Most of the insects which are found in the tubular leaves of this and similar plants enter into them voluntarily; but Sir James Smith mentions a curious fact, from which it appears that in some cases they are deposited by other species. One of the gardeners of the Liverpool Botanic Garden observed an insect, from the description one of the Crabronidæ, which dragged several large flies to the Sarracenia adunca, and, having with some difficulty forced them under the lid or cover of its leaf, deposited them in its tubular part which was half filled with water: and on examination all the leaves were found crowded with dead or drowning flies[532]. What was the object of this singular manœuvre does not seem very obvious. At the first glance one might suppose that, having deposited an egg in the fly, it intended to avail itself of the tube of the leaf instead of a burrow. Yet we know of no such strange deviation from natural instinct, which would be the more remarkable because the insect was European, while the plant was American and growing in a hot-house. And at any rate it does not seem very likely that the insect would commit her egg to the tube without having previously examined it; in which case she must have discovered it to be half full of water, and consequently unfit for her purpose.—It is not so wonderful that many large flies should, as Professor Barton informs us, drop their eggs into the Ascidia furnished with dead carcases: and it seems very probable that Dytisci oviposit in them; for the Squilla which Rumphius found there was probably one of their larvæ, this being the old name for them[533].
However problematical the agency of insects caught by plants as to their nutriment, there can be no doubt that many species perform an important function with regard to their impregnation, which indeed without their aid would in some cases never take place at all. Thus, for the due fertilization of the common Barberry (Berberis vulgaris) it is necessary that the irritable stamens should be brought into contact with the pistil by the application of some stimulus to the base of the filament; but this would never take place were not insects attracted by the melliferous glands of the flower to insinuate themselves amongst the filaments, and thus, while seeking their own food, unknowingly fulfill the intentions of nature in another department[534].
The agency of these little operators is not less indispensable in the beautiful tribe of Iris. In these, as appears from the observations of Kölreuter, the true stigma is situated on the upper side of a transverse membrane (arcus eminens of Haller) which is stretched across the middle of the under surface of the petal-like expansion or style-flag, the whole of which has been often improperly regarded as fulfilling the office of a stigma. Now as the anther is situated at the base of the style-flag which covers it, at a considerable distance from the stigma, and at the same time cut off from all access to it, by the intervening barrier formed by the arcus eminens, it is clear that but for some extraneous agency the pollen could never possibly arrive at the place of its destination. In this case the humble-bee is the operator. Led by instinct, or, as the ingenious Sprengel supposes, by one of those honey marks (Saftmaal) or spots of a different colour from the rest of the corolla, which, according to him, are placed in many flowers expressly to guide insects to the nectaries, she pushes herself between the stiff style-flag and elastic petal, which last, while she is in the interior, presses her close to the anther, and thus causes her to brush off the pollen with her hairy back, which ultimately, though not at once, conveys it to the stigma. Having exhausted the nectar she retreats backwards; and in doing this, is indeed pressed by the petal to the arcus eminens; but it is only to its lower or negative surface, which cannot influence impregnation. She now takes her way to the second petal, and insinuating herself under its style-flag, her back comes into close contact with the true stigma, which is thus impregnated with the pollen of the first visited anther: and in this manner migrating from one part of the corolla to another, and from flower to flower, she fructifies one with pollen gathered in her search after honey in another.—Mr. Sprengel found, that not only are insects indispensable in fructifying the different species of Iris, but that some of them, as I. Xiphium, require the agency of the larger humble-bees, which alone are strong enough to force their way beneath the style-flag: and hence, as these insects are not so common as many others, this Iris is often barren, or bears imperfect seeds[535].
Aristolochia Clematitis, according to Professor Willdenow, is so formed, that the anthers of themselves cannot impregnate the stigma; but this important affair is devolved upon a particular species of gnat (Cecidomyia pennicornis). The throat of the flower is lined with dense hair, pointing downward so as to form a kind of funnel or entrance like that of some kinds of mouse-traps, through which the insects may easily enter but not return: several creep in, and, uneasy at their confinement, are constantly moving to and fro, and so deposit the pollen upon the stigma: but when the work entrusted to them is completed, and impregnation has taken place, the hair which prevented their escape shrinks, and adheres closely to the sides of the flower, and these little go-betweens of Flora at length leave their prison[536]. Sir James Smith supposes that it is for want of some insect of this kind that Aristolochia Sipho never forms fruit in this country.
Equally important is the agency of insects in fructifying the plants of the Linnean classes Monoecia, Dioecia and Polygamia, in which the stamens are in one blossom and the pistil in another. In exploring these for honey and pollen, which last is the food of several insects besides bees[537], it becomes involved in the hair, with which in many cases their bodies seem provided for this express purpose, and is conveyed to the germen requiring its fertilizing influence. Sprengel supposes that with this view some plants have particular insects appropriated to them, as to the dioecious nettle Catheretes Urticæ, to the toad-flax Catheretes gravidus, both minute beetles, &c. Whether the operations of Cynips Psenes be of that advantage in fertilizing the fig, which the cultivators of that fruit in the East have long supposed, is doubted by Hasselquist and Olivier[538], both competent observers, who have been on the spot. Our own gardeners, however, will admit their obligations to bees in setting their cucumbers and melons, to which they find the necessity of themselves conveying pollen from a male flower, when the early season of the year precludes the assistance of insects. Sprengel asserts, that apparently with a view to prevent hybrid mixtures, insects which derive their honey or pollen from different plants indiscriminately, will during a whole day confine their visits to that species on which they first fixed in the morning, provided there be a sufficient supply of it[539]; and the same observation was long since made with respect to bees by our countryman Dobbs[540].
Thus we see that the flowers which we vainly think are
"... born to blush unseen,
And waste their fragrance on the desert air,"
though unvisited by the lord of the creation, who boasts that they were made for him, have nevertheless myriads of insect visitants and admirers, which, though they pilfer their sweets, contribute to their fertility.
I am, &c.