AMMOPHILA AND HER CATERPILLARS
BEFORE we had worked long on our Vespa family we were beguiled by
tempting opportunities into running after the solitary wasps. The
solitaries, so far as species are concerned, are immensely more
numerous than the socials; but they have only two sexes, and the males
and females usually see but little of each other after the mating
is over, although we occasionally find them living happily together
until the end of the season. In the early summer they begin to emerge
from the nest in which the eggs were laid the year before. Solitary
indeed they come into the world, the generation that gave them birth
having perished in the fall. For a time their career is one of unmixed
pleasure, and yet, free and unguided though they are, basking in the
sunshine, feeding on the flowers, or sleeping at night under some
sheltering leaf, they are hourly acquiring experience, so that when
the cares of life descend upon them they are no longer creatures
of mere instinct. With these sobering cares an almost absurdly
heavy sense of responsibility for future generations transforms the
hitherto happy–go–lucky females into grown–up wasps with serious views
on marketing and infant foods. Each one makes a separate nest and
provisions it by her own labor; and in many cases a new nest is made
for each egg. There is no coöperation among them; although in certain
genera, as Aphilanthops and Bembex, a number of individuals build
close together, forming a colony. The nests may be made of mud, and
attached for shelter under leaves, rocks, or eaves of buildings, or
may be burrows hollowed out in the ground, in trees or in the stems of
plants. The adult wasp lives upon fruit or nectar, but the young grub
or larva must have animal food; and here the parent wasp shows a rigid
conservatism, each species providing the sort of food that has been
approved by its family for generations, one taking flies, another bugs,
and another beetles, caterpillars, grasshoppers, crickets, locusts,
spiders, cockroaches, aphides, or other creatures, as the case may be.
When the egg–laying time arrives the female secures her prey, which
she either kills or paralyzes, places it in the nest, lays the egg
upon it, and then, in most cases, closes the hole and takes no further
interest in it, going on to make new nests from day to day. In some
genera the female maintains a longer connection with her offspring,
not bringing all the provision at once, but returning to feed the larva
as it grows, and leaving the nest permanently only when the grub has
spun its cocoon. The males never acquire this interest, so admirable
for the development of character, and aid little, if at all, in the
care of the family. The egg develops in from one to three days into a
footless, maggot–like creature which feeds upon the store provided for
it, increasing rapidly in size, and entering the pupal stage in from
three days to two weeks. In the cocoon it passes through its final
metamorphosis, emerging as a perfect insect, perhaps in two or three
weeks, or, in many cases, after the winter months have passed and
summer has come again.
Most graceful and attractive of all the wasps—“_taille effilée,
tournure svelte_,” as Fabre describes them, the Ammophiles, of all the
inhabitants of the garden, hold the first place in our affections.
Not so beautiful as the blue Pelopæus, nor so industrious as the
little red–girdled Trypoxylon, their intelligence, their distinct
individuality, and their obliging tolerance of our society make
them an unfailing source of interest. They are, moreover, the most
remarkable of all genera in their stinging habits, being supposed to
use the nicest surgical skill in paralyzing their caterpillars; and few
things have given us deeper pleasure than our success in following
the activities and penetrating the secrets of their lives. In our
garden we have two species of Ammophila, urnaria Cresson, and gracilis
Cresson, both of them being very slender–bodied wasps of about an inch
in length, gracilis all black, and urnaria with a red band around
the front end of the abdomen. A. polita and A. vulgaris, which look
much like urnaria, are common in the sandy fields west and south of
Milwaukee.
[Illustration: AMMOPHILA URNARIA CARRYING CATERPILLAR TO NEST]
During the earlier part of the summer we had often seen these wasps
feeding upon the nectar of flowers, especially upon that of the sorrel,
of which they are particularly fond; but at that time we gave them but
passing notice. One bright morning, however, we came upon an urnaria
that was so evidently hunting, and hunting in earnest, that we gave up
everything else to follow her. The ground was covered, more or less
thickly, with patches of purslain, and it was under these weeds that
our Ammophila was eagerly searching for her prey. After thoroughly
investigating one plant she would pass to another, running three or
four steps and then bounding as though she were made of thistledown and
were too light to remain upon the ground. We followed her easily, and
as she was in full view nearly all of the time we had every hope of
witnessing the capture; but in this we were destined to disappointment.
We had been in attendance on her for about a quarter of an hour when,
after disappearing for a few moments under the thick purslain leaves,
she came out with a green caterpillar. We had missed the wonderful
sight of the paralyzer at work; but we had no time to bemoan our loss,
for she was making off at so rapid a pace that we were well occupied in
keeping up with her. She hurried along with the same motion as before,
unembarrassed by the weight of her victim. For sixty feet she kept to
open ground, passing between two rows of bushes; but at the end of this
division of the garden, she plunged, very much to our dismay, into a
field of standing corn. Here we had great difficulty in following her,
since, far from keeping to her former orderly course, she zigzagged
among the plants in the most bewildering fashion, although keeping a
general direction of northeast. It seemed quite impossible that she
could know where she was going. The corn rose to a height of six feet
all around us; the ground was uniform in appearance, and, to our eyes,
each group of cornstalks was just like every other group, and yet,
without pause or hesitation, the little creature passed quickly along,
as we might through the familiar streets of our native town.
