THE ENTOMOLOGIST IN THE FOREST 89
action, whether sudden variations in temperature or
moisture, or (probably) the attacks of bacterial or
fungoid diseases, is at present unknown. The
study of the problem of the cause of the sudden
increase or decrease of groups of insects is scarcely
advanced beyond the purely speculative stage.
We have lingered too long, perhaps, with the
Lepidoptera, albeit there are other divisions (such
as the Tineina and Pyralides) equally worthy of
the attention of the Field Naturalist, and we must
now turn to some of the other groups of insects
which find in the forest a congenial home.
The parasitical tribes of the Hymenoptera have
been alluded to, but that great order includes many
families. The Bees (Apidae) and the Wasps
(Vespidae) are not so numerous in species or
individuals as one would wish, owing to the fewness
of sandy tracts and banks; there are, however,
some interesting species, including the Hornet
( Vespa crabro), and at least one species of aerial
nest-making wasp (Vespa norvegica), the beautiful
pendulous home of which is not very rare in goose-
berry and currant plantations. The rambler can
hardly fail to notice the large Wood Ant (Formica
rufa) in the late summer, crawling over the paths
and up the trunks of trees, and presenting a formid-
able front of distended mandibles and elevated
head when irritated or alarmed. A little following
up the lines of foraging ants will lead one to the nest
beneath some hawthorn bush, externally a large
conical heap of leaves and twigs, but within
designed and reared with all the wondrous skill of
some of Nature's best architects. (N.B. Do not
disturb the nest when you find it.)
One or two species of the larger Humble-bees
in the forest are very liable to be infested by that
extraordinary internal parasite, Sphaerularia bombi,
belonging to the Thread-worms or Nematoda.
When we were studying the subject years ago, it
was found that 60 or 70 per cent, at least, of the
specimens of Bombus terrestris caught while flying
round the rhododendron flowers at High Beach
were so infected. En passant, the parasites of