« EelmineJätka »
America not only bees, moths, and other large insects carry the pollen from one flower to another, but many flowers, like the Marcgravia, are specially adapted to secure the aid of small birds, particularly humming-birds, for this purpose. Amongst these, the “ palosabre," a species of Erythrina, a small tree, bearing red flowers,
FLOWER OF PALOSA BRE.
that grew in this valley, near the brook, often drew my attention. The tree blooms in February, and is at the time leafless, so that the large red flowers are seen from a great distance. Each flower consists of a single long, rather fleshy petal, doubled over, flattened, and closed, excepting a small opening on one edge, where the stamens protrude. Only minute insects can find access to the flower, which secretes at the base a honey-like fluid. Two long-billed humming-birds frequent it; one (Heliomaster pallidiceps, Gould), which I have already mentioned, is rather rare; the other (Phæthornis longirostris, De Latt.) might be seen at any time when the tree was in bloom, by watching near it for a few minutes. It is mottled brown above, pale below, and the two middle tail feathers are much longer than the others. The bill is very long and curved, enabling the bird easily to probe the long flower, and with its extensile cleft tongue pick up the minute insects from the bottom of the tube, where they are caught as if in a trap, their only way of exit being closed by the bill of the bird. Ch. VIII.)
FOXGLOVE AND HUMBLE-BEE.
Whilst the bird is probing the flower, the pollen of the stamens is rubbed in to the lower part of its head, and thus carried from one flower to fecundate another. The bottom of the flower is covered externally with a thick, fleshy calyx-an effectual guard against the attempts of bees or wasps to break through to get at the honey. Humming-birds feed on minute insects, and the honey would only be wasted if larger ones could gain access to it, but in the flower of the palosabre this contingency is simply and completely guarded against.
Many flowers have contrivances for preventing useless insects from obtaining access to the nectaries. Amongst our English flowers there are scores of interesting examples, and I shall describe the fertilisation of one, the common foxglove, on account of the exceeding simplicity with which this object is effected, and to draw the attention of all lovers of nature to this other branch of a subject on which the labours of Darwin and other naturalists have of late years thrown a flood of light. The pollen of the foxglove (Digitalis purpurea) is carried from one flower to another by the humble-bee, who, far more than the hive bee, that “improves each shining hour,” deserves to be considered the type of steady, persevering industry. It improves not only the hours of sunshine, but those of cloud, and even rain; and, long before the honey-bee has ventured from its door, is at work bustling from flower to flower, its steady hum changing to an importunate squeak as it rifles the blossoms of their sweets. The racemes of purple bells held up by the foxglove are methodically visited by it, commencing at the bottom flower, and ascending step by step to the highest. The four stamens and the pistil of the foxglove are laid closely
against the upper side of the flower. First a stamen on one side opens its anthers and exposes its pollen. The humble-bee, as it bustles in and out, brushes this off. Then another stamen exposes its pollen on the other side, then another and another; but not till all the pollen has been brushed off does the cleft-end of the pistil open, and expose its viscid stigma. The humblebee brushes off the pollen unto its hairy coat from the upper flowers of one raceme and carries it direct to the lowest flowers of another, where the viscid stigmas are open and ready to receive it. If the humble-bee went first to the upper flowers of the spike and proceeded downwards, the whole economy of this plant to procure cross fertilisation would be upset.* The open flower of the foxglove hangs downwards. The lower part, or dilated opening of the tube, is turned outwards, and has scattered stiff hairs distributed over its inner surface; above these the inside of the flower hangs almost perpendicularly, and is smooth and pearly. The large humblebee bustles in with the greatest ease, and uses these hairs as footholds whilst he is sucking the honey; but the smaller honey-bees are impeded by them, and when, having at last struggled through them, they reach the pearly, slippery precipice above, they are completely baffled. I passed the autumn of 1857 in North Wales, where the foxglove was very abundant, and watched the flowers throughout the season, but only once saw
* Darwin mentions having seen humble-bees visiting the flowering spikes of the Spiranthes autumnalis (ladies' tresses), and notices that they always commenced with the bottom flowers, and crawling spirally up sucked one flower after the other, and shows how this proceeding ensures the cross fertilisation of different plants.—“Fertilisation of Orchids,” p. 127.
SPIDERS THE PREY OF WASPS.
a small bee reach the nectary, though many were seen trying in vain to do so.
Great attention has of late years been paid by naturalists to the wonderful contrivances amongst flowers to secure cross fertilisation; but the structure of many cannot, I believe, be understood, unless we take into consideration not only the beautiful adaptations for securing the services of the proper insect or bird, but also the contrivances for preventing insects that would not be useful, from obtaining access to the nectar. Thus the immense length of the nectary of the Angræcum sesquipedale of Madagascar might, perhaps, have been completely explained by Mr. Wallace, if this important purpose had been taken into account.*
The tramway in some parts was on raised ground, in others excavated in the bank side. In the cuttings the nearly perpendicular clay slopes were frequented by many kinds of wasps that excavated round holes of the diameter of their own bodies, and stored them with sting-paralysed spiders, grasshoppers, or horse - flies. Amongst these they lay their eggs, and the white grubs that issue therefrom feed on the poor prisoners. I one day saw a small black and yellow banded wasp (Pompilus polistoides) hunting for spiders; it approached a web where a spider was stationed in the centre, made a dart towards it—apparently a feint to frighten the spider clear of its web; at any rate it had that effect, for it fell to the ground, and was immediately seized by the wasp, who stung it, then ran quickly backwards, dragging the spider after it, up a branch reaching to the
* "Natural Selection,” by A. R. Wallace, p. 272.
ground, until it got high enough, when it flew heavily off with it. It was so small, and the spider so heavy, that it probably could not have raised it from the ground by flight. All over the world there are wasps that store their nests with the bodies of spiders for their young to feed on. In Australia, I often witnessed a wasp combating with a large flat spider that is found on the bark of trees. It would fall to the ground, and lie on its back, so as to be able to grapple with its opponent; but the wasp was always the victor in the encounters I saw, although it was not always allowed to carry its prey off in peace. One day, sitting on the sand-banks on the . coast of Hobson's Bay, I saw one dragging along a large spider. Three or four inches above it hovered two minute flies, keeping a little behind, and advancing with it. The wasp seemed much disturbed by the presence of the tiny flies, and twice left its prey to fly up towards them, but they darted away immediately. As soon as the wasp returned to the spider, there they were hovering over and following it again. At last, unable to drive away its small tormentors, the wasp reached its burrow and took down the spider, and the two flies stationed themselves one on each side the entrance, and would, doubtless, when the wasp went away to seek another victim, descend and lay their own eggs in the nest.
The variety of wasps, as of all other insects, was very great around Santo Domingo. Many made papery nests, hanging from the undersides of large leaves. Others hung their open cells underneath verandahs and eaves of houses. One large black one was particularly abundant about houses, and many people got stung by