At last she paused and laid her burden down. Ah! the power that has led
her is not a blind, mechanically perfect instinct, for she has traveled
a little too far. She must go back one row into the open space that she
has already crossed, although not just at this point. Nothing like a
nest is visible to us; the surface of the ground looks all alike, and
it is with exclamations of wonder that we see our little guide lift two
pellets of earth which have served as a covering to a small opening
running down into the ground.
The way being thus prepared, she hurries back with her wings quivering
and her whole manner betokening joyful triumph at the completion of
her task. We, in the mean time, have become as much excited over the
matter as she is herself. She picks up the caterpillar, brings it to
the mouth of the burrow, and lays it down. Then, backing in herself,
she catches it in her mandibles and drags it out of sight, leaving us
full of admiration and delight.
How clear and accurate must be the observing powers of these wonderful
little creatures! Every patch of ground must, for them, have its own
character; a pebble here, a larger stone there, a trifling tuft of
grass—these must be their landmarks. And the wonder of it is that their
interest in each nest is so temporary. A burrow is dug, provisioned and
closed up, all in two or three days, and then another is made in a new
place with everything to learn over again.
From this time on to the first of September our garden was full of
these wasps, and they never lost their fascination for us; although,
owing to a decided difference between their taste and ours as to what
constituted pleasant weather, all our knowledge of them was gained by
the sweat of our brows. When we wished to utilize the cool hours of
the morning or of the late afternoon in studying them, or thought to
take advantage of a cloud which cast a grateful shade over the sun at
noonday, where were our Ammophiles? Out of sight entirely, or at best
only to be seen idling about on the flowers of the onion or sorrel. At
such a time they seemed to have no mission in life and no idea of duty.
But when the air was clear and bright and the mercury rose higher and
higher, all was changed. Their favorite working hours were from eleven
in the morning to three in the afternoon, and when they did work they
threw their whole souls into it. It was well that it was so, for they
certainly needed all the enthusiasm and perseverance that they could
muster for such wearisome and disappointing labor. Hour after hour was
passed in search, and often there was nothing to show at the end of
it. Urnaria hunted on bare ground, on the purslain, and most of all
on the bean–plants. These were examined carefully, the wasp going up
and down the stems and looking under every leaf; but the search was so
frequently unsuccessful that in estimating their work we are inclined
to think that they can scarcely average one caterpillar a day.
In this species, as in every one that we have studied, we have found a
most interesting variation among the different individuals, not only in
methods, but in character and intellect. While one was beguiled from
her hunting by every sorrel blossom she passed, another stuck to her
work with indefatigable perseverance. While one stung her caterpillar
so carelessly and made her nest in so shiftless a way that her young
could survive only through some lucky chance, another devoted herself
to these duties not only with conscientious thoroughness, but with an
apparent craving after artistic perfection that was touching to see.
The method employed by the Ammophiles in stinging their prey is more
complex than that of any other predatory wasp. The larvæ with which
they provision their nests are made up of thirteen segments, and
each of these has its own nervous centre or ganglion. Hence if the
caterpillar is to be reduced to a state of immobility, or to a state
so nearly approaching immobility that the egg may be safely laid upon
it, a single sting, such as is given by some of the Pompilidæ to their
captured spiders, will be scarcely sufficient. All this we knew from
Fabre’s “Souvenirs,” and yet we were not at all prepared to believe
that any plain American wasp could supply us with such a thrilling
performance as that of the Gallic hirsuta, which he so dramatically
describes. We were, however, most anxious to be present at the
all–important moment that we might see for ourselves just how and where
urnaria stings her victim.
For a whole week of scorching summer weather we lived in the bean
patch, scorning fatigue. We quoted to each other the example of
Fabre’s daughter Claire, who followed Odynerus with unfaltering zeal
until a sunstroke laid her low. We attended scores of wasps as they
hunted; we ran, we threw ourselves upon the ground, we scrambled along
on our hands and knees in our desperate endeavors to keep them in
view, sometimes with our eyes upon the wasps themselves and sometimes
pursuing their shadows, which, like those of coming events, were cast
before; and yet they escaped us. After we had kept one in sight for an
hour or more, some sudden flight would carry her far away, and all our
labor was lost.
At last, however, our day came. We were doing a little hunting on our
own account, hoping to find some larvæ which we could drop in view of
the wasps and thus lead them to display their powers, when we saw an
urnaria fly up from the ground to the underside of a bean leaf and
knock down a small green caterpillar. Breathless with an excitement
which will be understood by those who have tasted the joy of such a
moment, we hung over the actors in our little drama. The ground was
bare, we were close by and could see every motion distinctly. Nothing
more perfect could have been desired.
