Page:The American Cyclopædia (1879) Volume IX.djvu/322

 310 INSECTIVOROUS PLANTS INSECTS INSECTIVOBOrS PLA3VTS. In the article Dio- 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 dionma 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 dioncea than in any other plant, and it is provided with a special- ly devised apparatus for its gratification ; but there are other plants which destroy insects, and what is known of dionasa has put natu- ralists 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 no- ticed, but their occurrence was considered ac- cidental. It is now known that sundews cap- ture 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 es- cape 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 dioncea, 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., Decem- ber, 18T3) 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 num- bers 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 sur- face of the pitcher-like leaves is mostly cov- ered 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 in- tended 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 de- composed 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. INSECTS, six-footed articulated animals, the most beautiful, most active, and most highly organized of the invertebrata, in which, ana- tomically considered, they bear a remarkable analogy to birds among the vertebrates. Like birds they inhabit the air, earth, and water, have an extensive respiratory apparatus, and consequently a higher calorific and motor power than any other invertebrates. The number of species and of individuals is exceedingly great, and their metamorphoses are among the most interesting phenomena in nature. The class of insects includes all articulates having a distinct head, thorax, and abdomen, with antenna?, three pairs of feet, an aerial respiration by means of tracheae, a feebly developed circula- ting system, almost all being winged and under- going transformation. The cutaneous envelope forms a kind of external skeleton, generally of a horny consistence, formed principally of chitine ; it is made up of a considerable num- ber of pieces more or less movable on each other, and is frequently provided with hairs, which are sometimes sharp and barbed (as in the processionary caterpillars), producing con- siderable irritation when introduced into the human skin. The limbs, which are appendages of the thorax, are hollow tubes containing the muscles and nerves for their motion. The first segment constitutes the head, on which are placed the antennas, the eyes, and the oral ap- pendages. The antennee are composed of a va- riable number of joints, generally resembling delicate and flexible horns, plumed, serrated, clubbed, or foliated, according to genera and families ; they are principally organs of touch. The mouth in the chewing insects, like the beetles, cockroaches, and grasshoppers, con- sists of an upper middle piece or Icibrum with a mandible on each side, the latter being very hard, often toothed at the extremity ; the max- illa or under jaws are softer and of many pieces, with maxillary jointed palpi, and the central piece is the labium or under lip, also supporting jointed palpi. At the base of the