In the article Dio-n^ea the structure of the Venus's fly-trap has been described, and the recent discoveries in relation to its action have been briefly stated. The leaves of the dionea present a beautifully designed and most efficient insect trap, and while the fact of its catching insects had long been known and wondered at, it has only within a few years been demonstrated that the plant does not catch insects for amusement, but food. The insectivorous propensity is more strikingly manifested in dionaea than in any other plant, and it is provided with a specially devised apparatus for its gratification; but there are other plants which destroy insects, and what is known of dionaea has put naturalists upon a course of observation. The drosera or sundew, some species of which are found in almost all parts of the world, has its leaves studded with short hairs, each of which is tipped by a little globule of a clear liquid which, though it looks like a drop of dew, is so viscid as to be able to hold fast a small insect that alights upon the leaf. Dead insects upon the sundews have long been noticed, but their occurrence was considered accidental. It is now known that sundews capture insects with a motion quite certain in its results.
When an insect is caught by one or more of the sticky hairs, the other hairs upon the leaf incline toward it, and bring so many adhesive points in contact with it that escape is impossible; and the leaf itself curves and partly envelops the prey. In the case of one long, thread-leaved sundew (D. filiformis), the leaves actually coil around the insect. As in the case of dionaea, the action is excited by a piece of beef as well as by an insect, but to an inorganic substance, as a bit of chalk, it is indifferent. The observations of Mrs. Treat ("American Naturalist," Salem, Mass., December, 1873) show that when a fly is pinned at the distance of half an inch from the leaves, they will bend toward and reach it. The sar-racenias or pitcher plants, of which there is one species in the northern states and several in the southern, all have tubular leaves which contain water in which are found great numbers of dead insects. It is known that in some species at least there is near the opening of the pitcher a sweet secretion, which would .appear to be placed there for the purpose of attracting flies and other insects; indeed, the whole structure of the leaves of these plants shows that they are designed as insect traps.
The water, which in some species may be caught from the rains, is in others secreted by the plant, as the orifice is so covered by a hood that none can fall in; the interior surface of the pitcher-like leaves is mostly covered with fine sharp bristles which all point downward, and render it almost impossible for an insect that is once within to escape; if we add to this the attractive sweet liquid at the mouth of the pitcher, which has been observed in two species, we have a very complete insect trap; indeed, the large leaves of the species called trumpets (S. flava) are said to be used in houses as fly traps. The abundance of dead insects in the leaves of those pitcher plants which are protected by a hood may be cited in evidence that their habitual drowning of them is not accidental, but that the apparatus is intended to capture and destroy them. When the contents of one of these pitcher leaves is examined there are found insects in all stages, from those recently caught to those so far decomposed as to make it impossible to identify them. The leaves of the related California pitcher plant (Darlingtonia) are most effective traps; according to Mr. Robinson, the tubular leaves are for a good portion of their length filled with a nearly solid mass of putrescent insects.
The opinion that the animal matter thus abundantly secured by these plants serves to nourish them, though not demonstrated, is highly probable.