The wasp attacked at once, but was rudely repulsed, the caterpillar
rolling and unrolling itself rapidly and with the most violent
contortions of the whole body. Again and again its adversary descended,
but failed to gain a hold. The caterpillar, in its struggles, flung
itself here and there over the ground, and had there been any grass
or other covering near by it might have reached a place of partial
safety; but there was no shelter within reach, and at the fifth attack
the wasp succeeded in alighting over it, near the anterior end, and in
grasping its body firmly in her mandibles. Standing high on her long
legs and disregarding the continued struggles of her victim, she lifted
it from the ground, curved the end of her abdomen under its body, and
darted her sting between the third and fourth segments. From this
instant there was a complete cessation of movement on the part of the
unfortunate caterpillar. Limp and helpless, it could offer no further
opposition to the will of its conqueror. For some moments the wasp
remained motionless, and then, withdrawing her sting, she plunged it
successively between the third and the second, and between the second
and the first segments.
The caterpillar was now left lying on the ground. For a moment the wasp
circled above it, and then, descending, seized it again, further back
this time, and with great deliberation and nicety of action gave it
four more stings, beginning between the ninth and tenth segments and
progressing backward.
Urnaria, probably feeling—as we certainly did—a reaction from the
strain of the last few minutes, and a relief at the completion of her
task, now rested from her labors. Alighting on the ground close by, she
proceeded to smooth her body with her long hind legs, standing, in the
mean time, almost on her head, with her abdomen directed upward. She
then gave her face a thorough washing and rubbing with her first legs,
and not until she had made a complete and satisfactory toilet did she
return to the caterpillar.
We saw Ammophila capture her prey only three times during the whole
summer; but from these observations and from the condition of her
caterpillars taken at various times from nests, her method seems to be
wonderfully close to that of hirsuta, with just about the same amount
of variation in different individuals.
Thus in our second example, she stung the first three segments in
the regular order, the third, the second, and lastly (and most
persistently) the first. She then went on, without a pause, to sting
the fourth, fifth, sixth, and seventh, stopping at this point and
leaving the posterior segments untouched. In our first example, it will
be remembered, the middle segments were spared. The stinging being
completed, she proceeded to the process known as _malaxation_, which
consists in repeatedly squeezing the neck of the caterpillar, or other
victim, between the mandibles, the subject of the treatment being
turned around and around so that all sides may be equally affected.
[Illustration: AMMOPHILA URNARIA STINGING CATERPILLAR]
In our third case a caterpillar which we had caught was placed in front
of a wasp just after she had carried the second larva into her nest.
She seemed rather indifferent to it, passing it once or twice as she
ran about, but finally picked it up and gave it one prolonged sting
between the third and fourth segments. She then spent a long time in
squeezing the neck, pinching it again and again, after which it was
left on the ground; and as she showed no further interest in it we
carried it home for further study.
In the three captures, then, that came under our observation, all
the caterpillars being of the same species and almost exactly of the
same size, three different methods were employed. In the first, seven
stings were given at the extremities, the middle segments being left
untouched, and no malaxation was practiced. In the second, seven
stings again, but given in the anterior and middle segments, followed
by slight malaxation. In the third, only one sting was given, but the
malaxation was prolonged and severe.
Let us now compare these variations with those of Fabre. In his first
case the sting entered at twelve different points, beginning between
the first and second segments and progressing regularly backward. There
was no malaxation. In his second example the third, second, and first
segments were stung in the order given, and thereafter each succeeding
segment up to the ninth, nine stings being given in all, with careful
malaxation following. In his later experiments, which seem to have been
numerous, he found that as a usual thing all the segments were stung,
although the posterior three or four were occasionally spared, but that
the order in which they were operated upon, as well as the amount of
malaxation, was very variable.
Our conclusions, then, as to Ammophila’s methods of stinging agree
fairly well with those of Fabre; but there is one important exception.
In his cases the middle segments, upon one of which the egg is laid
in our species as well as in his, were invariably stung, and this he
considers a point of extreme importance. In one of our cases the middle
segments were not touched.
[Illustration: CATERPILLAR WITH EGG OF AMMOPHILA URNARIA]
The point in which our observations differ most widely from those of
Fabre is in the condition of the caterpillars after the stinging.
He seems to have found that they always lived a long time, but in a
motionless or nearly motionless state; and he dwells at length upon the
necessity of both of these conditions, since he believes that while the
wasp larva must have perfectly fresh food, any violent motion would
imperil its safety. As a matter of fact we found a wide variation in
the thoroughness with which the wasps performed their task. We had, in
all, fifteen caterpillars upon which urnaria had worked her will; and
while a few of them fulfilled to a nicety the conditions which Fabre
believes to be imperative, most of them were far from doing so. Some
of them lived only three days, others a little longer, while still
others showed signs of life at the end of two weeks. Urnaria stores
two caterpillars, and in more than one instance the second one died
and became discolored before the first one was entirely eaten. The
wasp larva did not, as might have been expected, find fault with this
arrangement, but proceeded to attack number two with good appetite, ate
it all up, and then spun its cocoon as though nothing unpleasant had
occurred.
The second condition was also violated. In one case the bite of the
newly hatched larva caused the caterpillar to rear upon end in so
violent a manner that it looked as though the little creature would
surely be dislodged. Another caterpillar kept up a continuous wriggling
without any external stimulation, and when it was touched it rolled
about almost as these larvæ do in a healthy state, and yet the egg was
not shaken off. The caterpillar which received but a single sting,
although not motionless, would have been a safer repository for the egg
than either of these. Others fulfilled Fabre’s condition perfectly,
lying immovable except when stimulated, and then responding only by a
slight quivering of the legs or skin.
