270 OAK GALLS AND GALL INSECTS
reproduction has become parthenogenetic, owing to a gradual
loss of males, there being only one generation a year. It is
probable that the process has been equally slow in other forms,
but is so far advanced that only females are known. In Rhodites
rosae we may conclude that males will soon be quite as extinct
as they at present are in the agamic forms of oak gall-makers.
Again, the receptaculum seminis is present both in the agamic
and sexual generations, though atrophied in the former. Unless
this can be explained by its being inherited in the agamic
generation from the sexual, but being functionless has gradually
become atrophied from want of use and may eventually dis-
appear, it seems to point to the fact that males were present at
some remote period. Before copulation can take place the
ovipositor of the female must be extruded. Copulation takes
place soon after the insects leave the galls, and the females sit
with the ovipositor extruded awaiting fecundation. But this
extrusion is carried out by the females of the agamic generations
also, though they have no males to fecundate them and repro-
duce parthenogenetically. Does not this apparently prove that
these agamic generations at one time had males, or could this act
come under the head of inherited instincts?
Some galls, especially bark galls such as Andricus radicis,
require two years in which to complete their generation cycle.
The rudiment of the gall is formed in the first year and further
development is arrested till the following spring, when the gall
formation is renewed with a fresh period of vegetative activity.
Maturity is reached in the autumn, the flies emerging in the
spring of the next year. These lay eggs in the buds, which give
rise to the sexual generation (A. trilineatus Htg.) which in turn
oviposit to form the A. radicis gall. Galls formed by the brood
appearing in the spring take a much shorter time to reach
maturity than those formed by the sexual generation.
Insects of the agamic generation appear in early spring
often in very cold weather, some forms, such as Biorhiza
terminalis, frequently ovipositing during the winter, when the
snow is on the ground and the temperature at freezing point;
consequently it is a good thing for them to be able to lay their
eggs at once, without previous connection with a male. The
sexual generation on the other hand appears in the summer
when the weather is warm, live longer, and so have more time
to spare for the process of continuing their species.