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Leaves remain shut for a longer time over insects, especially if the latter are large, than over meat. In many instances where they have remained for a long period over insects naturally caught, they were more or less torpid when they reopened, and generally so much so during many succeeding days that no excitement of the filaments caused the least movement. Vigorous leaves will sometimes devour prey several times, but ordinarily twice, or, quite often, once is enough to render them unserviceable.

What purpose the marginal spikes, which form so conspicuous a feature in the appearance of the plant, subserve was unknown until the genius of Darwin solved the mystery. It was he that showed that elongated spaces between the spikes, varying from one-fifteenth to one-tenth of an inch in breadth according to the size of the leaf, are left open for a short time before the edges of the lobes come into contact, consequent upon the intercrossing of the tips of the marginal spikes first, thus enabling an insect whose body is not thicker than these measurements to escape, when disturbed by the closing lobes and the increasing darkness, quite easily between the crossed spikes. Moderately sized insects, if they try to escape between the bars, will be pushed back into the horrid prison with the slowly closing walls, for the spikes continue to close more and more until the lobes are brought into contact. Very strong insects, however, manage to effect their release. It would manifestly be a great disadvantage to the plant to remain many days clasped over a minute insect, and as many additional days or weeks in recovering its sensibility, inasmuch as a very small insect would afford but little nourishment. Far better would it be for the plant to wait until a moderately large insect was captured, and to allow the little ones to escape, and this advantage is gained by the slow intercrossing of the marginal spikes, which, acting like the large meshes of a fishing-net, allow the small and worthless fry to pass through.