More Hunting Wasps

by Jean-Henri Fabre

Despite this evidence, I was anxious to enquire whether the reality corresponded with the previsions of the most elementary logic. Instances are not unknown in which the most sagacious deductions have been found to disagree with the facts. During the last few years, therefore, I have profited by my winter leisure to collect, from spots noted as favourable during the working-season, a few handfuls of cocoons of various Digger-wasps, notably of the Bee-eating Philanthus, who has just furnished us with an inventory of provisions. Surrounding these cocoons and thrust against the wall of the cell were the remnants of the victuals—wings, corselets, heads, wing-cases—a count of which enabled me to determine how many head of game had been provided for the larva, now enclosed in its silken abode. I thus obtained the correct list of provisions for each of the huntress' cocoons. On the other hand, I estimated the quantities of honey, or rather I gauged the receptacles, the cells, whose capacity is proportionate to the mass of the provisions stored. After making these preparations, registering the cells, cocoons and rations and putting all my figures in order, I had only to wait for the hatching-season to determine the sex.

Well, I found that logic and experiment were in perfect agreement. The Philanthus-cocoons with two Bees gave me males, always males; those with a larger ration gave me females. From the Tachytes-cocoons with double or treble that ration I obtained females. When fed upon four or five Nut-weevils, the Sand Cerceris was a male; when fed upon eight or ten, a female. In short, abundant provisions and spacious cells yield females; scanty provisions and narrow cells yield males. This is a law upon which I may henceforth rely.

At the stage which we have now reached a question arises, a question of major importance, touching the most nebulous aspect of embryogeny. How is it that the larva of the Philanthus, to take a particular case, receives three to five Bees from its mother when it is to become a female and not more than two when it is to become a male? Here the various head of game are identical in size, in flavour, in nutritive properties. The food-value is precisely in proportion to the number of items supplied, a helpful detail which eliminates the uncertainties wherein we might be left by the provision of game of different species and varying sizes. How is it, then, that a host of Bees and Wasps, of honey-gatherers as well as huntresses, store a larger or smaller quantity of victuals in their cells according as the nurselings are to become females or males?

The provisions are stored before the eggs are laid; and these provisions are measured by the needs of the sex of an egg still inside the mother's body. If the egg-laying were to precede the rationing, which occasionally takes place, as with the Odyneri (Cf. "The Hunting Wasps": chapters 2 and 8.—Translator's Note.), for example, we might imagine that the gravid mother enquires into the sex of the egg, recognizes it and stacks victuals accordingly. But, whether destined to become a male or a female, the egg is always the same; the differences—and I have no doubt that there are differences—are in the domain of the infinitely subtle, the mysterious, imperceptible even to the most practised embryogenist. What can a poor insect see—in the absolute darkness of its burrow, moreover—where science armed with optical instruments has not yet succeeded in seeing anything? And besides, even were it more discerning than we are in these genetic obscurities, its visual discernment would have nothing whereupon to practice. As I have said, the egg is laid only when the corresponding provisions are stored. The meal is prepared before the larva which is to eat it has come into the world. The supply is generously calculated by the needs of the coming creature; the dining-room is built large or small to contain a giant or a dwarf still germinating in the ovarian ducts. The mother, therefore, knows the sex of her egg beforehand.

A strange conclusion, which plays havoc with our current notions! The logic of the facts leads us to it directly. And yet it seems so absurd that, before accepting it, we seek to escape the predicament by another absurdity. We wonder whether the quantity of food may not decide the fate of the egg, originally sexless. Given more food and more room, the egg would become a female; given less food and less room, it would become a male. The mother, obeying her instincts, would store more food in this case and less in that; she would build now a large and now a small cell; and the future of the egg would be determined by the conditions of food and shelter.

Let us make every test, every experiment, down to the absurd: the crude absurdity of the moment has sometimes proved to be the truth of the morrow. Besides, the well-known story of the Hive-bee should make us wary of rejecting paradoxical suppositions. Is it not by increasing the size of the cell, by modifying the quality and quantity of the food, that the population of a hive transforms a worker larva into a female or royal larva? It is true that the sex remains the same, since the workers are only incompletely developed females. The change is none the less miraculous, so much so that it is almost lawful to enquire whether the transformation may not go further, turning a male, that poor abortion, into a sturdy female by means of a plentiful diet. Let us therefore resort to experiment.

I have at hand some long bits of reed in the hollow of which an Osmia, the Three-horned Osmia, has stacked her cells, bounded by earthen partitions. I have related elsewhere (Cf. "Bramble-bees and Others": chapters 2 to 5.—Translator's Note.) how I obtain as many of these nests as I could wish for. When the reed is split lengthwise, the cells come into view, together with their provisions, the egg lying on the paste, or even the budding larva. Observations multiplied ad nauseam have taught me where to find the males and where the females in this apiary. The males occupy the fore-part of the reed, the end next to the opening; the females are at the bottom, next to the knot which serves as a natural stopper to the channel. For the rest, the quantity of the provisions in itself points to the sex: for the females it is twice or thrice as great as for the males.

In the scantily-provided cells, I double or treble the ration with food taken from other cells; in the cells which are plentifully supplied, I reduce the portion to a half or a third. Controls are left: that is to say, some cells remain untouched, with their provisions as I found them, both in the part which is abundantly provided and in that which is more meagrely rationed. The two halves of the reed are then restored to their original position and firmly bound with a few turns of wire. We shall see, when the time comes, whether these changes increasing or decreasing the victuals have determined the sex.

Here is the result: the cells which at first were sparingly provided, but whose supplies were doubled or trebled by my artifice, contain males, as foretold by the original amount of victuals. The surplus which I added has not completely disappeared, far from it: the larva has had more than it needed for its evolution as a male; and, being unable to consume the whole of its copious provisions, it has spun its cocoon in the midst of the remaining pollen-dust. These males, so richly supplied, are of handsome but not exaggerated proportions; you can see that the additional food has profited them to some small extent.

The cells with abundant provisions, reduced to a half or a third by my intervention, contain cocoons as small as the male cocoons, pale, translucent and limp, whereas the normal cocoons are dark-brown, opaque and firm to the touch. These, we perceive at once, are the work of starved, anaemic weavers, who, failing to satisfy their appetite and having eaten the last grain of pollen, have, before dying, done their best with their poor little drop of silk. Those cocoons which correspond with the smallest allowance of food contain only a dead and shrivelled larva; others, in whose case the provisions were less markedly decreased, contain females in the adult form, but of very diminutive size, comparable with that of the males, or even smaller. As for the controls which I was careful to leave, they confirm the fact that I had males in the part near the orifice of the reed and females in the part near the knot closing the channel.