THE FLY-HUNTING WASP

YOU have read about the Wasps who store up paralyzed Caterpillars and Crickets for their babies’ food, then close up the cells and fly away; now you shall hear about a Wasp who feeds her children with fresh food from day to day. This is the Bembex, or the Fly-hunting Wasp, as I shall call her.

This Wasp digs her burrows in very soft, light sand, under a blazing sun and a blue sky. I go out and watch her sometimes on an unshaded plain where it is so hot that the only way to avoid sunstroke is to lie down at full length behind some sandy knoll, put one’s head down a rabbit-burrow, or provide one’s self with a large umbrella. The latter is what I did. If the reader will sit with me under the umbrella at the end of July, he will see the following sight.

A Fly-hunting Wasp arrives suddenly and alights, without any hesitation, at a spot which to my eyes looks exactly like the rest of the sandy surface. With her front feet, which are armed with rows of stiff hairs and remind one at the same time of a broom, a brush, and a rake, she works at clearing her underground dwelling. The insect stands on her four hind-legs, while the front ones first scratch and then sweep the shifting sand. She shoots the sand backwards so fast that it gushes in a curve like a stream of water, falling to the ground seven or eight inches away. This spray of dust is kept up evenly for five or ten minutes at a time by the swift, graceful Wasp.

Mingled with this dust are tiny bits of wood, decayed leaf stalks, particles of grit and other rubbish. The Wasp picks them up in her mouth and carries them away. This is really the purpose of her digging. She is sifting out the sand at the entrance to her home, which is all ready underground, having been dug some time before. The Wasp wishes to make the sand at the entrance to her burrow fine, light, and free from any obstacle, so that when she alights suddenly with a Fly for her children, she can dig an entrance to her home quickly. She does this work in her spare time, when her larva has enough food to last it for a while, so that she does not need to go hunting. She seems happy as she works so fast and eagerly, and who knows that she is not expressing in this way her mother’s satisfaction in watching over the roof of her house where her baby lives?

If we should take a knife and dig down into the sand where the Wasp-mother is scratching, we should find, first, an entrance corridor, as wide as one’s finger, and perhaps eight to twelve inches long, and then a room, hollowed out down below where the sand is damper and firmer. It is large enough to contain two or three walnuts; but all it does hold at present is a Fly, a golden-green Greenbottle, with a tiny white egg laid on the side. This is the Wasp’s egg. It will hatch out in about twenty-four hours, into a little worm, which will feed on the dead Fly. For the Fly is dead, and not paralyzed, as the food of other Wasp-babies often is.

At the end of two or three days the Wasp-grub will have eaten up the little Fly. Meanwhile the mother Wasp remains in the neighborhood and you see her sometimes feeding herself by sipping the honey of the field flowers, sometimes settling happily on the burning sand, no doubt watching the outside of the house. Every now and then she sifts the sand at the entrance; then she flies away for a while. But, however long she may stay away, she never forgets the young larva who has food enough to last only a short time; her mother’s instinct tells her the hour when the grub has finished its food and wants more. She therefore returns to the nest, which, you must remember, does not show in the least from the surface of the ground, as the shifting sand has filled in the entrance; she knows, however, exactly where to look for it; she goes down into the earth, this time carrying a larger piece of game. After leaving this in the underground room she again leaves the house and waits outside until the time comes to serve a third course. This is not long, for the little worm is getting a larger appetite all the time. Again the mother appears with another Fly.

For nearly two weeks, while the larva is growing up, the meals thus follow in succession, one by one, as needed, and coming closer together as the infant grows larger. Towards the end of the two weeks, the mother is kept as busy as she can be satisfying her hungry child, now a large, fat grub. You see her at every moment coming back with a fresh capture, at every moment setting out again upon the chase. She does not cease her efforts until the grub is stuffed full and refuses its food. I have counted and found that sometimes the grub will eat as many as eighty-two Flies.

I have wondered sometimes why this Wasp does not lay up a store of food, as the other Wasps do, close the door of her burrow and fly away, instead of waiting about, as she does so patiently. I realize that she does not do so because her Flies would not keep; they would spoil and be unfit for eating. But why does she kill the Fly instead of paralyzing it? Possibly because the Fly would not make a satisfactory preserved food; it is so slight and frail, it would shrivel up and there would be nothing of it; it must be eaten fresh to be worth anything. Another reason almost certainly is that the Fly has to be captured very quickly, on the wing. There is not time for the Wasp to aim her sting, as the Wasps do who are killing clumsy Worms or fat Crickets on the ground. She must attack with claws, mouth or sting wherever she can, and this method of attack kills at once.

It is not easy to surprise a Wasp hunting, as she flies far away from where her burrow lies; but one day I had a quite unexpected experience as I was sitting in the hot sun under my umbrella. I was not the only one to enjoy the shade of the umbrella. Gad-flies of various kinds would take refuge under the silken dome and sit peacefully on every part of the tightly stretched cover. To while away the hours when I had nothing to do, it amused me to watch their great gold eyes, which shone like carbuncles under my umbrella; I loved to follow their solemn progress when some part of the ceiling became too hot and obliged them to move a little way on.

One day, bang! The tight cover resounded like the skin of a drum. Perhaps an oak had dropped an acorn on the umbrella. Presently, one after the other, bang, bang, bang! Can some practical joker be flinging acorns or little pebbles at my umbrella? I leave my tent and look around: nothing! I hear the same sharp sounds again. I look up at the ceiling and the mystery is explained. The Fly-hunting Wasps of the neighborhood, who all eat Gad-flies, had discovered the rich game that was keeping me company and were impudently coming into my shelter to seize the Flies on the ceiling. Things were going to perfection: I had only to sit still and look.

