THE WOOD–BORERS
OUR two species of Trypoxylon are both slender–waisted black wasps,
albopilosum having bunches of snowy white hairs on the first legs, and
measuring three quarters of an inch in length, while rubrocinctum is a
little smaller, and, as the name implies, wears a red girdle.
Although these wasps are called wood–borers, they will use convenient
cavities in any material. When we went out to our summer cottage, in
the last days of June, 1895, we found many little wasps of the species
Trypoxylon rubrocinctum busily working about a brick smoke–house on the
place. Closer examination showed that in the mortar between the bricks
were many little openings leading back for a considerable distance,
which were occupied by the wasps. It would seem that these holes were
excavated by some other agency than the wasps themselves, as they were
so much too deep for their purposes that before using them they built
a mud partition across the opening about an inch from the outside of
the wall. Later we found nests of the same species in the posts which
support an upper balcony of the cottage; and here, too, the wasps made
use of holes which were already excavated.
In the following summer we found large numbers of these wasps at work
in a straw–stack. The stack had been cut off perfectly smooth on one
side, so that many thousands of the cut ends of the straws were exposed
to view, and these proved very attractive to rubrocinctum. This species
is very cosmopolitan in its tastes, for we found it utilizing the
small holes in the sticks of a woodpile. The straws made the daintiest
nesting–places, however, and were well adapted to our purposes, since
they could be drawn out of the stack and split lengthwise so that the
contents could be easily studied. The two halves could then be brought
together again without injuring the inhabitants, and thus we often
kept several sets under observation long enough to watch the changes
from the egg to the pupa. We found Trypoxylon albopilosum nesting in
holes made by beetles in posts and trees, but never in straws. A third
species, bidentatum, was very common, nesting in the stems of plants.
During the month of August we saw many individuals of this species
hunting for spiders on the blackberry bushes; but at this time we were
so much absorbed in Crabro stirpicola that we never followed them to
their homes.
Rubrocinctum was more conveniently studied, and through July and August
we watched the comings and goings of these little wasps. They were very
good–tempered, never resenting our close proximity nor our interference
with their housekeeping. By working hard they could prepare a nest,
store it with spiders, and seal it up all in the same day. This we have
seen them do in several instances. In other cases the same operation
takes three or four days. In the second summer that we worked with them
we found one very energetic mother that stored four nests in one day.
It had rained hard on the twenty–sixth of July, and no wasp works in
such weather. On the afternoon of the twenty–seventh we took a straw
just as the little mother was bringing in a spider. We opened it and
found that the innermost cell contained eight Epeirids, with an egg on
the abdomen of the last one taken in; the second cell was provisioned
with ten spiders, with the egg on the seventh, so that three had been
brought in after it was laid; the third cell had the egg on the last
spider, as did also the fourth. All of these eggs hatched on the
twenty–ninth,—the two outer ones, that were laid last, between eight
and nine o’clock in the morning, and the two that were laid earlier
between two and three in the afternoon. This was the biggest day’s
hunting that we have ever recorded for any of our wasps.
With both species (T. rubrocinctum and T. albopilosum), when the
preliminary work of clearing the nest and erecting the inner partition
has been performed by the female, the male takes up his station inside
the cell, facing outward, his little head just filling the opening.
Here he stands on guard for the greater part of the time until the nest
is provisioned and sealed up, occasionally varying the monotony of his
task by a short flight. As a usual thing all the work is performed by
the female, who applies herself to her duties with greater or with less
industry according to her individual character; but the male doubtless
discharges an important office in protecting the nest from parasites.
We have frequently seen him drive away the brilliant green Chrysis fly,
which is always waiting about for a chance to enter an unguarded nest.
On these occasions the defense is carried on with great vigor, the fly
being pursued for some distance into the air. There are usually two
or three unmated males flying about in the neighborhood of the nests,
poking their heads into unused holes, and occasionally trying to enter
one that is occupied, but never, so far as we have seen, with any
success, the male in charge being always quite ready and able to take
care of his rights. The males, however, made no objection when strange
females entered the nest, as they sometimes did by mistake, nor did the
females object to the entrance of a strange male when the one belonging
to the nest happened to be away; but in such cases the rightful owner,
on his return, quickly ejected the intruder. We often amused ourselves,
while we were watching the nests, by approaching the little male, as
he stood in his doorway, with a blade of grass. He always attacked it
valiantly, and sometimes grasped it so tightly in his mandibles that he
could be drawn out of the nest with it.
When the female returns to the nest with a spider the male flies out to
make way for her, and then as she goes in he alights on her back and
enters with her. When she comes out again she brings him with her, but
he at once reënters, and then, after a moment, comes out and backs in,
so that he faces outward as before.
