THE IRIS-WEEVIL

Plants, with their fruits, have been and still are the main sustenance of mankind. The ancient Paradise of which the eastern legends tell us had no other food-resources. It was a delicious garden with cool rivulets and fruits of every kind, including the apple that was to be so fatal to us. On the other hand, from a very early period, our ills sought to obtain relief by the virtues of simples, virtues that were sometimes real and sometimes, indeed most frequently, imaginary. Our knowledge of plants is thus as old as our infirmities and our need of food.

Our knowledge of insects, on the contrary, is quite recent. The ancients knew nothing of the lesser animals, did not even deign to glance at them. This disdain is by no means extinct. We are vaguely familiar with the work of the Bee and the Silk-worm; we have heard people speak of the industry of the Ant; we know that the Cicada sings, without having a very exact notion of the singer, who is confused with others; we have perhaps vouchsafed a careless glance to the splendours of the Butterflies; and with this, for [236]the immense majority, entomology begins and ends. What layman would risk naming an insect, even one of the more remarkable?

The Provençal peasant, who is pretty quick at observing things that have to do with the land, has a dozen expressions at the very most to denominate indiscriminately the vast world of insects, though he possesses a very rich vocabulary by which to describe plants. This or that bit of weed which one would think was known only to the botanists is to him a familiar object and bears a special name of its own.

Now the vegetarian insect is, as a rule, scrupulously faithful to its food-plant, so that, with botany and entomology going hand in hand, the beginner is spared many a hesitation. The plant exploited gives the name of the exploiting insect. Who, for instance, does not know the splendid yellow iris? The green cutlasses of its leaves and its yellow cluster of flowers are mirrored in the brooks. The pretty, green Tree-frog, swelling his throat into a bagpipe, sits and croaks in it at the approach of rain.

Come nearer. On its trivalvular capsules, which the heat of June is beginning to ripen, we shall see a curious sight. Here, a restless company of thick-set, rusty-red Weevils are embracing, separating and coming together again. They are working with their beaks and are busy mating. This shall be our subject for to-day.[237]

Our current language has not given them a name, but history has inflicted on them the fantastic appellation of Mononychus pseudo-acori, Fab. Literally interpreted and amplified, this means ‘the one-nailed insect of the mock acorus,’ acorus in its turn being derived from α, privative, and κόρη, the pupil of the eye. The grammarian’s scalpel, searching and dissecting the entrails of words, is liable, like the anatomist’s scalpel, to meet with strange adventures. Let us explain this scientific jargon, which at first sight seems utterly meaningless.

The plant helpful to those without pupils—that is to say, the weak-sighted—is the acorus, or sweet flag, which the medical science of antiquity prescribed for certain affections of the eyes. Its sword-shaped leaves bear some resemblance to those of the yellow iris. Ours, therefore, is the false acorus, a deceptive image of the famous medicinal plant.

As for the one nail, this is explained by the tarsi, the insect’s six fingers, each of which is armed with a single claw instead of the usual two. This strange exception certainly deserved to be pointed out; all the same, any one must prefer Iris-weevil to Mononychus pseudo-acori. Neglecting all pomp and ostentation, the everyday name does not topsy-turvify the mind and makes straight for the insect.

In June, I pluck some stems of yellow iris surmounted by their bunch of capsules, which are [238]already large and keep fresh and green for a long time. The exploiting Weevil goes with them. In captivity, under the trellis-work of a wire-gauze cover, the work proceeds just as it does beside the brook. Most of the insects, singly or in groups, stand at convenient points. With their rostrum plunged into the green hull, they sip and sup indefinitely. When they retire sated, a drop of gum oozes out which, after drying on the orifice of the well, marks the spot which they have drained.

Others are grazing. They attack the tender capsules and skin them almost down to the seeds. Despite their tiny size, they nibble gluttonously; when several of them are feasting together, they gnaw large areas; but they do not actually reach the seeds, the food reserved for the larvæ. Many of them stroll about, seem not to care for eating. They meet, tease one another for a moment and couple.

