88 EPPING FOREST
has of late years become a sight only too frequent
and distressing in our spring woodlands.
When we remember that each female moth lays
some hundreds of eggs, it is manifest that there
must exist some efficient controlling agents, or our
woodlands would succumb to the plagues of cater-
pillars in a few years. Birds doubtless devour
prodigious numbers of insects, but birds alone
would be utterly unable to cope with the myriads
upon myriads of such caterpillars as those we have
above mentioned. It is to the various kinds of
small hymenopterous flies, belonging to the Fami-
lies Ichneumonidae and Chalcididae, that we must
principally attribute this control. The eggs of these
parasites are deposited within the bodies of the
caterpillars, and the resulting grubs live and thrive
on the substance of their victims, ultimately de-
stroying them. It is a curious matter for specula-
tion whether over-protection, and the consequent
abnormal abundance of insectivorous birds, may
not actually lead to an increase in some years of
certain kinds of caterpillar pests. It is commonly
observed by collectors that when caterpillars are
"stung" they become sluggish, are easily shaken
off the branches, and, by reason of abandoning
their special methods of concealment, they are
easily detected. Healthy larvae are much more
difficult to find. It is quite conceivable that
birds may devour these sluggish, readily-seen larvae,
together with the contained parasites, leaving the
more concealed and healthy ones to continue
the race, and furnish forth a sufficient number of
moths in November and December to provide
another army of caterpillars in the spring. At the
same time the ratio of the parasites to the cater-
pillars would have been disturbed by the uncon-
scious selection of the birds in the previous season.
This selection may go on until the caterpillars
become a plague, or until some other natural con-
trolling action (which must exist, although in some
years latent) springs into force and restores the
equilibrium between the parasites and their " hosts."
What is the precise nature of this hidden controlling