Among the fifteen caterpillars that we have taken from the nests of
urnaria three kinds are represented, twelve of them belonging to one
species, two to the second, and one to the third.
The egg, which is laid upon the side of the sixth or seventh segment,
hatches in from two to three days; the larva spends from six days to
two weeks in eating, and then spins its pale yellowish cocoon.
[Illustration: NEST OF AMMOPHILA]
The nesting habits of urnaria closely resemble those of the other
members of the genus, as reported by various observers. The spot chosen
is in firm soil, sometimes in open ground, but much more frequently
under the leaves of some plant. The plan is a very simple one. A
tunnel of about an inch in length leads to the pocket in which the
caterpillars are stored. There is no hardening of the walls in any
part. We took pains to draw every nest that we opened, and there was a
very considerable variation in the minor details, such as the obliquity
of the entrance tunnel, the shape of the pocket, and the angle at
which the tunnel and pocket were joined.
The work is done with the mandibles and the first legs. When it has
proceeded so far that the wasp is partly hidden, she begins to carry
the earth away from the nest. In doing this she backs up to the edge
of the opening and, flying a little way, gives a sort of flirt which
throws the pellet that she carries in her mandibles to a distance. She
then alights where she is and pauses a moment before she runs back
to the hole, or, in some cases, darts back on the wing. We watched
the process of nest–making five times during the summer. In the first
instance Ammophila, having made her excavation, ran off and after some
search returned with a good–sized lump of earth. This she laid over
the opening, which was now entirely hidden. She then flew to the bean
patch close by, but after ten minutes she came back and looked at her
nest. It was so neatly covered as to be almost indistinguishable, but
to this fastidious little creature something seemed lacking. She pulled
away the cover, carried out three or four more loads, and then began to
search for another piece for closing. After a time she came hurrying
back with a lump of earth, but when close to the nest she concluded
that it would not do, dropped it, and ran off in another direction.
Presently she found one which fitted into the hole exactly, and after
placing it she brought a much smaller piece which she put above and to
one side. She then stood back and surveyed the whole, and it seemed to
us that we could read pride and satisfaction in her mien. She then flew
away, and we supposed that that stage of the work was completed. Upon
coming back two hours later, however, we found that she had been trying
some more improvements, as a number of little pellets had been piled
up over the nest. This wasp, by the way, never succeeded in finding a
caterpillar, since when we opened the burrow a few days later it was
still empty. Perhaps she came to some untimely end.
Of the other wasps that we saw making a temporary closure of their
nests, one wedged a good–sized stone deep down into the neck of the
burrow and then filled the space above, solidly, with smaller stones
and earth. Another placed two lumps of earth just below the surface
of the ground, filled the opening with pellets loosely thrown in, and
then kicked some light dust over the whole. The others used only two or
three lumps of earth, which they fitted neatly into the opening just
below the surface. Although it is usual for urnaria to leave her nest
closed while she is off searching for her prey, there is no invariable
rule in the matter, even for single individuals. Once having seen a
wasp dig her nest and close it up, we drew some radiating lines from
the spot, in the light dust that covered the place, that we might find
it again. When we returned, two hours later, the same wasp had made a
nest four or five inches distant from the first one, and had left it
wide open, while she had gone off to search for her caterpillar. She
had probably been alarmed by the marks that we had made, and had felt
it necessary to dig a new nest, but being in a hurry to lay her egg had
omitted the usual process of closing it. We witnessed the storing of
the caterpillar and the final closing.
From Fabre we learn that argentata and sabulosa close the nest as soon
as it has been made, at least when the provisioning is to be postponed
until the next day, while holosericea leaves it open until it is
completely stored. He suggests an explanation for this variation by
dwelling upon the inconvenience that would result if it were opened
every time that the wasp brought in a caterpillar, since holosericea
stores up five or six small larvæ instead of one or two large ones. But
what, then, shall be said of polita and yarrowii, which, while they
also store a number of small caterpillars, take pains to close and
conceal the entrance every time they come out? We see the same habit
in other genera where the mother continually passes in and out, as in
Bembex and Oxybelus.
Fabre thinks that hirsuta has the habit, unusual for Ammophila, of
catching her prey first and then digging the hole in which she bestows
it. As she takes only one large caterpillar she is thus relieved of the
necessity of closing the nest more than once.
As has been said, urnaria usually hunts a long time before she finds
her caterpillar, and one or two days may pass before anything is put
into the nest. During this prolonged search she often revisits the
spot, and thus keeps fresh the memory of its locality. As soon as the
first caterpillar is stored she lays an egg on it, and then closes the
nest as before. The second one may be brought in within a few hours;
but in one instance that came under our notice we feel sure that the
interval was as much as three days. We saw the interment of the second
caterpillar, and upon excavating, found on the first one a larva at
least a day old; we suppose that at least two days had elapsed between
the laying and the hatching of the egg.
