Insect life: Souvenirs of a naturalist

by Jean-Henri Fabre

A PARASITE—THE COCOON

I have just described the Bembex hovering, loaded with her prey, above the nest, and descending with a vertical flight—very slow, and accompanied by a plaintive hum. This cautious, hesitating mode of arrival might suggest that the insect was examining from above in order to find her door, and trying to recall the locality before alighting. But I shall show that there is another motive. In ordinary conditions, when nothing alarms her, she comes suddenly, without hovering or plaintive hum or hesitation, and alights at her threshold, or close by. So faithful is her memory that she has no need to search about. Let us find out the cause of the hesitating arrival just described.

The insect hovers, descends slowly, mounts again, flies off and returns, because serious danger threatens. That plaintive hum is a sign of anxiety, and is never produced unless there is peril. But who is the enemy? Is it I, sitting by and watching? Not in the least; I am quite unimportant—a block unworthy of notice. The dreaded enemy—the foe who must be avoided at any price—is on the ground, perfectly [[244]]still upon the sand, near the nest. It is a small Dipteron—nothing at all to look at—of inoffensive aspect. This petty fly is the terror of the Bembex. That bold assassin of Diptera, who so promptly twists the neck of colossal gadflies, full fed on blood from an ox’s back, dares not enter her home because she sees herself watched by another Dipteron—a mere pigmy, which would scarce make one mouthful for her larva.

Why not pounce on it and get rid of it? The Bembex flies fast enough to overtake it, and, small as it is, the larvæ would not disdain it, since they eat all and every Diptera. Yet the Bembex flies in terror before an enemy which one bite would hew in pieces. I really feel as though I saw a cat wild with terror before a mouse. The ardent pursuer of Diptera is driven away by a Dipteron, and that one of the smallest! I bow before the facts without any hope of ever comprehending this reversal of parts. To be able to get rid easily of a mortal enemy, who is meditating the ruin of your family, and who might make a feast for them—to be able, I say, to do this, and not to do it when the foe is there, within reach, watching you, defying you,—is the height of folly in an animal. Folly, however, is not rightly the word: let us rather talk of the harmony of creatures, for since this wretched little Dipteron has its small part to play in the great whole of things, the Bembex must needs respect it and basely flee before it,—otherwise long ago there would have been no more Dipteron of this species in the world.

Let us trace the history of this parasite. Among Bembex nests there are found, and that frequently, [[245]]some which are occupied at the same time by the larvæ of the Hymenoptera and by other larvæ—strangers to the family and greedily sharing their food. These strangers are smaller than the nursling of the Bembex—shaped like a tear, and of the colour of wine, from the food paste which can be seen through their transparent bodies. Their number varies from six to ten or more. They belong to a kind of Dipteron, as may be perceived from their form and from the pupæ which one afterwards finds in their place. The demonstration is completed by bringing them up one’s self in a box, where, fed daily with flies, and laid on sand, they turn into pupæ, whence issue the following year little Diptera—Tachinids of the genus Miltogramma.

This is the Dipteron which, when lying in wait near the burrow, awakens such alarm in the Bembex. Her terror is only too well founded. This is what happens in the dwelling. Around the heap of food which the mother wears herself out in providing in sufficient quantity, sit in company with the legitimate nursling from six to ten hungry guests, who put their sharp mouths into the general heap as unceremoniously as if they were at home. Concord seems to reign at table. I have never seen the legitimate larva take offence at the indiscretion of the strangers, nor observed these attempt to trouble its repast. All keep themselves together, and eat peaceably without annoying their neighbours.

So far all would be well, were it not that a grave difficulty arises. However active may be the mother-nurse, it is clear that she cannot meet such a consumption of food. She has to be incessantly on the [[246]]wing to feed one larva: what must happen if there are a dozen gluttons to provide for? The result of this enormous increase of family can only be want, or even famine, not for the larvæ of the Dipteron (which develop more rapidly than that of the Bembex, profiting by the days when abundance still reigns, their host being yet in early youth), but for the latter, who reaches the moment of metamorphosis without being able to make up for lost time. Besides, when the first guests become pupæ and leave the table free to it, others come, as long as the mother visits the nest, and complete its starvation.

In burrows invaded by numerous parasites the Bembex larva is undoubtedly much smaller than one would expect from the heap of food consumed, the remains of which encumber the cell. Limp, emaciated,—only half or a third of its proper size,—it vainly tries to spin a cocoon, the silk for which it has not got, and it perishes in a corner of the cell, amid the pupæ of guests more fortunate than itself. Or its end may be yet more tragic. Should provender fail, or the mother delay too long in returning with food, the Diptera devour it. I ascertained this black deed by bringing up the brood myself. All went well as long as food was plentiful, but if through neglect, or on purpose, the daily supply failed, next day or the day after I was sure to find the Diptera larvæ greedily rending that of the Bembex. Thus, when the nest is invaded by parasites, the legitimate larva is fated to perish either by hunger or a violent death, and this it is which makes the sight of Miltogramma prowling round the nest so odious to the Bembex. [[247]]

The Bembex is not the only victim of these parasites: the burrows of one and all of the mining Hymenoptera are invaded by Tachinids, especially by the Miltogramma. Various observers—notably Lepeletier de Saint Fargeau—have spoken of the manœuvres of these impudent Diptera; but as far as I know none have perceived the very curious case of parasitism at the expense of the Bembex—very curious, because the conditions are quite different. Nests of other Fossors are stored beforehand, and the Miltogramma drops an egg on the prey just as it is being carried in. The provender stored and her egg laid, the Hymenopteron closes up the cell where thenceforward live the legitimate larva and the strangers, unvisited in their prison. Thus, the robbery committed by the parasite is unknown to the mother, and must consequently remain unpunished.

With the Bembex it is quite otherwise. The mother constantly returns during the fortnight that she is bringing up the larva; she knows that her offspring is living among numerous intruders, who appropriate the greater part of the food; every time that she brings provender she touches and feels at the bottom of her den these detestable guests, who, far from contenting themselves with remains, seize what is best. She must perceive, however small her powers of arithmetic may be, that twelve are more than one; besides, she would discover this from the disproportion between the consumption of food and her means of hunting, and yet, instead of seizing these bold intruders and bundling them out, she serenely tolerates them. Tolerates! Why, she [[248]]feeds them and brings them their rations, and perhaps feels as much tenderness for them as for her own larva. It is a new version of the cuckoo story in yet more singular circumstances. The theory that the cuckoo, almost as big as a sparrowhawk and coloured like it, should look imposing enough to introduce an egg unresisted into the nest of the weak hedge-sparrow, and that the latter, overawed perhaps by the alarming look of her toad-faced nursling, should accept and care for the stranger, has something in its favour. But what shall we say of a sparrow which, turning parasite, should go with splendid audacity and intrust her eggs to the eyrie of a bird of prey—the nest of the sparrowhawk itself—the sanguinary devourer of sparrows? What should we say of the bird of prey who should accept the charge and bring up the brood tenderly? It is precisely thus that the Bembex acts,—she, a captor of Diptera who yet brings up other Diptera—a huntress who distributes food to a prey whose last repast will be her own disembowelled offspring! I leave to cleverer people the task of explaining these amazing relations.