SENSE OF DIRECTION

WE once made a number of experiments to discover in what way the social wasps came back to the nest on returning from their hunting expeditions. Were they endowed with some innate power which made memory of places unnecessary, and enabled them to fly in a straight line to any point they wished to reach, or did their return depend upon the more commonplace method of remembering the appearance of the country-side?

One morning at half past eight, we placed a wasp cage over the opening of a yellow-jacket hole that had been closed since the night before, and caught fifty-five workers, after which the nest was again closed, one of us taking the cage out on to the lake, while the other remained to watch for their return.

At seven minutes before nine, twenty of the wasps were liberated an eighth of a mile from shore near the end of the island. All, without exception, flew toward the island and away from the nest. Whether they settled could not be determined. The boat was then moved an eighth of a mile beyond the island to the north, where, at ten minutes after nine, the remaining wasps were set free. They seemed a good deal confused, and flew in all directions. Many returned to the boat and alighted, but soon flew away again. Two that settled on the boat were knocked into the water; but they instantly rose and circled up into the air until out of sight.

Of the fifty-five wasps that we set free, thirty-nine returned to the nest by ten o’clock, five of them belonging to the lot that flew to the island, since they soon found their bearings and came directly home, reaching the nest before the wasps of the second lot were liberated.

Of the thirty-five wasps that were set free at the second point, at least twenty started in wrong directions. Adding these to the first twenty, we have left only fifteen that appeared to know where to look for their home, and yet thirty-nine reached the nest in a little more than an hour from the time the first wasps were set free.

On another morning we caught thirty-eight workers and took them to a boat-house on the shore of the lake, in the second story of which was a large room with two good-sized windows; one looked west over the lake and away from the nest, the other east toward the nest, and both were wide open. The west window was the brighter, but the other was light, the sun being on that side of the house.

We placed the cage in the middle of this room and opened the door, stationing ourselves well to one side so as not to interfere with the movements of the wasps. They came out very naturally, pausing a moment before flying, and followed each other so slowly that we could easily see which window they went out by. Twenty-two flew through the west window away from the nest, and sixteen through the east toward the nest.

At another time we took fourteen wasps from the nest of a different species and carried them seventy-three yards to the southeast. The cage was opened so that they could fly out in any direction they chose, and they all started in a straight line for the nest. Later on the same day, we took forty-five from this nest, and set them free one hundred and seventy-six yards to the south. Seven flew north toward the nest, twenty-one south, eight west, and seven east, while the other two circled around as they rose higher and higher, until they were lost to view. None in this experiment returned to take a fresh start.

Again, we took twenty-three wasps three hundred yards southeast of the nest and liberated them in an open field; thirteen flew east or south away from the nest, seven west or northwest toward the nest, and four returned to the starting-place and seemed unwilling to venture out again.

These observations show that the two species of wasps with which we experimented have no sense of direction in the form of a mysterious additional sense, nor yet in the form of a power by which they keep a register of the turns and changes in a journey and are thus able to retrace their way. Our cage was of wire, and so open that they could see all about, as we carried them from place to place; yet when they flew out, they most frequently started in a wrong direction and toward a point that we had not passed. In many instances, however, these wasps returned to the nest, and it seems highly probable that as they rose higher and higher into the air, circling as they went, they discovered some lofty treetop or other object that had before served them as a landmark, and that in this way they were able to make their way home. Bee-keepers know that if young workers which, in strong hives, pass the first ten or fifteen days of their lives in feeding the larvÆ without going abroad, are taken out and set free only a short distance from home, they are unable to find their way back, and perish, while those that have passed beyond the nursing stage and have begun to do outside work may be carried long distances away and still readily regain the nest.

With the solitary wasps we attacked the problem from the other end. We observed what the social wasps did in attempting to return to the nest; with the solitaries, we watched them when, after making the nest, they prepared to leave it to go out into the fields or woods in search of food or prey, thinking that the procedure of different species under these circumstances would afford a clue to the faculty upon which they depended to find their way about. If they were furnished with an innate sense of direction they would not need to make a study of the locality of the nest in order to find the way back, but if they were without this sense it would be only common prudence to take a good account of their bearings before going far afield.

The sight of a bee or a wasp returning to its home from some far distant spot, without hesitation or uncertainty, is indeed marvelous. When we saw our first Ammophila perform this feat we were filled with wonder. How was it possible for her to hunt for hours, in all directions, far and wide, and then return in a direct line to a nest which had been so carefully covered over that every trace of its existence was obliterated?

