Insect life: Souvenirs of a naturalist

by Jean-Henri Fabre

Let us observe the tactics employed by the Tachinid, whose object is to confide her egg to the nest of the miner. It is an invariable rule that the fly should never penetrate into the burrow, even if left open and the owner absent. The crafty parasite would take good care not to entangle itself in a passage, where, having no possibility of flight, it might pay dearly for its effrontery. The only moment for its designs—a moment watched for with the greatest patience—is that when the Hymenopteron [[249]]enters the gallery, clasping her prey. At that instant, brief as it is, when the Bembex or any other miner has half her body within the entrance, and is about to disappear underground, the Miltogramma arrives on the wing, perches on the prey slightly, projecting beyond the hinder end of the Bembex, and while she is delayed by the difficulties of entering, the Miltogramma, with unparalleled promptitude, lays an egg on the prey, or two, or even three eggs, successively. The hesitation of the Bembex, embarrassed by her load, lasts but the twinkling of an eye; but that matters not—it is long enough for the fly to accomplish its misdeed without being dragged beyond the threshold. What must not be the suppleness of organs to achieve this instantaneous laying of the egg! The Bembex disappears, herself introducing the enemy, and the Tachinid goes and crouches in the sun, close to the burrow, and meditates fresh crimes. If one would make sure that the Dipteron’s eggs have really been deposited during this rapid manœuvre, it suffices to open the burrow and follow the Bembex to the bottom of her abode. The prey which one takes from her bears underneath at least one egg—sometimes more, according to the length of the delay at the entrance. These very minute eggs could only belong to a parasite, and if any doubt remained, you can bring up the brood in a box, and the result will be Diptera larvæ—later pupæ, and finally Miltogramma.

The fly shows wonderful sagacity in the moment selected by it—the only one which could permit of her carrying out her purpose with neither peril nor vain efforts. The Bembex, half-way through [[250]]the entrance, cannot see her enemy audaciously perched on the hind quarters of the prey, or, if she suspects the bandit’s presence, cannot drive it away, having no freedom of movement in the strait passage, and in spite of all precautions to facilitate speedy entrance, cannot always vanish underground with the celerity required, so quick is the parasite. In fact, this is the only propitious moment, since prudence forbids the Dipteron to penetrate into the den, where other Diptera, far stronger than itself, are served up as food for the larvæ. Outside, in the open air, the difficulty is insurmountable, so great is the vigilance of the Bembex. Let us give a moment to the arrival of the mother, when the nest is being watched by the Miltogramma.

Some of these flies—more or fewer, generally three or four—have settled on the sand and are quite motionless, all gazing at the burrow, the entrance of which they know very well, carefully hid though it be. Their dull-brown colour, their large crimson-red eyes, their intense stillness, have often made me think of bandits who, dressed in a dark material, with a red kerchief over their heads, are lying in wait to do some evil deed. The Hymenopteron comes, loaded with prey. Had she no anxieties she would alight straightway at her door. Instead, she hovers at a certain height, descends slowly and circumspectly, hesitates, and vibrates her wings, producing a plaintive hum denoting apprehension. She must have seen the malefactors. They too have seen the Bembex. The movement of their red heads shows that they are following her with their eyes; every gaze is fixed on the coveted [[251]]booty. Then come marches and counter-marches of cunning versus prudence.

The Bembex drops straight down with an imperceptible flight, as if she let herself sink gently, making a parachute of her wings. Now she is hovering just above the ground; the flies take wing, placing themselves one and all behind her,—some nearer, some farther,—in a geometrical line. If she turns round to disconcert them, they turn too, with a precision which keeps them all in the same straight line; if she advances, so do they; if she draws back, they draw back too, measuring their flight, now slow, now stationary, on that of the Bembex at the head of the file. They do not attempt to fling themselves on the desired object, their tactics being merely to hold themselves in readiness in the position of rearguard, so as to avoid any hesitation when the rapid final manœuvre shall come.