In one instance, with rubrocinctum, where the work of storing the nest
had been delayed by rainy weather, we saw the male assisting by taking
the spiders from the female as she brought them and packing them into
the nest, leaving her free to hunt for more. This was an especially
attentive little fellow, as he guarded the nest almost continuously
for four days, the female sometimes being gone for hours at a time. On
the last day he even revisited the nest three or four times after it
had been sealed up.
It is upon the female that the heaviest part of the work devolves.
As soon as she has put the nest in order she begins the arduous task
of catching spiders wherewith to store it. It usually takes her from
ten to twenty minutes to find a spider and bring it home, but she is
sometimes absent for a much longer time. When the spider has been
carried to the nest the process of packing it in begins. This occupies
some time, and apparently a good deal of strength,—the female pushing
it into place with her head, totally disregarding its comfort, all
the spiders that are caught being pressed and jammed together into a
compact mass. While she is busied in this way she makes a loud cheerful
humming noise. The number of spiders brought seems to depend upon
their size, in which quality they vary greatly, the largest ones being
six or eight times as large as the smallest. Rubrocinctum fills her
nest with from seven to fourteen, while the larger albopilosum brings
as many as twenty–five or thirty. Those that we examined represented
many different genera, and even different families, although they were
usually orb–weavers.
In a number of cases, during the first summer, after several spiders
had been stored, we gently drew them out with a bent wire. In one nest
in which there were five spiders, we found, two hours after they had
been stored, that three were alive and two were dead. In another, which
the wasp had just begun to seal up, were ten spiders. Three of these
were injured in being drawn out. Of the remainder four were alive and
three dead. On the anterior part of the dorsum of one of the living
spiders was the egg. It had probably been fertilized as the female
carried the male into the nest on her back.
When we discovered rubrocinctum in the straw–stack, we made many
observations as to the position of the egg and the number and condition
of the spiders. We found that the egg was always placed either on the
side or the back of the anterior part of the abdomen. The number of
spiders stored was, as we have already stated, from seven to fourteen.
A fact that interested us greatly was the remarkable accuracy shown by
the wasp in never selecting too large a spider for the calibre of the
straw. Oftentimes it was an extremely close fit, but it could always
be squeezed down. When they nested in posts they used at times much
larger prey. Unfortunately we never saw this species capture its prey,
nor could we prevail upon it to sting in captivity, but the number
of spiders that we found in straws was so large as to afford abundant
evidence concerning the degree of surgical skill possessed by the
wasps. P. marginatus and P. scelestus, in overpowering their large
fierce Lycosids, must sting when and where they can, but most of the
spiders taken by rubrocinctum are inoffensive creatures, and there is
so little need to be careful or adroit in dealing with them that she
has time and opportunity to sting the exact spot that will give the
best results.
[Illustration: TRYPOXYLON RUBROCINCTUM]
The concentration of the nervous system in the Arachnida would seem
to conduce very strongly to uniform results from the stinging of the
wasps. Unlike the larva used by Ammophila, with its chain of ganglia,
in the Araneidæ the whole central nervous system, including the brain
and the ventral cord, forms a single mass, pierced by the œsophagus.
The greater part of this mass, which lies behind the œsophagus,
represents the fused ventral cord from which the nerves radiate. It is
evident that a thrust given in almost any part of the ventral face of
the cephalothorax, or even on either side of the anterior half of its
edges, would reach the nervous centre. With these facts before us let
us turn to the notes made upon the condition of the spiders that had
been stung and stored up in the nests of the straw–stack. By the “first
cell” we mean the last one stored, which was naturally the first one
opened.
July 11. Opened a nest of _rubrocinctum_. The first cell contained
fourteen live spiders with a newly laid egg. Some of the spiders were
very lively, moving spontaneously. Second cell, ten spiders, one dead,
others alive, and an egg. Third cell, eight spiders, three dead and five
alive, and the egg.
July 12. In each of the first and second cells one spider has died since
yesterday, while in the third there is no change in their condition. The
egg in the third cell hatched at nine in the morning, and the one in the
second cell at three in the afternoon.
July 13. In the first cell all the spiders are dead but one, and in the
second, all but four, while in the third none are alive.
July 15. All the spiders in the second cell are dead.
July 16. The one spider in the first cell has outlived all the others,
but that, too, died to–day.
The record of another set of nests is as follows: On July eighth we
took a straw with a wasp as she went in with her spider. The cell was
not sealed up. It contained fourteen specimens of three species of
orb–weavers, and the egg was apparently just laid. The spiders were
pushed in very tightly, and the legs and abdomens were, in many cases,
bent to one side. All were limp, but alive. By July tenth, four were
dead; on July eleventh the egg hatched. By July thirteenth all of the
spiders were dead.