To say that she is a creature of instinct, however, is not quite fair to her ladyship’s intelligence, as a better acquaintance with her would prove. In reading much popular natural history one might suppose that the insects seen flying about on a summer’s day were a part of some great throng which is ever moving onward, those that are here to-day being replaced by a new set on the morrow. Except during certain seasons the exact opposite of this is true. The flying things about us abide in the same locality and are the inhabitants of a fairly restricted area. The garden in which we worked was, to a large extent, the home of a limited number of certain species of wasps that had resided there from birth, or having found the place accidentally, had settled there permanently. To make this matter clear let us suppose the case of an individual of A. urnaria. In June she spent her time in sipping nectar from the onion flowers or from the sorrel that grew on the border of the garden. In July came the days of her courtship and honeymoon, and these too were passed in going from flower to flower, from one part of the garden to another. Many a day we have followed her when she flew from blossom to blossom along a row of bean plants, turning, when she reached the end, and wending her way leisurely back along the next row. Then comes a day when we see her running over the ground and looking carefully under the weeds for a good nesting-place. At last a spot is selected and she begins to dig; but two or three times before the work is completed she goes away for a short flight. When it is done, and covered over, she flies away, but returns again and again within the next few hours, to look at the spot and, perhaps, to make some little alteration in her arrangements. From this time on, until the caterpillars are stored and the egg laid, she visits her nest several times a day, so that she becomes perfectly familiar with the neighborhood, and it is not surprising, after all, that she is able to carry her prey from any point in her territory in a nearly direct line to her hole—we say nearly direct, for there was almost invariably some slight mistake in the direction which made a little looking about necessary before the exact spot was found.

After days passed in flying about the garden—going up Bean Street and down Onion Avenue, time and time again—one would think that any formal study of the precise locality of a nest might be omitted; but it was not so with our wasps. They made repeated and detailed studies of the surroundings of their nests. Moreover, when their prey was laid down for a moment on the way home, they felt the necessity of noting the place carefully before leaving it.

Of the species that catch their prey before making the nest we have good examples in Pompilus quinquenotatus, the tornado wasp, and fuscipennis, the Pompilus with the red girdle.

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The tornado wasp may make her nest anywhere from one to ten feet from the spot on which she has deposited her spider, while fuscipennis never goes more than fourteen inches away. During the process of excavation both of these wasps pay several visits to the spider, and frequently they have difficulty in finding it. As an example of this kind of trouble we give a diagram of the course followed by an individual of fuscipennis after she had finished her nest, in trying to find her spider and in bringing it home. This and the other similar diagrams that are given are reductions of large tracings that were made on the spot. Although not absolutely correct they are exact enough for all practical purposes, since wherever there is an error it is necessarily in the direction of making the path pursued by the wasp appear shorter and less complex than it really was. The individual in question had placed her spider on a cucumber vine which lay on the ground, not hidden by leaves, but fully exposed to view. The nest was only eight inches away, but when it was finished and the wasp went to bring the spider, she found it only after a search of three minutes; and then when she went back to the nest she at first passed to one side and went some inches beyond, so that she had to retrace her steps.

Marchal notes that some wasps are very unskillful in finding their way about, showing by their errors and hesitations not only that they have no sense of direction, but that they are badly served by their memory and by what senses they have. He draws this conclusion from his own observations, one of which had for its subject Pompilus seriaceus, which nests, conveniently for him, in the walls of the rustic summer-house which he uses for a laboratory. A wasp of this species, having caught her spider, had a most wearisome experience in getting it to the nest, which had been previously excavated near the ground. She first carried it straight up, not only passing the opening, but going to the very top of the wall. Realizing that she had gone wrong, she laid it down, and after a prolonged hunt up and down, to the right and to the left, found the nest; but on leaving it again to go for the spider, she started in exactly the wrong direction, down instead of up; and not until forty minutes had been spent in searching alternately for spider and for nest did she finally bring the two together.

The best evidence that wasps depend upon a knowledge of the place in returning to their nests is given by the pains they take to acquire that knowledge. When Sphex ichneumonea was ready to dig her nest, she had great difficulty in finding a place that suited her. Many a spot was merely looked at and passed by, while others that seemed more attractive were left after they had been excavated for a little way. At last, the nest dug, she was ready to go out and seek for her store of provisions; and now came a most thorough and systematic study of the surroundings. The nests that had been partly made and then deserted had been left without any circling. Evidently she was conscious of the difference and meant, now, to take all necessary precautions against losing her way. She flew in and out among the plants, first in narrow circles near the surface of the ground, and now in wider and wider ones as she rose higher in the air, until at last she took a straight line and disappeared in the distance. Very often, after one thorough study of the topography of her home has been made, a wasp goes away a second time with much less circling or with none at all.

