"They lay wait as he that setteth snares".
- Jeremiah v. 26.
The story of the trap-setting and insect-eating plants is a more than twice-told tale. The pitcher-plant, which beguiles the hapless fly to his drowning in its vase-shaped leaves, baited on the outside with nectar-bearing glands, and filled with water; the Venus's fly-trap, which shuts up on him and crushes him; the sundew (Drosera), which chokes him in a sticky secretion, are all known, at least by pictures and descriptions, to the tyro in botanic study. And we have learned that they all have good and sufficient reasons for thus dealing with the hapless flies. For, as Darwin has pointed out, these plants usually grow rooted in moss, or in very sandy and barren soil. Insect-eating leaves are probably a device to supply the plant with nitrogen by means of the foliage, in circumstances where the roots prove powerless for the purpose.
Fig. 84. - Spreading dogbane (Apocynum androscemifolium).
The insect slaughter which they carry on has the same excuse -as the animal slaughter of the abattoir. It is killing for food, and the insects which these plants catch are honestly eaten and digested. But in the infinite analogy of the vegetable world we find what seems a curious parallel to killing for sport. There are a few native flowers which entrap insects simply and solely, it appears, for the deed's own sake. The prisoners serve no apparent use in the plant's economy, nor do their poor little corpses nourish the plant's life. A botanist who let his imagination run away with him might accuse the guileless-looking flowers of that savage joy in another creature's pain which drew our forefathers in crowds to the badger-drawings and bear-baitings of bygone times.
One of these flower tormentors is the spreading dogbane (Apocynum androsaemifolium) (Fig. 84), which is common all summer, along shady roadsides and around the borders of thickets, in the Northern and Eastern states. The plant is about three feet high, erect and branching. The flowers are nearly as large as single blossoms of the lily of the valley, and when closely examined are seen to be very beautiful.
The corolla is bell-shaped, and cleft at the edge into five slender points. Its deep pink-veining suggests nectar, and the insect visitor is not disappointed, for at its base are five nectar-bearing glands. These stand in a ring around the pistil, and in a larger circle, outside the ring of honey-glands, are the five stamens. The anthers stand erect, and in shape are like arrow- or spear-heads (Fig. 85). Corresponding to the two points at the base of a spear-head there are, at the base of each anther, two little hard horns, and the stamens ring so closely about the pistil that horn is beside horn all around the circle.
On the inside of the corolla, near its base, are five triangular callosities, with their points up. These are placed in such a way as to alternate, with the stamens, and stand a little below them, so that the two hard points at the bases of two neighboring anthers, and the hard tip of the callosity - three little horns - come together like the teeth of a trap. There are no fewer than five places inside the flower's cup where these traps are set, and inside the circle of traps are the glands which contain nectar (Fig. 85).
The blossom is visited by bees and flies, but its favorite guests, says Muller, are butterflies. It cements its pollen to their tongues, and thus compels them to carry it away with them to other dogbane flowers.
Fig. 85. - Trap of the spreading dogbane.
a, the flower with its calyx and corolla removed, showing the stamens and honey-glands; b, the opened corolla, showing the callosities; c, a trap seen from the side; d, the circle of traps, seen from above.
The fly-caller seems unable to sip the nectar except by running his proboscis in between the long anthers, and just above the horny excrescences on the corolla. When he attempts to withdraw, after drinking his fill, the three points lock together, like the jaws of a trap, holding the tip of his proboscis in durance vile. If the winged captive is big and strong, he gets free with a long and a vigorous pull. But small flies are often held prisoners till they die, probably from starvation. Sometimes one may see three or four of these hapless victims on one full-blooming plant of spreading dogbane.
Among the prisoners one may often see a little summer-fly of dudish aspect, with body ringed with alternate bands of bronze and gold and wings of gauze shot with opaline colors. To what end is this bright little fellow sacrificed? Held as he is by the tip of his proboscis, his body does not come into contact with the plant, and hence it cannot be digested by the vegetable juices, as are the corpses of the sundew's victims. The only possible justification the dogbane can furnish for his taking off is that he has trespassed upon the butterfly's preserves. For this intrusion he is dealt with as severely as poachers were under the forest-laws of feudal England.
Fig. 86. - Common milkweed (Asclepias cornuti).
There is another variety of dogbane, the Indian-hemp, or Apocynum cannabinum, which bears smaller blossoms than the androsaemifolium, blooms somewhat later, and is more widely distributed over the country. This flower has no callosities in its corolla, sets no scares for insect victims, and is apparently quite innocent of the crimes which one is inclined to lay to the charge of its first cousin.
The common milkweed (Asclepias cornuti) (Fig. 86) also imprisons insects, which sometimes die in captivity, and do no apparent good to the plant by their deaths. They have, however, invited misfortune, for though the milkweed is rich in honey and is visited by a large and miscellaneous company, it can be fertilized, apparently only by bees, and perhaps by a few large flies.