WORKERS IN CLAY
THE nests of Pelopæus cœruleus and Pelopæus cementarius, our two
mud–daubers, are common under eaves and in other sheltered places,
and many a country boy on opening them has been astonished to find
that they do not contain wasps, but are crammed with spiders. Let them
alone, however, and the wasps will arrive, for somewhere in the mass is
an egg; and when it hatches the spiders will serve as breakfast, dinner
and tea for the larva, until the change from the Arachnida to the
Hymenoptera has been accomplished. Poor spiders! it is a wonder that
there are any left, such thousands and tens of thousands are destroyed
by these tremendously energetic enemies.
Of what is Pelopæus thinking as, humming loudly, she jams her paralyzed
and benumbed victims into her little cylindrical tubes? If only we
could get inside of that little head! If only we could be wasps for a
day, and then come back and tell about it, how much vain speculation
would be saved! We can understand her when she soars gayly into the
blue, the sunshine flashing from her brilliant wings; we too have felt
the delight of health and freedom. She is still comprehensible when,
at the close of day, she and her sisters quarrel for the favorite
sleeping–places among the carvings of the porch pillars; but we
cannot follow her mental processes when, at the moment of building,
she surrenders herself to the mysterious sway of instinct, doing she
knows not what, but doing it joyously, and preserving through it all
the precious possession of her own individuality. Every aspect speaks
of pleasure as these wasps gather at well or spring, and, singing
contentedly, stand on their heads to gather their loads of mud. Briskly
and gayly they fly back and forth, pausing at the nest long enough to
pat the soft building material into shape. A single load makes half a
ring at the larger part of the nest or a whole one at the bottom; and
since one dries before the next is put on, the contour of each ring is
visible when the tube is done, giving a very artistic effect. This is
only accident, however; the wasp cares nothing about the beauty of the
structure, for her next step is to daub the whole with lumps of mud,
the walls being thus thickened and strengthened. About forty loads are
necessary for each cell, and to build and provision one is a good day’s
work.
It is strange enough that with no one to teach her Pelopæus knew how
to make her cell; but now she must do her hunting, and it is stranger
still that she should be impelled to catch nothing but spiders. How
does she know a spider from a fly, and why should she prefer one to the
other? Not so unreasonable as some wasps, however, she demands nothing
further than that her prey shall belong to this great group, and passes
lightly over differences of species and genera. Her powerful sting fits
her to cope with anything she may meet; but as the size of the cell
must be taken into consideration, and the victim must be carried home
on the wing, she is on the lookout for something not too large. Here
then she ceases to be an automaton, and to some extent makes use of her
wits.
How does Pelopæus seize her spider? When and how many times is it
stung? Is the wound given with discrimination, a certain point in the
ganglion being pricked, so that the spider may be paralyzed, but not
killed? Is there any malaxation?
These were important questions to us, and we were therefore greatly
excited over our first hunt. One of the blue wasps came flying along,
alighted on our cottage wall, and began her search, creeping into
corners and cracks and investigating cottony lumps of web. In a few
moments a small Epeira strix (the only species to be found on the
cottage) was dislodged, and at once dropped to the floor of the porch.
The wasp paid no further attention to it, but went on with her search.
Three more spiders, one after the other, were disturbed and dropped to
the floor without being followed. The fifth one discovered was a little
larger than the others, and was seized by the jaws and first legs of
the wasp before it had time to escape. It was then rolled into a ball,
or at least so it appeared, and stung, then rolled a little more and
stung again, and then carried off. We had scarcely drawn breath after
this performance when a second wasp appeared. This one dislodged two
spiders, and then caught a third, which was seized and stung without
any rolling, and then instantly borne away. A third wasp seized the
first spider that she found, and started on her flight at the same
moment, stinging it on the wing.
So the game went on, while we waxed warm with the excitement and
fascination of the chase. As the hours went by some of the yellow
mud–daubers appeared, adding to the interest of the scene, although
we could not see that their method differed in the least from that of
cœruleus.
Rarely did they succeed in catching a spider until they had dislodged
two or three. Sometimes the spiders were followed as they dropped, and
were caught on the floor, but oftener the wasp let them escape and
continued her search on the wall. At the moment of capture we could see
that she bent her abdomen under and inflicted a sting, but although we
concentrated our attention on the point we could not be sure as to just
what part was touched. It is our impression that this first sting was
given anywhere, at random, with the object of producing a condition of
temporary quiet in the victim, so that the next part of the operation
could be carried on with deliberation.
The second step in the procedure was commonly for the wasp to alight
upon some neighboring object, usually the branch of a bush or tree, and
sting the spider a second time, being evidently in no haste; but the
difficulty of following her as she flew, and her habit of alighting
above our range of vision, made it almost impossible to see just what
she did. She certainly remained on the branch for some moments, either
resting quietly or rolling the spider around and around, and had every
opportunity to sting it as carefully as she wished; but we afterward
found that she followed no exact method, since two thirds of the
spiders were killed at the moment of capture, and most of the others
died within a week, while a few lived for thirty–five or forty days. In
this study we opened five hundred and seventy–three cells and handled
over two thousand spiders, watching over them from day to day with a
magnifying glass, that no sign of life might be neglected.
