THE VEGETARIAN INSECTS
Alone of living creatures, civilized man knows how to eat, by which I
mean to say that he treats the affairs of the stomach with a certain
pomp and circumstance. He is an expert cook and an artist in delicate
sauces. He celebrates his meals with luxurious plate and crockery. He
officiates at table like a high-priest; he practises rites and
ceremonies. At his banquets he calls for music and flowers, that he may
masticate his portion of dead flesh in splendour.
Animals do not display these eccentricities. They merely feed, which,
after all, may very well be the true means of avoiding deterioration.
They take nourishment; and that, for them, is enough. They eat to live,
whereas many of us live, above all, to eat.
Man’s stomach is a pit in which all things edible are engulfed. The
stomach of the vegetarian insect is a fastidious laboratory to which
nothing but appointed mouthfuls are allowed to find their way. Each
guest at the vegetarian banquet has its plant, its fruits, its pod, its
seed, which it eagerly exploits, disdaining other victuals, though they
may be of equal value.
The carnivorous insect, on the other hand, has no narrow specialities
and devours any kind of flesh. The Golden Carabus finds the
caterpillar, the Mantis, the Cockchafer, the Earthworm, the Slug or any
other kind of game to his taste. The Cerceres collect, for their grubs,
bags of Weevils or Buprestes, without distinction of species. The
Bruchus, [66] on the other hand, will touch nothing but her pea or her
bean; the Golden Rhynchites [67] only her sloe; the Spotted Larinus
[68] only the sky-blue ball of her little thistle; the Nut-weevil [69]
only her filbert; the Iris-weevil [70] only the capsule of the yellow
water iris. And so with other insects. The vegetarian is a
short-sighted specialist; the meat-eater an emancipated generalizer.
Some years ago, with a success which delighted the observer that I am,
I changed the diet of various carnivorous larvæ. To those which lived
on Weevils I gave Locusts; to those which lived on Locusts I gave
Flies. My nurslings unhesitatingly accepted the food unknown to their
race and were none the worse for it; but I would not undertake to rear
a caterpillar with the first sort of leaves that came to hand: it would
starve sooner than touch them.
Animal matter having undergone a more thorough refinement than
vegetable substances, enables the stomach to pass from one dish to
another without gradually becoming accustomed to each, whereas
vegetable food, being comparatively refractory, calls for an
apprenticeship on the part of the consumer. To turn Sheep’s flesh into
Wolf’s flesh is an easy matter: a few minor transmutations are enough;
but to make mutton out of grass is a complicated process of digestive
chemistry, for which the ruminant’s four stomachs are none too many.
The carnivorous insect is able to vary its diet, all sorts of game
being of equal value.
Vegetable food involves other conditions. With its starches, oils,
essences and spices and often with its poisons, each plant tried would
be a perilous innovation, to which the insect, repelled by the first
mouthfuls, would never consent. How greatly preferable to these
dangerous novelties is the invariable dish consecrated by ancient
custom! This, no doubt, is why the vegetarian insect is faithful to its
plant.
How is this division of the earth’s abundance among its consumers
effected? We can hardly hope to understand the problem; it is too far
beyond our methods of research. The most that we can do is, by
experimental methods, to explore this corner of the unknown a little,
to seek to discover how far the insect’s diet is fixed and to note its
variations, if any. This will give us data which the future will employ
to carry the problem farther.
Towards the end of the autumn, I had placed in the vivarium two couples
of the Stercoraceous Geotrupes, with an ample heap of provender
obtained from the Mules. I had no plans as regards my captives; I had
put them there because it was an old habit of mine never to lose an
opportunity. Chance had set them within my reach; chance would do the
rest.
With the sumptuous provision which I had bestowed upon them, the
Geotrupes had had plenty wherewith to attend to their domestic affairs.
They were overlooked all the winter, without any further intervention
on my part. On the approach of spring, curiosity impelled me, in a
leisure moment, to inspect them. It had been raining as hard through
the sides of the cage, which consisted of a metal trellis, as it had in
the streets; and, as the water could not trickle away through the
wooden floor, the soil in the vivarium had turned to mud.
The sausages of food prepared by the parents were numerous, in spite of
everything, but in a shocking state. Soaked by the rain, drenched to
the very centre by continual infiltration, they fell into fragments if
I moved them. Nevertheless, each contained, in the tattered chamber
beneath it, an egg laid about the end of autumn; and this egg, spared
by the ice-cold mud of winter, was so plump, so healthy and glossy,
that an imminent hatching seemed evident.