When the provisioning is completed the time arrives for the final
closing of the nest; and in this, as in all the processes of Ammophila,
the character of the work differs with the individual. For example, of
two wasps that we saw close their nests on the same day, one wedged
two or three pellets into the top of the hole, kicked in a little
dust, and then smoothed the surface over, finishing it all within five
minutes. This one seemed possessed by a spirit of hurry and bustle, and
did not believe in spending time on non–essentials. The other, on the
contrary, was an artist, an idealist. She worked for an hour, first
filling the neck of the burrow with fine earth which was jammed down
with much energy,—this part of the work being accompanied by a loud and
cheerful humming,—and next arranging the surface of the ground with
scrupulous care, and sweeping every particle of dust to a distance.
Even then she was not satisfied, but went scampering around, hunting
for some fitting object to crown the whole. First she tried to drag a
withered leaf to the spot, but the long stem stuck in the ground and
embarrassed her. Relinquishing this, she ran along a branch of the
plant under which she was working and, leaning over, picked up from the
ground below a good–sized stone; but the effort was too much for her,
and she turned a somersault on to the ground. She then started to bring
a large lump of earth; but this evidently did not come up to her ideal,
for she dropped it after a moment, and seizing another dry leaf carried
it successfully to the spot and placed it directly over the nest. A
third instance of the final closing of the nest was intermediate
between these two, the work occupying twenty minutes. The wasp first
put a plug well down, then dropped in several large pellets, brushed in
a quantity of fine earth, and finally smoothed the surface over.
We had another much less worthy example, one, indeed, that went to
the extreme of carelessness. We first saw her in the morning carrying
her caterpillar across the field. She frequently dropped it and ran
or flew to a little distance, and when she took it again the venter
was sometimes up and sometimes down, whichever way it happened. Her
nest was a very poor affair just beneath the surface, and after the
caterpillar was carried in, it was visible from above. She filled the
hole with loose particles of earth and then scratched the surface of
the ground a little in a perfunctory sort of way, as different as
possible from the painstaking labor that we had been accustomed to
in her sisters. That afternoon we opened the nest and removed its
contents. The next morning we saw this wasp bringing home her second
caterpillar. She was much puzzled and disturbed at the destruction
of her nest, and hunted for it for an hour and a half, leaving the
caterpillar on the ground near by. We could not help feeling sorry that
we had interrupted the contented routine of her life. She finally gave
up in despair, and we took possession of the deserted caterpillar.
Just here must be told the story of one little wasp whose individuality
stands out in our minds more distinctly than that of any of the others.
We remember her as the most fastidious and perfect little worker
of the whole season, so nice was she in her adaptation of means to
ends, so busy and contented in her labor of love, and so pretty in
her pride over the completed work. In filling up her nest she put her
head down into it and bit away the loose earth from the sides, letting
it fall to the bottom of the burrow, and then, after a quantity had
accumulated, jammed it down with her head. Earth was then brought from
the outside and pressed in, and then more was bitten from the sides.
When, at last, the filling was level with the ground, she brought a
quantity of fine grains of dirt to the spot, and picking up a small
pebble in her mandibles, used it as a hammer in pounding them down
with rapid strokes, thus making this spot as hard and firm as the
surrounding surface. Before we could recover from our astonishment at
this performance she had dropped her stone and was bringing more earth.
We then threw ourselves down on the ground that not a motion might be
lost, and in a moment we saw her pick up the pebble and again pound
the earth into place with it, hammering now here and now there until
all was level. Once more the whole process was repeated, and then the
little creature, all unconscious of the commotion that she had aroused
in our minds,—unconscious, indeed, of our very existence and intent
only on doing her work and doing it well,—gave one final, comprehensive
glance around and flew away.
[Illustration: AMMOPHILA URNARIA USING STONE TO POUND DOWN EARTH OVER
NEST]
We are claiming a great deal for Ammophila when we say that she
improvised a tool and made intelligent use of it, for such actions are
rare even among the higher animals; but fortunately our observation
does not stand alone, although we supposed this to be the case at the
time that it was made. Some weeks later, seeing a note of a similar
occurrence by Dr. S. W. Williston, of Kansas University, we wrote to
him on the subject. In his reply he said that he had waited for a year
before venturing to publish his observation, fearing that so remarkable
a statement would not be credited. His account is so interesting that
we quote it at length.
Even the casual observer, to whom all insects are bugs, cannot help but
be struck by the great diversity and number of the fossorial Hymenoptera
of the plains. Water is often inaccessible, trees there are few or
none, and only in places is the vegetation at all abundant. A much
larger proportion of insects, hence, find it necessary to live or breed
in holes in the ground, than is the case in more favored localities.
Especially is this the case with the Hymenoptera, great numbers and many
species of which thus breed in excavations made by themselves.
