32 THE ESSEX NATURALIST
the exterior. In addition to these primary parts there are also
a sac for the reception and storage of the spermatozoa and a pair
of accessory glands, both of which enter the main oviduct.
These accessory glands secrete the fluid which attaches the egg
to the surface of the plant. Each ovary consists of a number
of cylindrical tapering units, which start as a fine thread and
increase in diameter until they unite with the lateral ducts. At
the posterior end of the thread is a chamber in which new cells
are produced, some of which develop into eggs, the others acting
as nurse cells and supplying food to the egg cells. The last egg
in the ovariole is the largest, and it is here that it receives the
shell, which, with its characteristic pattern, is formed by the
cells lining the ovariole. It is only when the egg enters the
median oviduct that it receives the sperm to fertilise it, this being
made possible by the formation of a small plate called the
micropyle which has a number of fine tubes through which
the sperm can pass.
The egg-laying habits of butterflies are very varied : some
species will only oviposit on the upper surface of a leaf, while
others select the under surface, some select the flower stalks and
others the base of a dormant bud: whatever the position it is
constant for the species. It must be remembered that butterflies
feed upon the nectar of flowers and often the flowers are not
those of the plant upon which the larvae feed, hence when the
time for egg-laying comes the insects must search for and find
the right plant and also the right place for the eggs. Some
species place a large number of eggs together, others always
deposit them singly. I have watched a number of species
ovipositing. The only butterfly which simply drops her eggs is
the Marbled White; this she does whilst in flight, among the
rough grasses on which the larvae feed, and the eggs, deposited
in August, do not hatch until the following April. The Silver-
washed Fritillary places her eggs on any surface that is handy;
I have seen her oviposit on dead leaves and once upon the trunk
of a tree. In this species the eggs are laid in the autumn: the
newly hatched larvae eat the egg shell and then hibernate until
the spring, when they start to feed upon the violet.
Butterflies' eggs vary in shape; they may be almost spherical
as in the Marbled White, of a cylindrical shape as in the Small
Tortoiseshell, of a pyramidal form as in the Brimstone, or shaped
like a sea urchin as in the Blues: in all these the horizontal
section is a circle. The eggs of those Skippers which place their
ova in the sheath between the blade and the stem of grasses are
distinct from all other butterflies' eggs; they seem to be pressed
out of shape, being oval and flattened.
The shell may be almost smooth as in the egg of the Swallow-