What shall I give the grubs when they come out? I dare not count on the
remnants of the regulation sausages, reduced to bales of fibre by the
rains. As well give the new-born larvæ an old rope’s-end. What is to be
done? We will resort to a crazy artifice and serve a dish of our own
invention, one absolutely unknown to the Geotrupes.
The mess prepared for my larvæ is made of leaves decaying on the
ground: hazel-, cherry-, mulberry-, elm-, quince-leaves and others. I
steep them in water to soften them and then shred them like fine-cut
tobacco. The egg is placed at the bottom of a test-tube; and I pack a
column of my vegetable mince-meat on the top. For purposes of
comparison, other eggs are similarly lodged, but with a thankless
ration of the normal preserves soaked by the rains.
Hatching occurs during the first week in March. I have before my eyes,
when it leaves the egg, the larva which astonished me so greatly when I
first realized, many years ago, that it was a cripple. In once more
referring to this strange abnormality, I will confine myself to a few
words on the subject of the head, which is remarkably bulky, swollen as
it is by the motor muscles of the mandibular shears with broad, flat
blades, notched at the tip and bearing a strong spur at the base. It is
enough to see this dental armoury to recognize the new-born grub as one
that will not object to tackling ligneous fibres. With such a
mincing-machine, a bit of straw must be a luxury.
I watch the grubs taking their first mouthfuls. I expected to see them
hesitating, searching uneasily through these unaccustomed victuals,
such as no Geotrupes, it seems to me, can ever have used. Nothing of
the sort: this eater of dung-sausages accepts the dead-leaf-sausages
off-hand and so enthusiastically that I am convinced at the first trial
of the success of my queer experiment.
The grub finds before it, to begin with, the main nerve of a leaf. It
seizes it, turns it over and over with its palpi and fore-legs and then
gently nibbles one end of it. The whole of it goes down. Other morsels
follow, large or small indifferently. There is no picking and choosing:
what the mandibles encounter they crunch. And this goes on
indefinitely, always with an unimpaired appetite, so that the insect
attains the perfect stage without a check. When the back is black as
ebony and the belly an amethystine violet, I set my Geotrupes at
liberty. I am filled with amazement by what he has taught me.
An inverse experiment was essential. A Dung-beetle thrives on rotten
leaves; shall I be equally successful in rearing an eater of vegetable
refuse on dung? From the heap of dead leaves accumulated in a corner of
the garden for mould, I obtain a dozen half-grown larvæ of the Golden
Cetonia. I install them in a glass jar, with no other food than
Mule-droppings which have acquired the proper consistency by a few
days’ exposure to the air on the high-road. The stercoral ration is
welcomed by the future rose-dweller. I cannot see any signs of
hesitation or repugnance. When half-dry, the Mule’s fibrous scraps are
consumed as readily as the leaves brown with decay. A second jar
contains larvæ fed in the normal fashion. There is no difference
between the two groups in the matter of appetite and healthy looks. In
both cases the metamorphosis is properly accomplished.
This double success gives food for thought. Certainly the Cetonia-grub
would have nothing to gain if it thought fit to abandon its heap of
dead leaves in order to exploit the Mule-droppings in the road; it
would be leaving inexhaustible abundance, pleasant moisture and perfect
security in exchange for a scanty, perilous diet, trampled underfoot by
the passers-by. It will not commit this act of folly, however alluring
the bait of a new dish.
It is not the same thing with the larva of the Geotrupes. In the open
fields, the droppings of beasts of burden, without being scarce, are
not by any means to be met with everywhere. They are found chiefly on
the roads, which, encrusted with macadam, offer an insuperable obstacle
to burrowing. On the other hand, half-rotten dead leaves accumulate
everywhere in inexhaustible quantities. What is more, they abound on
loose soil, which is easily excavated. If they are too dry, there is
nothing to prevent their being carried down to such a depth that the
moisture of the soil will give them the requisite pliability. An insect
is not a Geotrupes, an earth-borer, for nothing. A silo sunk a few
inches deeper than the ordinary burrows would make an excellent
steeping-vat.
Since the Geotrupes-grubs thrive on a column of rotten leaves, as my
experiments have proved, it would seem that the maker of dung-sausages
would gain greatly by slightly modifying her trade and substituting
fermented leaves for stercoral matter. The race would be the better for
the change and would become more numerous, since there would be plenty
of food in perfectly safe places.