While packing specimens on an open space, uncovered by buffalo grass,
in the extreme western part of Kansas, the early part of last July, the
attention of a friend and myself was attracted by the numerous wasps
that were constantly alighting upon the ground. The hard, smooth baked
surface showed no indications of disturbance, and it was not till we
had attentively watched the insects that we learned what they were
doing. The wasp is a very slender one, more than an inch in length,
with a slender, pedicellate abdomen; it is known to entomologists as
Ammophila yarrowii Cres. They were so numerous that one was distracted
by their very multiplicity, but, by singling out different individuals,
we were enabled to verify each detail of their operations. An insect,
alighting, ran about on the smooth, hard surface till it had found a
suitable spot to begin its excavation, which was made about a quarter
of an inch in diameter, nearly vertical, and carried to a depth of
about four inches, as was shown by opening a number of them. The earth,
as removed, was formed into a rounded pellet and carefully carried to
the neighboring grass and dropped. For the first half of an inch or so
the hole was made of a slightly greater diameter. When the excavation
had been carried to the required depth, the wasp, after a survey of
the premises, flying away, soon returned with a large pebble in its
mandibles, which it carefully deposited within the opening; then,
standing over the entrance upon her four posterior feet, she (I say
she, for it was evident that they were all females) rapidly and most
amusingly scraped the dust with her two front feet, “hand over hand,”
back beneath her, till she had filled the hole above the stone to the
top. The operation so far was remarkable enough, but the next procedure
was more so. When she had heaped up the dirt to her satisfaction, she
again flew away and immediately returned with a smaller pebble, perhaps
an eighth of an inch in diameter, and then standing more nearly erect,
with the front feet folded beneath her, she pressed down the dust all
over and about the opening, smoothing off the surface, and accompanying
the action with a peculiar rasping sound. After all this was done, and
she spent several minutes each time in thus stamping the earth so that
only a keen eye could detect any abrasion of the surface, she laid
aside the little pebble and flew away to be gone some minutes. Soon,
however, she comes back with a heavy flight, scarcely able to sustain
the soft green larva, as long as herself, that she brings. The larva
is laid upon the ground, a little to one side, when, going to the spot
where she had industriously labored, by a few, rapid strokes she throws
out the dust and withdraws the stone cover, laying it aside. Next,
the larva is dragged down the hole, where the wasp remains for a few
minutes, afterwards returning and closing up the entrance precisely as
before. This, we thought, was the end, and supposed that the wasp would
now be off about her other affairs, but not so; soon she returns with
another larva, precisely like the first, and the whole operation is
again repeated. And not only the second time, but again and again, till
four or five of the larvæ have been stored up for the sustainment of
her future offspring. Once, while a wasp had gone down the hole with a
larva, my friend quietly removed the door stone that she had placed by
the entrance. Returning, she looked about for her door, but not finding
it, apparently mistrusted the honesty of a neighbor, which had just
descended, leaving her own door temptingly near. She purloined this
pebble and was making off with it, when the rightful owner appeared and
gave chase, compelling her to relinquish it.
The things that struck us as most remarkable were the unerring judgment
in the selection of a pebble of precisely the right size to fit the
entrance, and the use of the small pebble in smoothing down and packing
the soil over the opening, together with the instinct that taught them
to remove every evidence that the earth had been disturbed.
Since the Ammophiles of our species make their nests first and then do
their hunting it follows that they must sometimes carry their prey for
a considerable distance. The most ambitious attempt of this kind that
we ever witnessed was made by gracilis.
The wasp was first seen carrying a large green caterpillar, which
projected at both ends beyond her own body, across the potato field at
the lower end of the garden. We could not tell how far she had already
brought it, but judging by the direction from which she was coming, and
by the fact that we had never seen that species of caterpillar in the
garden, she had probably come through the fence from the woods beyond.
She moved along briskly over the remaining part of the potato field,
and then through an adjoining bean patch into the corn field. This had
been a place of much anxiety to us earlier in the summer; but now the
corn had been stacked and we could follow her without difficulty. So
far she had been going due south; but now she made a turn and plunged
into the long, tangled grass which grew around and among some large,
overgrown raspberry bushes. To keep track of her here seemed a hopeless
task, but we resolved to do our best, and followed anxiously after.
The wasp worked her way along about two inches above the ground and
very much below the top of the grass, clinging to the blades with her
feet and making surprisingly good progress. When half way through
the raspberry bushes she carried the caterpillar up on to a branch,
deposited it there, and after circling about to take her bearings, flew
away, doubtless to visit her nest and to make sure that she was going
in the right direction.
We, ourselves, were very glad of the chance to rest our tired eyes
and nerves from the strain of following her. The journey, so far, had
occupied nearly an hour, at almost every instant of which it had been
exceedingly difficult to keep her in view. But for our united efforts
we should certainly have failed.
While standing guard over the caterpillar we noticed that it moved its
head from side to side, showing that the first segment could not have
been severely stung, as is usually the case in the work of urnaria.
In five minutes the wasp returned, and, with the air of feeling that
everything was right, picked up her burden and carried it laboriously
through the remaining bushes and then through the grassy space that
edged the garden, as far as the rail fence which separated this part
of the grounds from the woods. Without a pause she climbed on to this
fence to the height of the second rail, passed through, and flew down
on the further side. Here she paused a moment, perhaps to take breath,
and we looked at each other in some dismay. Whither was she leading us?