When Pelopæus has filled her cell, she seals it up and makes another
close to it, clusters of from six to twenty being found in one spot.
Any especially desirable place is used by great numbers; and they make
a lively scene, working eagerly at their nests, dashing off for more
mud or bringing in their victims. All animated by the same compelling
instinct, they are still individuals, and the character of each enters
into her work. One picks up the first spider she sees, no matter how
tiny it may be, and makes twenty–five or thirty journeys before her
cell is filled, while another seems to have a calculating turn of mind,
using four or five big spiders instead of a quantity of small ones.
Has she made a note of the calibre of her cell, and determined to save
herself trouble by looking farther and selecting the largest ones that
will go in?
[Illustration: HORIZONTAL CELLS OF THE MUD–DAUBER]
Most of them place their cells vertically; but a few prefer the
horizontal position, while still others, undecided as to the matter
of direction, make clusters in which some are horizontal and
others upright. Occasionally there is a remarkable innovation in
building–material, as where in a group of fifteen, four cells in the
centre were constructed of pure white plaster, forming a striking
contrast to the surrounding mud color. One wasp built an entire cluster
after an original fashion, following the beaten track until the cell
was completed, and even bringing mud enough to daub it over, as her
sisters were doing, but sticking it all in one spot, so that when
the group was complete irregular lumps were attached here and there,
leaving visible the elegant architecture of the individual cells. Did
she think they were too pretty to spoil? or was she merely one of those
radical spirits that rebel against conventionality and demand change
for the sake of change? It is these variations that furnish Natural
Selection with its materials; but rigid as may be the rules regarding
the non–survival of the unfit, we find that the race of Pelopæus
still produces many absent–minded wasps, that after spending hours in
carefully constructing their nests, seal them up empty, forgetting to
put in the spiders or to lay the egg.
When a cell is sealed, the mother wasp ceases to take an interest
in it, but she has done all that is necessary. In two or three days
the egg hatches, after which the larva spends ten or fifteen days in
eating, and then spins its cocoon. Here it remains, perhaps for only
a few weeks,—for there are two or three generations in one season,—or
perhaps through the long months of winter.
Fabre gives a most entertaining account of a French species of Pelopæus
which nests in the wide–mouthed chimneys of the peasant. Undisturbed
by the steam of washing day or the bustle of dinner–getting, the wasp
enters the open door, passes unconcerned among the human inhabitants,
and makes her cells against the smoky bricks, out of reach of the
flames. This species kills her prey at the moment of capture, by which
act she falls in the estimation of Fabre, who respects a wasp in
proportion to the nicety with which she delivers her sting. He says,
however, that at least she follows a logical method in turning to
account these spiders, menaced with early decay. In the first place
the prey is multiplied in each cell. The piece actually attacked by
the larva is soon a disorganized mass, likely to decay speedily; but
it is small and is consumed before decomposition can advance, for when
a larva once attacks a spider it does not leave it for another. The
others then remain intact, which is enough to keep them fresh during
the short period of larval life. When, on the contrary, the prey
consists of a single large piece, it is necessary that the organic life
should be maintained, and a special art must also be observed in eating
it. It is well then that Pelopæus is inspired to take numerous small
pieces. The egg, moreover, is always placed on the first spider brought
in, whether the storing of the nest is completed within a few hours, or
whether, as in some cases, it occupies several days; and this M. Fabre
considers a very happy arrangement.
The French Pelopæi differ from ours at nearly every point. Ours kill
only about two thirds of their victims, many of the others being
paralyzed so perfectly that they live for two or three weeks. Again,
ours, instead of placing the egg upon the first spider, almost
invariably lay it upon the last one brought in. Another point of
difference is that our larvæ frequently start in by eating up the
soft abdomens, like children who first devour the plums in their
pudding, returning later to the tough parts that are left, a rash and
reprehensible course of action of which their better–taught French
cousins are never guilty. When one comes to compare the two sets of
facts furnished by the two groups of species, the deductions which
Fabre has drawn as to the importance of the instincts of the French
group are seen to be unfounded. The American species violate nearly
every principle which he considers necessary to their existence, and
yet they flourish and multiply. For our part we find nothing in the
actions of Pelopæus that needs to be explained—nothing that is not well
adapted to the conditions under which each species works. The measure
of praise or blame which we mete out to these depredators is merely a
way of saying whether we would or would not follow their methods in
provisioning our houses and rearing our children. Perhaps we would
always use large spiders and would always have them fresh; but it is
evident that tastes differ, and the matter is so purely a subjective
affair that it will have to go unsettled. In any event, whether her
victims be strong or feeble, old or young, big or little, fresh or dry,
they certainly serve admirably in enabling Pelopæus to rear brood after
brood, and to people the different parts of the earth with abundant
representatives of her kind.