1909. | LIPE-HISLORY OF THE AGRIONID DRAGONFLY. 277 
The lamellee of Agrion—I have been unable to find a character 
to distinguish the nymphs of A. pulchellam and puella from one 
another—are indistinguishable at first from those of Jschnura, 
unless possibly by certain small patches of pigment; but about 
the 6th, 7th, or 8th stage, sooner or later, according to the size 
of the nymph, they have become sufficiently wide and rounded at 
the apex to distinguish them, 
These organs, into which the lateral tracheal trunks of the 
body pass and in which they break up into minute branches, so 
that the whole lamella is filled with them, are said to have ¢ 
respiratory function. I have already referred to the ianeul 
mittent blood-stream in these organs. Whether it is correct to 
describe it as intermittent I am not sure, but I have several 
times noticed that in a nymph taken from the tumbler and 
placed under the microscope no circulation was to be detected 
until after the lapse of some time, and it has occurred to ine that 
the circulation ih the lamellse was noticeable only when the air 
in the water was becoming exhausted. 
These organs are not, ‘however, indispensable, as the nymph 
can live quite well without them. In the final stage they are 
often without them, having apparently cast them off. In the 
earlier stages, however, if a lamella is removed it is replaced by 
degrees. If two or three days previous to a moult a lamella is 
removed, no replacement takes place until the second succeeding 
moult. As soon as the nymph escapes from the old skin, a very 
thin pointed sac, more or less circular in transverse section, is 
visible in place of the lamella previously removed. This sac is 
much smaller than the other Jamelle and contains at first no 
trace of trachee. It takes two, three, or even four moults before 
a new lamella is indistinguish: ble from the uninjured ones. The 
process of renewal is not "sudden as it appears to be. The wound 
caused by removing a lamella is apparently quickly closed and 
the formation of the new lamella commences within the “ shell,” 
I believe at the time when a new nymph-skin is forming within 
the old one. Presumably ib forms in a crumpled condition like 
the imaginal wings within the nymph wing-cases, and as soon as 
the pressure of the outer shell is released, the new lamella expands 
asa sac, becoming turgid owing to fluid enteri ing it from the body. 
A leg i is replaced i in the same way as a lamella, taking two or 
three stages to attain perfection, appearing immediately after the 
moult as a thin-walled sac which gradually thickens and hardens. 
I have paid very little attention to the replacement of lost 
parts, but it seems to involve two processes: first, the formation 
of a thin-walled sac in the form of or tending towards the form 
of the organ to be renewed, and, secondly, a re-formation within 
the sac of the muscular and tracheal systems, 
WING DEVELOPMENT, 
I have already mentioned that the first sign of the pleural 
ridges appears after the nymph has completed its third or fourth 