Every moment a Wasp would enter, swift as lightning, and dart up to the silken ceiling, which resounded with a sharp thud. Some rumpus was going on aloft, where so lively was the fray that one could not tell which was attacker, which attacked. The struggle did not last long: the Wasp would soon retire with a victim between her legs. The dull herd of Gad-flies would not leave the dangerous shelter. It was so hot outside! Why get excited?

“One day, bang!”

Let us watch the Wasp as she returns to the burrow with her capture held under her body between her legs. As she draws near her home, she makes a shrill humming, which has something plaintive about it and which lasts until the insect sets foot to earth. The Wasp hovers above the sand and then dips down, very slowly and cautiously, all the time humming. If her keen eyes see anything unusual, she slows up in her descent, hovers for a second or two, goes up again, comes down again and flies away, swift as an arrow. We shall see in a few moments what it is that makes her hesitate. Soon she is back again, looks at things once more from a height, then comes down slowly and alights at a spot which looks exactly like the rest of the sandy surface.

I think she has landed more or less on chance, and will now look about for the entrance to her home. But no; she is exactly over her burrow. Without once letting go her prey, she scratches a little in front of her, gives a push with her head, and at once enters, carrying the Fly. The sand falls in, the door closes, and the Wasp is at home. It makes no difference that I have seen this Wasp return to her nest hundreds of times; I am always astonished to behold the keen-sighted insect find without hesitation a door which does not show at all.

The Wasp does not always hesitate in the air before alighting at her house, and when she does, it is because she sees her nest is threatened by a very grave danger. Her plaintive hum shows anxiety; she never gives it when there is no peril. But who is the enemy? It is a miserable little Fly, feeble and harmless in appearance, whom we have mentioned in another chapter. The Wasp, the scourge of the Fly-tribe, the fierce slayer of large Gad-flies, does not enter her home because she sees herself watched by another Fly, a tiny dwarf, who would make scarcely a mouthful for her larvæ.

I feel just as I should if I saw my Cat fleeing in terror from a Mouse. Why does the Wasp not pounce upon the little wretch of a Fly and get rid of her? I do not know. It must be because this wretched little Fly has her tiny part to play in the universe, as well as the Wasp. These things are ordered somehow, in a way we do not understand.

As I shall mention elsewhere, this is the Fly that lays her eggs on the game the Wasp puts in the nest for her own baby; and the Fly’s offspring eat the food of the Wasp-grub, and sometimes eat the grub itself, if provisions are scarce. The way the Fly manages her business is interesting. She never enters the Wasp’s burrow, but she waits with the greatest patience for the moment when the Wasp dives into her home, with her game clasped between her legs. Just as she has half her body well within the entrance and is about to disappear underground, the Fly dashes up and settles on the piece of game that projects a little way beyond the hinder end of the Wasp; and while the latter is delayed by the difficulty of entering, the former, with wonderful swiftness, lays an egg on the prey, or even two or three in quick succession. The hesitation of the Wasp, hampered by her load, lasts but the twinkling of an eye. No matter: the Gnat has accomplished what she wished to, and now she goes and squats in the sun, close to the burrow, and plans fresh deeds of darkness.

A number of these Flies, usually three or four, are apt to station themselves on the sand at one time near a burrow, of which they well know the entrance, carefully hidden though it be. Their dull-brown color, their great blood-red eyes, their astonishing patience, have often reminded me of a picture of brigands, clad in dark clothes, with red handkerchiefs around their heads, waiting in ambush for an opportunity to hold up some travelers.

It is when the poor Wasp sees these brigands that she hesitates. At last she comes nearer, however. The Midges then take flight and follow behind the Wasp. If she turns, they turn also, so as to keep exactly behind her; if she advances, they advance; if she retreats, they retreat. She cannot keep them off. At last she grows weary and alights; they also alight, still behind her. The Wasp darts off again, with an indignant whimpering; the Midges dart after her. The Wasp tries one more way to get rid of them. She flies far away at full speed, hoping that they will follow and lose their way. But they know too much for that. They settle down on the sand again near the burrow and wait for her to come back. Come she does; the pursuit begins all over again; the mother’s patience is worn out, and at last they have a chance to lay their eggs as she goes into the burrow.

We shall end our chapter with the story of the Wasp-grub to whom no accidents happen, into whose burrow no nasty Fly-eggs enter. For two weeks it eats and grows; then it begins to weave its cocoon. It has not very much silk in its body to use for this, so it uses grains of sand to strengthen it. First it pushes away the remains of its food and forces them into a corner of the cell. Then, having swept its floor, it fixes to the different walls of its room threads of a beautiful white silk, forming a web which makes a kind of scaffold for the next work.

It then weaves a hammock of silk in the center of the threads. This hammock is like a sack open wide at one end and closed at the other in a point. The grub, leaning half out of its hammock, picks up the sand almost grain by grain with its mouth. If any grain found is too large, it is thrown away. When the sand is sorted in this way, the grub brings some into the hammock in its mouth, and begins to spread it in an even layer on the lower side of the hammock-sack; it adds grains also to the upper side, fixing them in the silk as one would place stones in putty.

The cocoon is still open at one end. It is time to close it. The grub weaves a cap of silk which fits the mouth of the sack exactly, and lays grains of sand one by one upon this foundation. The cocoon is all finished now, except that the grub gives some finishing touches to the inside by glazing the walls with varnish to protect its delicate skin from the rough sand. It then goes peacefully to sleep, to wait for its transformation into a Wasp like its mother.