THE LIFE OF A SOLITARY WASP
OF all "the Tribes of my Frontier" none are more deserving of notice than the solitary wasp. Their ways are of even greater interest than those of the social hymenoptera, whose praises have been so admirably sung by Maeterlinck, Grant Allen, and others. Perhaps it is the lonely life led by the solitary wasps that gives them so much character; for character they certainly have. " So whimsical," writes Burroughs, " so fickle, so forgetful, so fussy, so wise, and yet so foolish, as these little people are; such victims of routine and yet so individual, such apparent foresight and yet such thoughtlessness, at such great pains to dig a hole and build a cell, and then at times sealing it up without storing it with food or laying the egg, half finishing hole after hole, and then abandoning them without any apparent reason ; sometimes killing their spiders, at other times only paralysing them; one species digging its burrow before it captures its game, another catching its prey and then digging the hole; some of them hanging the spider up in the fork of a weed to keep it away from the ants while they work at the nest, and running to it every few minutes to see that it is safe; others laying the insect on the ground while they dig; one species walking backwards and dragging its spider after it, and when the spider is so small that it carries it in its mandible, it still walks backwards as if dragging it, when it would be much easier to walk forward. A curious little people, leading their solitary lives and greatly differentiated by their solitude, hardly any two alike, one nervous and excitable, another calm and unhurried ; one careless in her work, another neat and thorough; this one suspicious, that one confiding; ammophila using a pebble to pack down the earth in her burrow, while another species uses the end of her abdomen -: verily a queer little people, with a lot of wild nature about them, and a lot of human nature too."
A multitude of solitary wasps are found in Madras, many of which invade our houses and build their nests inside them. One of these, one of the Eumenidae, recently forced herself upon my notice. She is known to entomologists as Rhynchium brunneum. She has no popular name. I use the pronoun " she" advisedly, for among wasps the male is an unimportant creature. He is smaller than the female, and takes no part in the construction or the provisioning of the nest.
The female of this particular wasp is about three-quarters of an inch long, her waist is short and thick, her body is brownish red in colour, marked posteriorly by three black bands which run across the body. Her glassy wings are of a brownish-yellow hue. Thus her garments are neither very beautiful nor very showy. She is clad in quiet, businesslike clothes which are quite in keeping with her calm, industrious habits.
A lady wasp of this species came, a little over a month ago, into a bedroom through an open window and began at once to look about her for a suitable site for her nest. Her attention was soon attracted by a wooden bed. In this she found some ideal nesting-places -: the holes in the upright posts intended to receive the poles for mosquito curtains. Having elected to nest in these six-inch-deep cavities, the wasp promptly set to work to prepare them for her eggs.
She flew out of the window, to return in a few minutes, carrying between her front legs a pellet of mud, fully half the size of her body. She herself had prepared this pellet by means of her jaws and saliva out of dust collected on the roadside. She flew with it into the cavity, and proceeded to line it with mud. Having utilized her load, the industrious insect flew off and returned with a second load, and a third, and a fourth.
In a short time she had lined the hole, and the mud soon set as hard as mortar. I believe that directly the nest is lined the wasp lays an egg in it, but of this I cannot be sure, for it is impossible to see what is going on at the bottom of a hole six inches deep and less than an inch wide. It is therefore possible that the egg was laid at a later stage in the proceedings. The nest has now to be provisioned, for when the grub emerges from the egg in its underground cell it will need food. Accordingly the wasp mother goes forth to seek provender for her offspring upon which she will in all probability never set eyes. Consider for a moment the significance of this. We have, here, an insect toiling all day long for her offspring which she will never see. I do not think that she even knows that her eggs will give rise to young wasps. She toils for the benefit of these because that strange internal force which we call instinct compels her to do so. She knows not what she is doing, yet no human parent could work harder in the interests of her offspring. Analogy would lead us to think that the female wasp loves her children. Yet this is impossible. The question thus arises therefore in the case of the higher animals, how much of their solicitude for their offspring is due to affection and how much to blind instinct?
The grub which the egg will produce is both carnivorous and voracious, and, what is more, it must be fed upon fresh meat. Here, then, is a difficult problem which the wasp has to solve: how to provide fresh meat for her offspring. It is obviously useless to kill some creatures and place them underground, for by the time the young one hatches out the food will have become putrid. If, on the other hand, she catch some feeble creatures and put them alive into the nest, they will wriggle and struggle, so that, if they do not damage the egg, they will at least knock it away from them. This would be fatal were it to take place, for the grub, when it first emerges from the egg, is so weak that it cannot move by so much as a hair's breadth, so that it will starve to death unless it is hatched right in the midst of its food-supply.
