10 
of Celithemis is usually accompanied by the male, who helps 
her to escape when menaced by the open mouth of a hungry 
fish. Several hundred eggs are often laid by a single female. 
The period of incubation varies with the season and also 
independently of it. In midsummer, eggs of some species hatch 
in from six to ten days, while others, laid in autumn, do not 
hatch until the following spring. In the same lot of eggs the 
period of incubation may vary, even in midsummer, from a 
week to more than a month. 
The apparent abundance of nymph and imago is far from 
corresponding, the difference in some cases being quite surpris- 
ing. Of the nymph of Celithemis eponina, we have secured only 
a few examples; yet the imago is a familiar sight everywhere 
about the Station. The species of Sympetrwm are common and 
familiar dragon-flies; but we have obtained only a few of the 
nymphs. This genus probably breeds in swampy places, where 
the vegetation is so dense, the water so shallow, and the mud 
so deep as to make collecting very difficult. On the other hand, 
Kpicordulia princeps is abundant and widely distributed as a 
nymph, but the imago is not commonly taken. Nymphs of 
Macromia, Progomphus, and Hagenius are not at all rare in 
streams; the imagos are considered very rare or almost 
unobtainable. In the Gomphide are numerous similar exam- 
ples. This discrepancy may be due either to the swift, high, or 
prolonged flight of the imago, or to the shortness of its life ; 
but in some cases it is almost inconceivable how the imagos 
can vanish so completely as they do. 
Walsh (63, p. 239) makes some remarkable statements as 
to the relative proportion of the sexes in Gomphus. In some 
species he found four males to every female, and in another 
two or three females to each male, and he asserts that this is 
the case in freshly emerged material. Mr. Needham is of the 
opinion that in nymphs generally there is no notable excess of 
either sex, but that in the imago an excess of males may occur 
because of the destruction of the females by fishes in species the 
females of which oviposit unattended by the males. Itis prob- 