If the Geotrupes does nothing of the kind, if it has never even
attempted to do so, apart from my artificial methods of rearing, it is
because the regimen is not determined merely by the appetites of the
consumers. Economic laws regulate the diet and each species has its
portion, in order that nothing shall be left unused in the treasury of
unorganizable matter.
Let us consider a few examples. The Death’s-head Hawk-moth (Acherontia
atropos, Linn.) has the leaves of the potato for her caterpillar’s
portion. She is a foreigner, who seems to have come from America
together with her food-plant. I have tried to rear her caterpillar on
various plants belonging, like the potato, to the family of the
Solanaceæ. Henbane, datura and tobacco were obstinately refused,
despite the acute hunger displayed when the normal food was served.
The violent alkaloids with which these plants are saturated may perhaps
explain this refusal. We will therefore keep the true genus Solanum and
we will replace the too active poisons by solanin, which is not so
virulent. The leaves of the tomato (Solanum lycopersicum), the
egg-plant (S. melongena), the black-berried nightshade (S. nigrum), the
orange-berried nightshade (S. villosum), a native of New Zealand, and
the common bittersweet of our country-sides (S. dulcamara) are, on the
other hand, accepted with the same relish as the potato.
These contradictory results leave me perplexed. Since the caterpillar
of the Death’s-head Hawk-moth requires food flavoured with solanin, why
are certain species of the same genus Solanum gluttonously devoured and
others refused? Can it be because the dose of solanin is unequal, being
weaker here and more abundant there? Or are there other reasons? I am
utterly at a loss.
The magnificent caterpillar of the Spurge Hawk-moth, La Belle, as
Réaumur calls it, knows nothing of these inexplicable preferences. It
welcomes any species whose wounds exude the sap of the tithymals, the
white milky liquid with the fiery flavour. In my neighbourhood it is
often found on the tall spurge of these parts, Euphorbia characias; but
it is just as happy on smaller species, such as the narrow notch-leaved
spurge (Euphorbia serrata) and Gerard’s spurge (E. Gerardiana).
Under my bell-jars it thrives on the first spurge that comes to hand.
Anything except these caustic foods, which no other caterpillar would
accept, it abhors. It turns away in disdain from the insipid lettuce of
our gardens, from peppermint, from the Cruciferæ, rich in sulphurous
juices, the caustic ranunculus and other more or less highly flavoured
plants. It will have nothing but the spurge, whose milky sap would
corrode any gullet but its own. An insect that can feed with pleasure
on such acrid fare must obviously be predisposed that way.
For that matter, consumers devoted to pungent flavours are not scarce.
The grub of Brachycerus algirus is as fond of aioli as the Provençal
peasant; it thrives and grows fat in a clove of garlic, without other
nourishment.
What is more, I have found the larvæ of I know not what insect on Nux
vomica, the terrible poison with which our municipal authorities
flavour the sausages used for destroying Wolves and stray dogs. These
strychnine-eaters have certainly not accustomed themselves by degrees
to this terrible diet: they would perish at the first mouthful, if they
had not a specially constructed stomach at their service.
This exclusive taste for such or such a vegetable, sometimes harmless
and sometimes poisonous, has many exceptions. Some vegetarian insects
are omnivorous. The destructive Locust nibbles every green thing; our
common Grasshoppers eat the tips of any sort of grass without
distinction. Kept in a cage to divert the children, the Field Cricket
feasts on a leaf of lettuce or endive, new foodstuffs that help it to
forget the tough grasses of his meadows.
In April, on the green banks by the road-side, we meet with squads of
an ugly, fat, bronze-black creature, which, when we tease it, plays the
Tortoise, shrinking into a ball. It walks heavily on six feeble legs,
while the end of the intestine, becoming a supplementary foot, acts as
a lever and pushes it forward. It is the larva of a large black
Chrysomela (Timarcha tenebricosa, Fab.), an unpleasant Beetle which, in
self-defense, disgorges an orange spittle.
I amused myself last spring by following a flock of these larvæ to
their grazing-grounds. The favourite plant was one of the Rubiaceæ, the
cheese-rennet (Galium verum), in the stage of young shoots. Various
other plants were eaten no less readily on the way, including
especially Cichoriaceæ such as Pterotheca nemansensis, Chondrilla
juncea or gum-succory, and cut-leaved podospermum (P. laciniatum), and
Leguminosæ such as Medicago falcata, or yellow medick and Trifolium
repens, or white clover. The acrid flavours did not in the least
discourage the flock. A Gerard’s spurge was met with, trailing its
flower on the ground. A few larvæ stopped and nibbled the tender tops
as eagerly as the clover. In short, the fat crippled larva varies its
meal greatly.
