258 MR. F. BALFOUR-BROWNE ON THE [ Mar. 2, 
being marked by the successive moults. I have then gone into 
some detail with regard to the development of the form of the 
nymph—that is, morphologically rather than anatomically; and 
in this part of the subject one or two interesting facts as to the 
manner of growth of the external parts have been observed. 
This detail has brought to light a certain want of agreement 
between the stages as marked by the moults and the stages in the 
development of these external parts; and I have therefore, in 
conclusion, reviewed the question from the point of view of the 
significance of the moult. This latter part of the paper is very 
incomplete, since, although a number of facts on the subject of 
the moult in insects are known, there is insufficient material at 
present upon which to base any really satisfactory theory. How- 
ever, this is merely forestalling the conclusion. 
THe PRoNYMPH. 
The Agrionid egg is an elongated body, rounded at one end and 
bluntly pointed at the other, the point being a small pedicel. In 
Agrion pulchellumand Ischnura elegans its length is about 1°3mm., 
but the size no doubt varies in the different species. The egg is 
inserted, as already explained, in the soft tissues of aquatic plants, 
the pedicel end being the last to leave the oviduct and being 
therefore nearer to the surface of the leaf in which the egg is 
deposited. 
The period of incubation varies somewhat in length according 
to temperature, apart from the fact that all the eggs of a batch do 
not hatch on the same day. Eggs laid inthe beginning of August 
took just less than four weeks Tatlone the rly of fhen hatched, 
while eggs laid later in the year did not hatch for more than five 
weeks. 
For some time previous to hatching the position of the nymph 
within the shell is easily made out. The head lies at the pedicel 
end of the egg, the antenne, first maxille, and labium being 
directed backward on the ventral side of it. The first and second 
pairs of legs le straight out along the ventral side of the abdomen, 
while the third pair are bent on themselves at about the junction 
of the tibiz and tarsi, the latter lying forward below the tibie. 
The posterior abdominal segments curve round in the shell 
ventrally, the 10th occupying the rounded end of the shell. The 
caudal lamellee lie forward along the ventral side, between the legs 
and the shell. : 
For at least three days before the egg is ready to hatch the heart 
can be seen beating, at first slowly but getting faster as the time 
approaches for the emergence of the pronymph. The posterior 
end of the heart is the part which develops and pulsates first, and 
although in the free nymph this is in the 9th abdominal segment, 
in the egg it appears to lie in the 10th segment. 
The first sign of the approaching emergence of the pronymph 
is the cracking of the shell immediately beneath the pedicel. At 