Let us see how the wasp solves the problem. She presently returns carrying a thin greenish caterpillar quite as long as herself. She flies with it into the nest. She carries it lengthwise, grasping it with all her six legs. Having placed it in the cell, she flies out of the window and soon returns with another caterpillar of the same kind. When this is safely deposited in the nest she goes off for a third. Let us now take out and examine one of these caterpillars. It is apparently alive and unwounded, but, if alive, it is certainly completely paralysed, since it never makes the slightest motion. It is therefore evident that the wasp has done something to it. Has she killed it or merely paralysed it ?
Leon Dufour, who first studied the ways of the hymenopteron Cerceris, which stores the nest with weevils, was of opinion that the wasp killed her prey and injected into it some antiseptic liquid to keep it fresh during the weeks or days her eggs took to hatch.
The great French entomologist Fabre, whose work, "Insect Life" (of which there is an English edition), every one should read, discovered that the antiseptic theory is incorrect and that the wasp only paralyses its prey. He proved conclusively that the wasp merely pricks the motor nerve centres of her victim and thus completely paralyses it. He actually saw a Cerceris wasp perform the operation. As she was returning with a paralysed weevil, Fabre snatched it away from her with pinchers, instantly throwing a living weevil in exchange. "The manoeuvre," writes Fabre, "succeeded perfectly. As soon as the Cercercis felt the prey slip under her body and escape her, she stamped with impatience, turned round, and, perceiving the weevil which had replaced hers, flung itself upon it and clasped it in order to carry it away. But she promptly perceived that this prey was active, and then the drama began, and ended with inconceivable rapidity. The Cerceris faced her victim, seized its proboscis with her powerful jaws, and grasped it vigorously, and while the weevil reared itself up, pressed her forefeet hard on its back as if to force open some ventral articulation. Then the tail of the murderess slid under the Cleonus, curved and darted its poisoned lancet swiftly two or three times between the first and second pair of feet. In a twinkling all was over. Without one convulsive movement, with no motion of the limbs, such as accompany the death of an animal, the victim fell motionless for ever, as if annihilated.
" It was at once wonderful and terrible in its rapidity. Then the assassin turned the weevil on its back, placed herself body to body with it, her legs on either side of it, and flew off. Three times I renewed this experiment the same scene always occurred."
In like manner does the wasp Rhynchium, of which we are speaking, paralyse her victim, with, however, one difference. There is in the weevil but one motor centre, so that the wasp has only to stab it in one place in order to completely paralyse it; a caterpillar, how¬ever, is a composite creature, having several motor centres; hence it has to be stabbed in three places before it is rendered quiescent -: in the neck, in the hind part of the thorax, and in the abdomen. The first stroke gives the front part of the body its quietus, the second paralyses the front pro-legs, and the last stills for ever the movements of the hind pro-legs. The wasp has a wonderful knowledge of the anatomy of caterpillars ! " It is," writes Fabre, " in this triple blow that the infallibility, the infused science, of instinct, appear in all their magnificence."
These words are in the main true, but more recent investigations have shown that instinct is, in this case, not absolutely infallible. The wasp does sometimes make a " boss shot." It occasionally happens that a stab fails to reach the nerve ganglion. When the wasp has stored the cell with eight caterpillars she closes it by roofing it with mud. I believe that eight is the number of caterpillars she allows to each egg, but there again I speak not with certainty.
These observations were made at random and were often interrupted. After the cell had been closed there was still plenty of room left in the hole in the bedpost ; in this space the wasp laid another egg, killed more caterpillars, and then closed the cell with mud, making the top of the roof flush with the summit of the post. She then proceeded to stock the hole in the bed-post, behaving in precisely the same way as before. Having completed the second nest, she forthwith began to line the third hole with mud, and was stocking it with caterpillars, when I cut short her life. I had to sacrifice her in the interests of science, in order to find out the species to which she belonged.
Five days after she had closed the first nest I opened it, and found that all the caterpillars had disappeared, and that a great fat white grub, fully an inch in length, had taken their place. This had emerged from the egg, and then devoured all the caterpillars. The length of time that the eggs require to hatch varies with different species, and is often considerably longer than the time occupied by the Rhynchium egg.
The larva soon passes into the pupal state. It does not spin a cocoon as the silkworm moth does. The transformation into the imago or adult occupies less than three weeks. As there is no cocoon, one might, if the creature could live in the light, watch the wonderful metamorphosis actually taking place, but light appears to kill the pupa. About seventeen days after the egg had been laid I dug out another pupa. It had assumed the adult wasp-like form, was almost white in colour, and looked what it was -: an unfinished wasp.
Over the last cell I tied a piece of muslin to make a cage into which the imago would have to fly on leaving the nest, but I might have spared myself the trouble. Twenty-five days after the closing of the cell, I noticed that a hole had been gnawed in the muslin, and, looking into the nest, I saw a hole through the roof of the cell and knew that the wasp had flown. She had been able to adapt herself to circumstances. She had used her jaws, with which instinct had taught her to rasp away the roof of her cell, to cut a hole in the muslin, and thus gained her liberty.