of these plates a trachea runs in a longitudinal 
direction, giving off branches at each side. The remaining gills, 
of which there are four pairs, are situated on the dorsal surface of 
the fourth, fifth, sixth, and seventh abdominal segments. They 
are thin and delicate, somewhat ovoid in form, and. are fringed 
with long filaments on the margins. The trachea divides near the 
base of each plate into several branches, the sub-divisions of which 
are continued into the marginal filaments, z.e., each filament receives 
a single branch. The plates on the fourth segment are about half 
the length of the large protecting plates, those on the posterior 
segments gradually diminishing in size from the fourth to the 
seventh segments, the last being about ;45 of an inch in length. 
In the full-grown nymph each of these plates covers to some extent 
the posterior plate contiguous to it, and their terminal filaments are 
intermingled when the insect is not engaged in the visible act of 
respiration. ‘This provision prevents the introduction of foreign 
matter between the plates, a protection which appears to be neces- 
sary on account of their delicacy and the burrowing propensity of 
the nymph. 
When the nymph respires, the large plates on the third segment 
are raised and fixed against the edges of the second segment at an 
angle of about 50° above the dorsal surface. They never vibrate. 
The four posterior pairs of membranous gills also are raised, and 
are kept in rapid vibration until they are lowered again. The first 
pair are held nearly parallel to the protecting pair, whilst vibrating 
through a small angle. The second pair are more depressed, the 
third still more so, the last pair being raised not more than 20° or 
25°above the dorsum. The number of vibrations which I counted 
during one observation amounted to about 250 per minute. The 