If the examination of the objects about the nest makes no impression upon the wasp, or if it is not remembered, she ought not to be inconvenienced nor thrown off her track when weeds and stones are removed and the surface of the ground is smoothed over; but this is just what happens. Aporus fasciatus entirely lost her way when we broke off the leaf that covered her nest, but found it, without trouble, when the missing object was replaced. All of the species of Cerceris were extremely annoyed if we placed any new object near their nesting-places. One Ammophila refused to make use of her burrow after we had drawn some deep lines in the dust before it. The same annoyance is exhibited when there is any change made near the spot upon which the prey of the wasp, whatever it may be, is deposited temporarily. We learned from experience how important it was not to disarrange the grass or plants on such occasions. The wasps are in many cases so prudent as to conceal their booty among the leaves; and this made it very inconvenient to keep our eyes upon the captured prey, as was quite necessary if we wished to follow it on its travels. To avoid the discomfort of lying on the ground or of twisting the neck at some impossible angle for half an hour at a time, we sometimes gently moved the intercepting objects to one side; but even such a slight change cost us dear in time and patience, as it threw the wasp out of her bearings and made it difficult for her to recover her treasure. We recall one exceedingly warm day in September when we were delayed in this way for forty minutes, when she would have seized the spider and gone on her way without a pause had we not interfered.ill288

Very often the wind would shake the plant so that the spider or caterpillar would fall to the ground. Under these circumstances the wasp was not at all disconcerted, but, on not finding her prey where she had left it, dropped at once to where it was lying. This is probably only an extension of their ordinary habits. A wasp that takes spiders learns to follow them as they drop from the web on being disturbed. In this they are evidently guided by sight, but perhaps they are also aided by the sense of smell under other conditions,—to the extent, at least, of recognizing the place upon which their prey has lain. With so much to build upon, it is easy to see how natural selection may have perfected the habit. We are delaying a long time over details, but we feel that to invoke an unknown sense is permissible only after a careful study of the daily life of the animals in question has left the problem unsolved.ill289

Among the wasps that first make the nest and then provision the larder, Astata bicolor is one of the most interesting. She makes a permanent abiding-place, and probably uses it until all of her eggs are laid. It is evident that since she comes and goes many times during the several weeks of her occupation, she does not need to make a prolonged study of the environment at every departure. Her first survey, just after the nest is completed, is most thorough; and, as a usual thing, when she first comes out on each succeeding morning, she reviews the situation more or less carefully. Individuals differ in this respect, however, some studying their local habitat much more than others. In this as well as in all other matters our observations are in complete accord with those of Sir John Lubbock, who says: “Indeed, many of my experiences seem to show not only a difference of character in the different species of ants, but that even within the limits of the same species there are individual differences between ants, just as between men.”[8]

This little bug-hunting Astata bicolor made her study in a different way from Sphex ichneumonea. She first flew from the nest to a spot near by and settled there, returning, after a moment, to the nest, or else flying to another resting-place. After pausing in a number of places (in the case of the one followed in the diagram, thirteen), she finished by a rapid zigzag flight. Another wasp of this genus, unicolor, differed from bicolor in not returning to the nest from the different resting-places, and in walking from one to another of them instead of flying, although the last part of the study was made on the wing.

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Cerceris deserta was one of the wasps that objected strongly to our presence, and she also made a great deal of fuss about leaving her nest. Nearly all the species circle before leaving a spot to which they intend to return, but deserta begins her flight with a series of short zigzags in the form of a half circle on one side of the nest. C. nigrescens, too, begins with semicircles, while C. clypeata flies entirely around and around the opening. The contrast between the deliberate movements of Astata and the rapid flight of Cerceris is very striking.

We have now given a sufficient number of instances, from widely separated genera, to show the care that is taken by wasps to acquaint themselves with the surroundings of their nests. It has also been shown that in spite of all this care they frequently have trouble in finding their way about. All these facts have led us to conclude that wasps are guided in their movements by their memory of localities. They go from place to place quite readily because they are familiar with the details of the landscape in the district they inhabit. Fair eyesight and a moderately good memory on their part are all that need be assumed in this simple explanation of the problem.