We had now been following her for over an hour, and she looked equal
to as much again as she started off once more, rapidly this time, for
the grass was short here and the traveling was easy. Soon, however,
it became evident that things were going wrong, although we could not
determine what was the matter. The caterpillar was laid down while the
wasp absented herself for six minutes. She returned and carried it for
fifteen minutes, and then left it for half an hour. Once more she came
back, and carried it for ten minutes, and then she flew away. It was
now four o’clock, and we had been following her since two. We watched
over the caterpillar for an hour longer, but saw no more of the wasp.
Did she become discouraged at the magnitude of her task? It would have
been a thousand times easier for her to have dug her nest close by the
place of capture, but perhaps she had one larva already stored with her
egg upon it. The caterpillar was carried two hundred and sixty–one feet
while we watched her, with an unknown distance at each end to complete
the line between the place of capture and the nest. She could scarcely
have lost her way, since at every return she proceeded on her journey
in one general direction without any hesitation. It seems probable
then that she had hunted too far afield, and did not realize, when she
started with her booty, what an undertaking it would be to carry it to
the nest. We once saw A. vulgaris have a similar experience. She was
running along with a small green caterpillar, but became discouraged
either at the difficulty of finding her nest, or at the distance she
had to cover. She would carry the caterpillar a little way, drop it,
circle about a while, and then pick it up again; but finally she gave
up the whole undertaking and flew away.
The affairs of Ammophila must frequently go wrong, since in still
another of our few examples we saw much trouble and labor wasted. The
wasp, in this case an urnaria, captured her caterpillar successfully
and proceeded to carry it off. She was far from being in a hurry,
going along for a foot or so, and then making a long pause, during
which she would lay it down and either circle above it, perhaps to
take bearings, or spend the time in cleaning herself off, stroking and
smoothing every part of her body with the utmost care and deliberation.
Her stops were so frequent and so lengthy that nearly an hour was
occupied in going about twenty–five feet. When, at last, the nest was
reached, the plug was removed from the entrance and the caterpillar
dragged in, but almost immediately the wasp came out backwards with the
point of an egg projecting from the extremity of her abdomen. She ran
around and around the nest in a distracted way four or five times and
then went back, dragged the caterpillar out, and carried it away. The
egg came out further and further, and finally dropped on the ground
and was lost. The wasp, carrying the caterpillar, led us a long dance,
in a great semicircle over the field, coming back to the nest at last.
Instead of going in, however, she was about to start off on another
tour when we took her prey from her and placed it in the nest. The wasp
remained in the neighborhood for over an hour, but finally disappeared.
The nest was not closed, and when we dug it up on the following day it
contained only the caterpillar that we had put in.
We could usually enter into the feelings of the Ammophiles and
understand the meaning of their actions; but we were puzzled once,
when we saw an urnaria that had stored her second caterpillar and
closed her nest permanently, spend the rest of her morning in hunting.
Why in hunting? She had not dug a nest, she could not lay another egg
at once, she did not want a caterpillar, for when we offered her one
she stung it and then left it lying on the ground. The sun was bright,
the sorrel–blossoms invited her. Surely it would have been the part of
a rational wasp to have passed the rest of the day in feasting and fun.
We have said that urnaria stores two caterpillars, but this rule is
not without its exception. It was on the last day of the summer that
on a visit to our dear and fruitful potato field, we came upon a wasp
of almost double the ordinary size, that made, when flying, a loud hum
that at once attracted attention. She was just completing and closing
her nest, and we determined to watch and see what kind of a victim she
would bring in, as it seemed improbable that this great creature would
content herself with the ordinary fare of the species. The opening to
the nest measured half an inch in diameter.
It was eleven o’clock when she flew away. At half past twelve she
reappeared, coming from the direction of the woods, opened her nest,
and took out a few more pellets. Then she flew to a bush which grew
against the fence, three feet away, and following her quickly we saw
an immense green caterpillar placed high up on a branch. It must have
taken both strength and perseverance to lift this heavy weight so far
from the ground. She seized it at once and carried it down, not flying,
as these wasps sometimes do when they are descending with a burden, and
then dragged it into her nest, where it fitted rather tightly. This
nest was so shallow and so obliquely directed that the caterpillar was
plainly visible after it had been taken in.
After she had laid her egg she crawled out, getting past the
caterpillar with some difficulty, and closed the nest. There was
certainly no room for any further store of provisions, and from the
size of the caterpillar we judged that it would furnish sufficient
nourishment even for the offspring of this wasp. We were, therefore,
not surprised, upon opening the nest two days later, to find that
nothing more had been brought. We have said that the wasp larvæ spend
from six days to two weeks in eating. To be more exact, all that we
watched, with the exception of the one which developed from the egg
of this big creature, ate from six to eight days and then spun their
cocoons; but this one seemed determined to reach the size of its
mother, and ate continuously for fourteen days. Of course long before
this time had expired the remnant of the caterpillar had become a dry,
dark–colored mass which looked little likely to tempt the appetite, but
the great larva ate away with unabated relish, gradually acquiring the
color and almost the thickness of the caterpillar it had destroyed.