Sometimes, wearied out by their obstinate pursuit, the Bembex alights, and the flies instantly settle on the sand, still behind her, and keep quite still. She rises again, with a sharper hum—the sign no doubt of increasing indignation; the flies follow her. One last means remains to throw the tenacious Diptera off the track; the Bembex flies far away—perhaps hoping to mislead the parasites by rapid evolutions over the fields. But the crafty flies are not taken in; they let her go, and settle down again on the sand round the burrow. When the Bembex returns the same manœuvres begin again until the obstinacy of the parasites has exhausted her prudence. At a moment when her vigilance fails, the flies are [[252]]instantly there. Whichever is at the most favourable point drops upon the vanishing prey, and the thing is done—the egg is laid.

There is ample evidence that the Bembex is conscious of danger, and knows how disastrous for the future of her nest is the presence of the hated fly; her long efforts to throw the parasites off her track, her hesitation and flights, leave not a doubt on the subject. How is it then, I ask myself once more, that the enemy of Diptera should allow herself to be annoyed by another Dipteron—a tiny robber, incapable of the least resistance, which, if she chose, she could destroy instantly? Why, when once free from the prey which hampers her, does she not pounce on these ill-doers? What is needed to exterminate the evil brood around her burrow? Merely a battle which would take but a few instants. But the harmony of those laws which govern the preservation of species will not have it so, and the Bembex will always allow herself to be harassed without ever learning from the famous “struggle for life” the radical means of extermination. I have seen some which, pressed too closely, let fall their prey and flew off wildly, but without any hostile demonstration, although dropping their game left them full liberty of action. The prey, so ardently desired a moment earlier by the Tachinidæ, lay on the ground at the mercy of them all, and not one cared about it. It had no value for the flies, whose larvæ need the shelter of a burrow. It was valueless also to the Bembex, who came back, felt it for an instant and left it disdainfully. The little break in her custody of it had rendered her suspicious of it. [[253]]

Let us end this chapter by the history of the larva. Its monotonous life offers nothing remarkable during the two weeks while it eats and grows. Then comes making a cocoon. The slight development of silk-producing organs does not allow of a dwelling of pure silk, like those of the Ammophila and Sphegidæ—made of several wrappers which protect the larva, and later the nymph, from damp in the ill-protected, shallow burrow during autumn rains and winter snows. Yet this Bembex burrow is in worse conditions than those of the Sphex, being made at a depth of only a few inches in very permeable soil. To fashion a sufficient shelter the larva supplements by its industry the small amount of silk at its disposal. With grains of sand artistically put together and connected by silky matter, it constructs a most solid cocoon—impenetrable to damp.

Three general methods are employed by fossorial Hymenoptera to construct the dwelling in which metamorphosis is to take place. Some hollow burrows at a great depth under a shelter, and then the cocoon consists of a single wrapper, so thin as to be transparent. Such is the case with Philanthidæ and Cerceris. Others are content with a shallow burrow in open ground; but in that case they have silk enough for manifold wrappings of the cocoon, as with Sphegidæ, Ammophila, and Scolia; or if the quantity be insufficient, they use agglutinated sand—as, for instance, the Bembex and Palarus. One might take a Bembex cocoon for a solid kernel, so compact and resistant is it. The form is cylindrical—one end rounded, the other pointed. [[254]]The length is about two centimetres. Outside it is slightly wrinkled and coarse, but within the walls are smoothed by a fine varnish.

Rearing at home has enabled me to follow every detail of the construction of this curious piece of architecture—a real strong box which can brave all the severity of the weather. First of all the larva pushes away the remains of its feast into a corner of the cell, or the compartment arranged for it in a box with paper partitions. Having cleared a space, it affixes to the walls of its abode threads of a beautiful white silk, forming a spidery web which keeps the heap of food-remains at a distance, and serves as scaffolding for the work to come.

This work consists of a hammock, suspended far from anything that can defile it, in the centre of threads stretched from wall to wall. Fine, beautiful white silk is the only material used. The shape is that of a sack open at one end, with a wide circular orifice, closed at the other and ending in a point; a fisherman’s basket gives a very fair idea of it. Then the edges of the aperture are permanently kept apart by numerous threads fastened to the neighbouring walls. The tissue of the bag is extremely fine, allowing all that the grub does to be seen.