Travelling by long stages, with their waggons drawn by shaggy Oxen and rolling on solid wheels cut out of the trunks of trees, the emigrants from Central Asia brought to our uncultivated tracts first the bean, then the pea and finally the cereal, that eminent stand-by against hunger. They taught us the care of herds and the use of bronze, of which the first metal implements were made. [[203]]Thus did the dawn of civilization rise over Europe.
With the bean did those ancient pioneers bring us, involuntarily, the insect which disputes its possession with us to-day? There is room for doubt; the Bruchus seems to be a native. I find her at least levying tribute on divers Leguminosæ of the country, spontaneous plants which have never tempted man’s appetite. She abounds in particular on the great broad-leaved everlasting pea (Lathyrus latifolius), with its magnificent clusters of flowers and its long and handsome pods. Its seeds are not large, are much smaller than those of our peas; but, gnawed to the very skin, as they always are by their occupants, they are each sufficient to the welfare of its grub.
Note also their considerable number: I have counted more than twenty to the pod, a wealth unknown to the garden pea, even in its most prolific state. Thus the superb perennial is generally able, without much loss, to feed the family entrusted to its pod.
Where the everlasting pea is lacking, the Bruchus none the less continues her habitual flux of germs on another legumen, of similar flavour but incapable of nourishing all the grubs, as for instance on the broad-podded vetch (Vicia peregrina) or the common vetch (V. sativa). The number of eggs remains high even on these insufficient pods, because the original plant offered a copious provender, [[204]]whether by the multiplicity or by the large size of the seeds. If the Bruchus is really a foreigner, we may accept the bean as her first victim; if the insect is a native, let us accept the everlasting pea.
Some time in the remote past the pea reached us, gathered at first in the same prehistoric garden-patch which already supplied the bean. Man found it a better food than the horse-bean, which is very much neglected to-day after doing such good service. The Weevil was of the same opinion and, without quite forgetting her broad bean and her everlasting pea, generally pitched her camp on the garden pea, which became more widely cultivated from century to century. To-day we have to go shares: the Bruchus takes what she wants and lets us have her leavings.
The insect’s prosperity, born of the abundance and quality of our products, from another point of view spells decadence. For the Weevil as for ourselves, progress in the matter of food and drink does not always mean improvement. The race fares better by remaining frugal. On her horse-bean, on her everlasting pea, the Bruchus founded colonies in which the infant mortality was low. There was room for all. On the pea, a delectable sweetmeat, the greater part of the guests die of starvation. The rations are few and the claimants legion.
We will linger over this problem no longer. [[205]]Let us inquire into the grub which has become the sole owner of the pea through the death of its brothers. It has had no part in that decease; chance has favoured it, that is all. In the centre of the pea, a luxurious solitude, it performs a grub’s duty, the one and only duty of eating. It gnaws the walls around and enlarges its cell, which it always fills completely with its fair round belly. It is a plump and shapely creature, glistening with health. If I tease it, it turns lazily in its cell and wags its head. This is its way of complaining of my rudeness. Let us leave it in peace.
The anchorite thrives so well and so fast that, by the dog-days, it is already making ready for its coming liberation. The adult has not the necessary tools to open for herself her way out of the pea, which is now quite hard. The larva knows of this future helplessness and provides against it with consummate art. With its strong jaws it bores an exit-shaft, absolutely circular, with very clean-cut sides. Our best ivory-carvers could produce nothing neater.
To prepare the door of escape in advance is not enough; we must also think of the tranquillity essential to the delicate work of the nymphosis. An intruder might enter through the open door and work mischief upon the defenceless nymph. This opening must therefore be kept shut. And how? Here is the device.
The grub boring the exit-hole eats the floury [[206]]matter without leaving a single crumb. On reaching the skin of the seed, suddenly it stops short. This semitranslucent membrane is the screen protecting the chamber in which the metamorphosis takes place, the door that defends the cabin against ill-intentioned intruders. It is also the only obstacle which the adult will encounter at the time of moving. To lessen the difficulty of forcing it out, the grub takes the precaution of carving a groove of least resistance inside the skin, all around the circumference. The perfect insect will only have to heave with its shoulders, to strike a blow or two with its head, in order to raise the lid and knock it off, like the lid of a box. The exit-hole shows through the transparent skin of the pea in the shape of a large circular spot, darkened by the obscurity within. What happens below cannot be seen, hidden as it is behind a sort of ground-glass window.