More Hunting Wasps

by Jean-Henri Fabre

Is this enough to dispose of the very improbable supposition that the determination of the sex depends on the quantity of food? Strictly speaking, there is still one door open to doubt. It may be said that experiment, with its artifices, does not succeed in realizing the delicate natural conditions. To make short work of all objections, I cannot do better than have recourse to facts in which the experimenter's hand has not intervened. The parasites will supply us with these facts; they will show us how alien the quantity and even the quality of the food are from either specific or sexual characters. The subject of enquiry thus becomes double, instead of single as it was when I plundered one cell in my split reeds to enrich another. Let us follow this double current for a little while.

An Ammophila, the Silky Ammophila (Cf. "The Hunting Wasps": chapter 13.—Translator's Note.), which feeds on Looper caterpillars (Known also as Measuring-worms, Inchworms, Spanworms and Surveyors: the caterpillars of the Geometrid Moths.—Translator's Note.), has just been reared in my refectory on Spiders. Replete to the regulation point, it spins its cocoon. What will emerge from this? If the reader expects to see any modifications, caused by a diet which the species, left to itself, had never effected, let him be undeceived and that quickly. The Ammophila fed on Spiders is precisely the same as the Ammophila fed on caterpillars, just as man fed on rice is the same as man fed on wheat. In vain I pass my lens over the product of my art: I cannot distinguish it from the natural product; and I defy the most meticulous entomologist to perceive any difference between the two. It is the same with my other boarders who have had their diet altered.

I see the objection coming. The differences may be inappreciable, for my experiments touch only a first rung of the ladder. What would happen if the ladder were prolonged, if the offspring of the Ammophila fed on Spiders were given the same food generation after generation? These differences, at first imperceptible, might become accentuated until they grew into distinct specific characters; the habits and instincts might also change; and in the end the caterpillar-huntress might become a Spider-huntress, with a shape of her own. A species would be created, for, among the factors at work in the transformation of animals, the most important of all is incontestably the type of food, the nature of the thing wherewith the animal builds itself. All this is much more important than the trivialities which Darwin relies upon.

To create a species is magnificent in theory, so that we find ourselves regretting that the experimenter is not able to continue the attempt. But, once the Ammophila has flown out of the laboratory to slake her thirst at the flowers in the neighbourhood, just to try to find her again and induce her to entrust you with her eggs, which you would rear in the refectory, to increase the taste for Spiders from generation to generation! Merely to dream of it were madness. Shall we, in our helplessness, admit ourselves beaten by the evolutionary effects of diet? Not a bit of it! One experiment—and you could not wish for a more decisive—is continually in progress, apart from all artifices, on an enormous scale. It is brought to our notice by the parasites.

They must, we are told, have acquired the habit of living on others in order to save themselves work and to lead an easier life. The poor wretches have made a sorry blunder. Their life is of the hardest. If a few establish themselves comfortably, dearth and dire famine await most of the rest. There are some—look at certain of the Oil-beetles—exposed to so many chances of destruction that, to save one, they are obliged to procreate a thousand. They seldom enjoy a free meal. Some stray into the houses of hosts whose victuals do not suit them; others find only a ration quite insufficient for their needs; others—and these are very numerous—find nothing at all. What misadventures, what disappointments do these needy creatures suffer, unaccustomed as they are to work! Let me relate some of their misfortunes, gleaned at random.

The Girdled Dioxys (D. cincta) loves the ample honey-stores of the Chalicodoma of the Pebbles. There she finds abundant food, so abundant that she cannot eat it all. I have already passed censure on this waste. (Cf. "The Mason-bees": chapter 10.—Translator's Note.) Now a little Osmia (O. cyanoxantha, Perez) makes her nest in the Mason's deserted cells; and this Bee, a victim of her ill-omened dwelling, also harbours the Dioxys. This is a manifest error on the parasite's part. The nest of the Chalicodoma, the hemisphere of mortar on its pebble, is what she is looking for, to confide her eggs to it. But the nest is now occupied by a stranger, by the Osmia, a circumstance unknown to the Dioxys, who comes stealing up to lay her egg in the mother's absence. The dome is familiar to her. She could not know it better if she had built it herself. Here she was born; here is what her family wants. Moreover, there is nothing to arouse her suspicions: the outside of the home has not changed its appearance in any respect; the stopper of gravel and green putty, which later will form a violent contrast with its white front, is not yet constructed. She goes in and sees a heap of honey. To her thinking this can be nothing but the Chalicodoma's portion. We ourselves would be beguiled, in the Osmia's absence. She lays her eggs in this deceptive cell.

Her mistake, which is easy to understand, does not in any way detract from her great talents as a parasite, but it is a serious matter for the future larva. The Osmia, in fact, in view of her small dimensions, collects but a very scanty store of food: a little loaf of pollen and honey, hardly the size of an average pea. Such a ration is insufficient for the Dioxys. I have described her as a waster of food when her larva is established, according to custom, in the cell of the Mason-bee. This description no longer applies; not in the very least. Inadvertently straying to the Osmia's table, the larva has no excuse for turning up its nose; it does not leave part of the food to go bad; it eats up the lot without having had enough.

This famine-stricken refectory can give us nothing but an abortion. As a matter of fact, the Dioxys subjected to this niggardly test does not die, for the parasite must have a tough constitution to enable it to face the disastrous hazards which lie in wait for it; but it attains barely half its ordinary dimensions, which means one-eighth of its normal bulk. To see it thus diminished, we are surprised at its tenacious vitality, which enables it to reach the adult form in spite of the extreme deficiency of food. Meanwhile, this adult is still the Dioxys; there is no change of any kind in her shape or colouring. Moreover, the two sexes are represented; this family of pigmies has its males and females. Dearth and the farinaceous mess in the Osmia's cell has had no more influence over species or sex than abundance and flowing honey in the Chalicodoma's home.

The same may be said of the Spotted Sapyga (S. punctata (A parasitic Wasp. Cf. "The Mason-bees": chapters 9 and 10.—Translator's Note.)), which, a parasite of the Three-pronged Osmia, a denizen of the bramble, and of the Golden Osmia, an occupant of empty Snail-shells, strays into the house of the Tiny Osmia (O. parvula (This bee makes her home in the brambles. Cf. "Bramble-dwellers and Others": chapters 2 and 3.—Translator's Note.)), where, for lack of sufficient food, it does not attain half its normal size.

A Leucopsis (Cf. "The Mason-bees": chapter 11.—Translator's Note.) inserts her eggs through the cement wall of our three Chalicodomae. I know her under two names. When she comes from the Chalicodoma of the Pebbles or Walls, whose opulent larva saturates her with food, she deserves by her large size the name of Leucopsis gigas, which Fabricius bestows upon her; when she comes from the Chalicodoma of the Sheds, she deserves no more than the name of L. grandis, which is all that Klug grants her. With a smaller ration "the giant" is to some degree diminished and becomes no more than "the large." When she comes from the Chalicodoma of the Shrubs, she is smaller still; and, if some nomenclator were to seek to describe her, she would no longer deserve to be called more than middling. From dimension 2 she has descended to dimension 1 without ceasing to be the same insect, despite the change of diet; and at the same time both sexes are present in the three nurselings, despite the variation in the quantity of victuals.