I do not succeed in observing the method of laying, which, however, must be much the same as that of the other Weevils who use a sound. The mother apparently bores a well with her rostrum; she then turns and places the egg in position by means of her oviscapt. I have seen larvæ quite recently hatched. The vermin occupy the interior of a seed whose substance is becoming organized and beginning to grow firm.

At the end of July, I open some capsules brought [239]on the same day from the banks of the stream. In most of them the insect occurs in the three forms of larva, nymph and adult. Each of the three cells of the fruit contains a row of some fifteen seeds, flat and pressed tightly one against the other. The grub’s portion consists of three contiguous seeds. The one in the middle is entirely consumed, excepting the husk, which is too tough; the two at either end are simply bitten into. The result is a house with three rooms, the central one shaped like a ring, the two outer ones dug cup-wise.

With its fifteen seeds, each compartment of the fruit is therefore able to shelter five larvæ at most, providing them with a fitting ration and a detached villa which does not interfere with the neighbours. However, on the back of the capsule, we count, for each cell, about twenty perforations, the edge of which is marked by a little wart either of gum or of some brown substance. These are so many soundings made by the Weevil’s rostrum.

Some of these have to do with the feeding: they are the refreshment-bars at which the colonists of the capsule have taken a snack. The others relate to the laying of the eggs and the placing of them, one by one, in the midst of the victuals. Outwardly there is nothing to distinguish the speck which marks a refreshment-bar from that which marks a cradle; therefore it is impossible, by merely counting the borings, to tell exactly how [240]many eggs have been confided to the capsule. Let us strike an average. Of the twenty punctures in one shell, let us consider ten as relating to the eggs. These would be twice as many as the cell could feed. What then has become of the surplus?

Here we are reminded of the Weevil who scatters over her pea-pod an excessive number of eggs, out of all proportion to the provisions which it contains. In the same way, on the iris, the pregnant mother takes no stock of the rations; she peoples the already populated and fills the overflowing. Her procreative fury does not reckon with the future. Let those thrive who may.

We can understand Verbascum thapsus allowing itself forty-eight thousand seeds when the germination of a single one would suffice to maintain the species: its distaff is a treasure-house of food by which a host of consumers will profit. But we cannot understand the Pea-weevil, the Iris-weevil and many others who, though not exposed to a serious thinning, nevertheless produce excessive families without taking into account the resources at their disposal.

For lack of room on the seed-capsule of the iris, of the ten guests in one shell four or five at most will survive. As for the disappearance of the rest, we need not seek the cause in the massacring of rivals, though the struggle for existence is fruitful in such crimes. The Weevil’s grub is too pacific [241]a creature to wring the neck of those which get in its way. I prefer the explanation which I gave in the case of the Pea-weevil. The late-comers, finding the best places taken, allow themselves to die without striving to dislodge the others. For those first installed, a plentiful board and life; for those which lag behind, famine and death.

In August the adults begin to appear outside the seed-pods of the iris. The larva has not the talent which the Pea-weevil’s grub possesses: it does not, by patient nibbling, make any sort of preparation for the exodus. It is the perfect insect itself that contrives the exit-way, which consists of a round hole bored through the tough husk of the seed and the thick wall of the fruit. Finally, in September, the capsules of the iris turn brown and the three valves become unfastened; the house threatens to fall to pieces. Before it becomes untenable, the last occupants hasten to clear out, each by its round window. They will spend the winter in the neighbourhood, under some kind of shelter; then, when spring returns and the iris is yellow with flowers, the colonizing of the capsules will begin all over again.

The flora of my district, not far from the spots frequented by our insect, in addition to the yellow iris comprises three other species. On the neighbouring hills, among the rock-roses and the rosemaries, the dwarf iris abounds (I. chamæiris, Bertol.), with flowers of varying colour: they [242]are sometimes purple, sometimes yellow or white and sometimes attired in a mixture of the three hues. The plant is barely a hand’s-breadth in height, but its flowers are quite as large as those of the other species.