Examples abound of insects equally omnivorous of vegetable substances;
there is no need to linger over them. Let us pass on to the exploiters
of woody materials. The larva of Ergates faber lives exclusively in
decayed pine-stumps; the hideous caterpillar of the Moth
inappropriately known as the Cossus eats into old willow-trees, in
company with the Ægosoma.
These two are specialists.
The lesser Capricorn, Cerambyx cerdo, entrusts her grubs to the
hawthorn, the sloe, the apricot-tree and the cherry-laurel, all of
which trees or shrubs belong to the family of the Rosaceæ. She varies
her domain a little, while remaining faithful to woody vegetation
characterized by a faint flavour of prussic acid.
The Zeuzera, or Leopard-moth, a large and beautiful white Moth with
blue spots, is more general. She is the scourge of most of the trees
and shrubs in my enclosure. I find her caterpillar chiefly in the
lilac-tree; also in the elm, the plant-tree, the quince, the
guelder-rose, the pear-tree and the chestnut. In these, always working
upwards, it bores itself straight galleries which turn a branch the
thickness of a good-sized bottle-neck into a fragile sheath soon broken
by the winter wind.
To return to the specialists: the Shagreen Saperda exploits the black
poplar and accepts nothing else, not even the white poplar; the Spotted
Saperda has the elm for its domain; the Scalary Saperda is faithful to
the dead cherry-tree. [71] The Great Capricorn lodges her grubs in the
oak, sometimes the English oak and sometimes the evergreen oak, or
ilex. This last Beetle, being easily reared with slices of pear for
food and sticks of wood in which to establish her family, lends herself
to an experiment of some interest.
I collect the eggs which the mother’s pointed, groping oviduct has
slipped into the irregular crevices of the bark. The number obtained
enables me to make a variety of tests. Will the new-born larvæ accept
the first wood that offers after they are hatched. That is the problem.
I select freshly-cut billets measuring two or three fingers’-breadths
in diameter. They include the ilex, elm, lime, robinia, cherry, willow,
elder, lilac, fig, laurel and pine. To avoid falls, which would confuse
the new-born grubs if they had to wander about in search of the spot at
which to bore, I do my best to imitate the natural conditions. The
mother Capricorn lodges her eggs, one at a time, here and there in the
fissures of the bark, fixing them with a thin varnish. I cannot gum the
eggs in this way: my glue would perhaps endanger the vitality of the
egg; but I can resort to the firm support of a furrow. With the point
of a penknife I make this furrow, that is to say, a tiny cleft into
which the egg sinks half-way. This precaution succeeds admirably.
In a few days the eggs hatch without falling off, each at the spot
decided by the point of my penknife. I watch in amazement the first
wriggles of the feeble little creature’s body, the first strokes of its
plane, as it attacks the thankless material, the bark and the wood,
still dragging its white egg-shell behind it. By the following day,
each grub has disappeared beneath a fine sawdust, the result of the
work accomplished. The mound is still very small, matching the weakness
of the excavator. Let us leave the grub at work. For a fortnight we see
the mound grow bigger and bigger, until it is almost the size of a
pinch of snuff. Then everything stops. The amount of sawdust does not
increase, except in the oak-billet.
This activity at the outset, which is everywhere the same, in media
differing so greatly in aroma and flavour, would lead us to suppose, at
first thoughts, that the young Cerambyx is endowed with a highly
complaisant stomach and can feed on the fig-tree, oozing with acrid
milk, the laurel, aromatic with essential oils, and the pine, saturated
with resin, as well as on the oak, seasoned with tannin. Reflection
persuades us that we are mistaken. The little creature is not engaged
in eating: it is toiling to make itself a deep lodging in which it can
feast in peace.
When examined through the lens, the sawdust confirms our theory: this
dust has not passed through the digestive canal; it has played no part
in feeding the grub. It is only so much meal, crumbled by the
mandibles, and nothing more.
When appetite has come and the requisite depth has been reached, the
grub at last begins to eat. If it finds the traditional food ready to
its teeth, the sap-wood of the oak, with its astringent flavour, it
gorges itself and proceeds to digest. If it finds nothing of the sort,
it abstains from eating. This is certainly the reason why the heap of
sawdust grows larger on the billet of oak but remains indefinitely
stationary on the others.
