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 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.
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 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 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 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 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 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 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, an aberration caused
by the tedium of captivity. I have found in the harmas [64] 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 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.