Ammophila polita, which we have never seen in the country, is very
common in the sandy fields to the south of Milwaukee. On the tenth of
September, in bright clear weather, we found half a dozen individuals
working within a few rods of each other, their method being similar to
that of A. yarrowii, described by Dr. Williston, and having an especial
interest, as it shows a transition stage between the wasps that
provide the store of food all at once and those that feed their young
all through the larval period. Urnaria rarely flies with her prey;
but this wasp, although her caterpillars, are not very much smaller,
and she herself is no larger, carries her booty lightly on the wing,
alighting only occasionally to run a few steps. She has to do more work
than urnaria, taking five or six caterpillars instead of two, and this
method of progression has the advantage of rapidity.
The first wasp that we saw was just alighting with a medium–sized
green caterpillar near a partly closed nest. When disturbed she flew
away, but soon returned, dropped her prey half an inch from the nest,
proceeded to clear the opening, ran inside to see that all was right,
and then backed in with the caterpillar. Emerging after a few minutes,
she placed a small pebble in the doorway, which was thus partly closed,
and flew away. She brought three more caterpillars at intervals of
thirty minutes, and then, after wedging a pebble into the neck of
the opening, she began to fill it in solidly, scratching in dirt and
packing in lumps of earth which were brought in her mandibles. We
did not allow her to complete this operation, as it would have made
excavation more difficult, but caught her and dug out the nest. The
tunnel ran down obliquely for five inches, being two inches below the
surface at the pocket. In it we found a wasp larva, which was at least
three days old, and four caterpillars. There were no signs of the
banqueting which must have already taken place. We carried this larva
home with us, and it ate the caterpillars up clean, finishing with a
fifth which we supplied from another nest, and going into its cocoon
on September sixteenth. The caterpillars all wriggled about on the
slightest stimulation, and remained in this lively state until they
were eaten. They belonged to four different species.
In a second nest to which food was being carried, we found four
caterpillars and a larva about three days old, all the conditions being
like those in the other example. Evidently the larva had been fed from
day to day, since four or five days must have elapsed since the making
of the nest.
Westwood states that Ammophila, when she has captured her prey, walks
backward, dragging it after her;[1] but in all the cases that came
under our notice she went forward, the caterpillar being grasped near
the anterior end, in her mandibles, and either lifted above the ground
or allowed to drag a little if long and heavy. It is usually held
venter up, but in one case, in which the wasp, while carrying it to her
nest, frequently laid it down and picked it up again, it was held with
the venter down or up indifferently.
The all–important lesson that Fabre draws from his study of the
Ammophiles is that they are inspired by automatically perfect
instincts, which can never have varied to any appreciable extent from
the beginning of time. He argues that deviation from the regular rule
would mean extinction. For example, if the wasp should sting ever so
little to one side of the median line the prey would be imperfectly
paralyzed and the egg would consequently be destroyed; or a sting in
the wrong place might cause the death of the caterpillar and thus the
death of the wasp larva, which, he thinks, can be nourished only by
perfectly fresh food.
The conclusions that we draw from the study of this genus differ
from these in the most striking manner. The one preëminent,
unmistakable, and ever present fact is variability. Variability in
every particular,—in the shape of the nest and the manner of digging
it, in the condition of the nest (whether closed or open) when left
temporarily, in the method of stinging the prey, in the degree of
malaxation, in the manner of carrying the victim, in the way of closing
the nest, and last, and most important of all, in the condition
produced in the victims of the stinging, some of them dying and
becoming “veritable cadavers,” to use an expressive term of Fabre’s,
long before the larva is ready to begin on them, while others live long
past the time at which they would have been attacked and destroyed if
we had not interfered with the natural course of events. And all this
variability we get from a study of nine wasps and fifteen caterpillars!
In his chapter on “Méthode des Ammophiles” Fabre says that each species
has its own tactics, allowing no novitiate. “Not one could have left
descendants if it were not the handy workman of to–day. Any little
slip is impracticable when the future of the race depends upon it.” And
yet we find that the prey may be stung so slightly that it can rear
and wriggle violently or so severely that it dies almost at once, and
in neither case is a break made in the generations of the Ammophiles,
since in the former the egg or larva is so firmly fastened as to keep
its hold, while in the latter the dead and decomposing caterpillar is
eaten without dissatisfaction or injury.
Nor do we, in gathering evidence for the evolution of the instincts of
these wasps, need to rely entirely upon our own observations. Fabre
himself gives many facts which point in the same direction, but he
draws a line between those actions which are the result of mechanical
and unvarying instinct and those which come within the sphere of
reason, and in relation to which the insect must consider, compare,
and judge. Yet this line, even in the light of his own work, is so
extremely variable, needing readjustment with every new species and
often among the individuals of the same species, that it loses for
others the meaning which it has for its author. He himself speaks of
certain individuals of the genus Sphex which refuse to be duped when
he withdraws their prey to a distance. These, he says, are the élite,
the strong–headed ones, which are able to recognize the malice of the
action and govern themselves accordingly, but these revolutionists, apt
at progress, he goes on to say, are few in numbers. The others, the
conservators of old usages and customs, are the majority, the crowd.
Yes, but is it not always the strong–minded few that direct the destiny
of a race?