On the same hills, at points where the rains have left a little moisture, the spurious iris (I. spuria) forms a glorious carpet. It is tall, slender-leaved and decked with flowers of rare beauty. Lastly, near the brook where I have been observing the Iris-weevil, is the Gladwyn iris, or leg-of-mutton iris (I. fœtidissiina, Lin.), whose leaves, when bruised, give a faint scent of mutton and garlic. Its seeds are a fine orange-red, a specific characteristic which recurs not elsewhere.

Altogether, without counting such foreigners as may have found their way into the flower-gardens around, we see four varieties of native iris at the Weevil’s disposal. They have the same sort of capsules, all equally bulky and equally rich in seeds, whose properties as food cannot differ much. Moreover the four plants flower at the same season. And of these four, which would permit her greatly to extend her race, the Weevil invariably selects the yellow iris. I have never found the insect established in the capsules of one of the other three.

For what reasons does she prefer niggardly uniformity to varied abundance? The tastes of the adult insect and those of the larva must have [243]something to say to the choice. The adult feeds on the fleshy hull of the capsules; the grub, on the other hand, lives entirely on the seeds, which are not yet hardened and are full of juice. Are the appetites of the adult insect satisfied with the fruit of any kind of iris? This can be tested.

Under the trellis-work of a wire cover, I place before the Weevil some green capsules of different origins. Jumbled up with the fruits of the yellow iris are those of the dwarf iris, the leg-of-mutton iris and the spurious iris. To these I add some foreign capsules, those of the pale Turkey iris (I. pallida, Lam.) and of the great bulbous iris (I. xiphoides, Ehrh.), which differs so greatly from the others by the bulb which takes the place of the usual rhizome.

Well, all these fruits are accepted as eagerly as those of the yellow iris. The Weevil riddles them with punctures, strips them bare, pierces them with windows. The capsules of my choosing and those from the banks of the stream, which are normally used, often lie side by side; the consumer makes no distinction between them, but goes without hesitation from one to the other, attacking them with a zeal which is in no wise impaired by the novelty of the dish. It considers everything good to eat, so long as it comes from an iris of some sort or other.

And this is not, as one might reasonably suppose, [244]an aberration caused by the tedium of captivity. I have found in the harmas1 on the tall stalks of the pale Turkey iris, a group of our Weevils feeding together on the green capsules. Whence came they, these pilgrims observed for the first time between my four walls? How did they learn, these colonists from the moist river-banks, that an iris which provided excellent eating was flowering amid the aridities of my acre of pebbles? At any rate, they left no part of the young capsules intact. The food discovered suited them very well. It was therefore impossible for me to profit by this windfall in order to ascertain whether the unfamiliar plant would serve for the establishment of the family.

Apart from the genus Iris, are there any other plants, its near botanical relations, whose fruits are accepted? I have vainly tried the trivalvular capsules of the corn-flag (Gladiolus segetum, Gawl.) and the globular capsules of two asphodels (Asphodelus luteus, Lin. and A. cerasiferus, Gay). The Weevil would have none of them. At most she dipped her rostrum into the green capsules of the yellow asphodel, the common Jacob’s staff. She tasted and then moved away. The dish was not to her liking; and hunger was unable to overcome her obstinate disdain. She would die of starvation [245]sooner than touch victuals unhallowed by tradition.

It goes without saying that I found nothing in the way of eggs on the corn-flag or the two asphodels. What the insect regards as unfit for its own consumption is a fortiori refused when the grub’s food is concerned. Nor was I any luckier with the various irises which I tried, the yellow iris excepted. Are we to attribute this refusal to the insect’s captivity? No, for the capsules of the yellow iris were colonized fairly well under my wire covers. The fact is that, as soon as the establishment of the family comes into question, the Weevil abstains entirely from anything that is contrary to habit and remains firmly faithful to the laws and customs of the ancients. In short, I have never found the Weevil established elsewhere than in the capsules of the yellow iris, however appetizing the appearance of the others, especially those of the dwarf iris, which are exceedingly fleshy and very numerous in the spring.