Miss Ormerod, in the very interesting report which she has
just published on injurious insects during the year 1881 (pub- lished by Swan Sonnenschein and Co.), gives an interesting description of the oak leaf-roller moth (Tortrix viridana) and caterpillar, which is the one that has done so much mischief to our oaks during the present and the past year. She tells us that, besides the rooks and black-headed sea-gulls, which (with the too rare nightjars) are the most efficient of the winged de- stroyers of this moth and caterpillar, one of its most formid- able enemies is a carnivorous caterpillar (Cosmia trapezina), which feeds upon the caterpillar of the oak leaf-roller. Miss Ormerod appears to regard these Trapezina caterpillars as almost as important for the purpose or clearing off the Tortriz viridana caterpillar, as the insectivorous birds themselves, and she remarks that the worst of the insectivorous birds is that they feed on the insects which are useful to us, no less than on the insects which are pernicious, whereas this carnivorous cater- pillar apparently feeds only on our insect enemies. Hence she discourages the unlimited multiplication of small birds, though deprecating any reduction of them below their " natural aver- age." But what is their " natural average P"
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