What do they do in their little galleries, these grubs subjected to a
strict fast in the absence of suitable victuals? In March, six months
after the hatching, I look into the matter. I split the billets. There
they are, the little grubs, no larger, but still lively, swaying their
heads to and fro if I disturb them. This persistence of life in such
puny creatures deprived of food rouses our astonishment. It reminds us
of the grubs of the Attelabus-beetle, which, subjected to the drought
of summer in their little kegs made of a strip of oak-leaf, cease
eating and slumber, half-dead, for four or five months, until the
autumn rains have softened their food.
When I myself produced rain, a thing not beyond my power, so far as the
needs of a grub are concerned, when I softened the rigid kegs and made
them edible by a brief immersion in water, the recluses used to return
to life, begin to eat again and continue their larval development
without further check. Similarly, after six months’ fasting in the
heart of inacceptable sticks, the Capricorn grubs would have recovered
their strength and activity if I had removed them and put before them a
freshly-cut billet of oak. I did not do it, so certain did the success
of the experiment appear.
I had other schemes in view. I wished to learn how long this arrested
life could be prolonged. A year after the hatching, I examined my
specimens again. This time I have gone too far. All the grubs are dead,
reduced to dark brown granules; only those in the oak are alive and
already well-grown. The experiment is conclusive; the Great Capricorn
has the oak for her domain; any other tree is fatal to her grub.
Let us recapitulate these details, to which it were easy to add
indefinitely. Among the vegetarian insects are some that are
omnivorous, by which we mean that they are able to feed on a great
variety of plants, but not on all indifferently: that goes without
saying. These consumers of miscellaneous foodstuffs are in the
minority. The others specialize, some more and others less strictly.
One guest at the great banquet of the animal world requires a vegetable
family, a group, a genus flavoured with certain alkaloids; another
needs a given plant, sometimes faintly and sometimes highly flavoured;
a third demands a seed, apart from which nothing is of use to it; and
others require their pod, bud, or blossom, their bark, root or bough
respectively. So it is with one and all. Each insect has its exclusive
tastes, narrowly limited, to the point of refusing the close equivalent
of the thing accepted.
Lest we lose our way in the inextricable throng at the entomological
banquet let us consider separately our two Capricorns, Cerambyx heros
and C. cerdo. No two creatures could be more alike than these two
long-horned Beetles; the lesser is the very picture of the greater. Let
us also consider the three Saperdæ mentioned above. They are the same
shape, as though they had been turned out of similar moulds, so much so
that we should confound them if differences of size and above all of
colour did not proclaim them to be of separate species.
The theorists tell us that our two Capricorns and their congeners
spring from a common stock, ramified in various directions by the
action of the centuries. In the same way, our three Saperdæ and the
others are variations of a primitive type. The ancestors of the
Capricorns, the Saperdæ and the Longicorns in general are in their turn
descended from a remote precursor, who herself was descended from etc.,
etc. One more plunge into the darkness of the past and we shall soon
reach the origins of the zoological series. What begins at all? The
Protozoon. How? With a drop of albumen. The whole succession of living
creatures has gradually proceeded from this first clot of protoplasm.
As an effort of the imagination, this is magnificent. But the
observable facts, which alone are worthy of admission to the stern
records of science, the facts corroborated by experiment, cannot keep
pace with the Protozoon. They tell us that, as food is the primordial
factor of life, digestive capacities should be handed down by atavistic
inheritance even more than are the length of the antennæ, the colour of
the wing-cases and other details of quite secondary importance. To
bring about the present state of affairs, in which the diet is so
varied, the precursors must have eaten a little of everything. They
ought to have bequeathed to their descendants an omnivorous regimen,
which is a notable cause of prosperity.
A common origin would inevitably lead to a common diet. Instead of
this, what do we see? Each species has its narrowly limited tastes,
which have no reference to the tastes of the cognate species. If they
are related through a common ancestry, it is absolutely impossible to
understand why, of our two Capricorns, one is allotted the oak and the
other the hawthorn and the cherry-laurel; why, of our three Saperdæ,
the first demands the black poplar, the second the elm and the third
the dead cherry-tree. This gastric independence loudly proclaims
independence of origin. And simple common sense, not always welcome to
the adventurous theorists, is of the same opinion.