It is unnecessary to give the history of other nests in detail, since
these facts make it clear that there is a great variation in the degree
of severity with which the spiders are stung, so that while with some
the paralysis is complete, with others it is only partial. Some were
killed outright, others lived two or three days, while still others
survived for two weeks. Compared with the work of the Pelopæi it would
seem that a smaller number of the spiders are killed at once, while a
larger number die after the lapse of a few days. None of the victims
of Trypoxylon live so long as the most perfectly paralyzed spiders of
the mud–daubers. Two of them lived ten and fifteen days respectively,
while with Pelopæus one survived until the thirty–eighth and one until
the fortieth day.
The egg requires from forty to sixty hours for its development, and the
larva feeds for seven or eight days before spinning its cocoon. Those
that we watched usually disposed first of the abdomen and then of the
cephalothorax; sometimes they would consume several abdomens before
attacking the other parts. After the body was devoured the legs were
picked up and eaten. When the supply of food was generous, portions of
the spiders were sometimes left untouched. The cocoons resembled in
general appearance and structure those of Pelopæus.
When a female returns with her load she usually hunts about for a few
moments before finding her nest, sometimes entering, first, two or
three that are empty or are occupied by other wasps; but we do not
wish to cast any reflection upon the sense of locality of a creature
that is able to find one particular straw out of the many thousands in
an expanse of stack twenty feet high by twelve wide. We ourselves can
testify, from experience, to the extreme difficulty of the task.
After the storing process is completed the female seals up the nest
with mud. In the case of one rubrocinctum that we were watching, she
began to close the opening at four in the afternoon and finished her
work just thirty minutes later. In this time she made ten journeys for
mud, bringing it in pellets in her mandibles. In another case, also a
rubrocinctum, the female, after bringing so many spiders that the cell
was full up to the very door (which we saw in no other case), went away
without closing it, and never returned. The male seemed uneasy at her
conduct, and several times flew away, staying an hour or two and then
returning; but after a time he too deserted the nest. Whether some evil
fate overtook the female or whether there was some failure of instinct
on her part, can only be conjectured; but the latter hypothesis is not
untenable, since out of seventy–six nests that we had under observation
seven were cleaned out and prepared and were then sealed up empty. We
have often found similar cases among the nests of the blue mud–dauber
wasps, where it is not a very uncommon thing for the absent–minded
females to build their pretty little cylindrical nests with infinite
care and patience, and then to seal them up without putting anything
inside.
Cocoons of rubrocinctum that were gathered in the month of August
remained over the winter and hatched in May and June.
Almost as interesting as rubrocinctum is the slightly larger species,
T. albopilosum. This wasp has a great liking for the posts that support
the balcony of our cottage, a preference that is very convenient for
us, as it enables us to sit in the shade and watch their doings at our
ease.
One afternoon as we sat, literally, at our posts, a female of
albopilosum came humming along, looking very important and energetic,
as though she had planned beforehand exactly what to do. She entered an
empty hole, head first, and at once began to gnaw at the wood, kicking
it out backwards with considerable violence. After a few minutes she
changed her method of work, and began to carry out loads of wood dust
in her mandibles, dropping it in little showers just outside the nest,
and then hastening back. In forty minutes she carried out, in this way,
upwards of fifty loads. She then flew away, but returned in ten minutes
with a male. She alighted, he took his place on her back, and they went
in together.
After a time they came out and both flew away, but the next morning
they came back and the nest was stored.
[Illustration: MALE TRYPOXYLON AWAITING THE FEMALE]
In this species the male does not always come out of the nest when
the female brings a spider, the nest being enough larger than in
rubrocinctum to accommodate them both comfortably. As a usual thing,
however, he enters on the back of the female. The spiders brought by
albopilosum are larger than those used by rubrocinctum. They sometimes
bring such heavy specimens of Epeira insularis that they are carried
with difficulty, the wasp alighting and dragging the spider into the
hole instead of flying directly in as usual.
We watched a number of albopilosum nests during the second summer,
finding them in several instances through the loud humming of the
female while she was pushing the spiders into her hole. From our not
very extensive study of the spiders taken by this species we are of the
opinion that some are killed at the moment of capture, while others
that are only paralyzed die in the nest from day to day.
Mr. W. H. Ashmead has noted that albopilosum stores its nest with
aphides, but in the cases that we observed they used only spiders.
There can be no mistake on this point, as we more than once took the
spider from the wasp as she was entering the nest. In a recent letter
Mr. Ashmead says that his notes were made in the field, and that he
probably mistook some closely allied species for this one.
We are not as familiar with the habits of T. bidentatum as with those
of the other two, but we have a few notes relating to the female. This
little worker is the smallest of the three, and like her sisters is a
confirmed spider–hunter. Once, when out among the raspberry bushes, we
had the good fortune to witness a capture. The wasp seized the spider,
as it rested on a leaf, by the top of the cephalothorax, and, holding
it firmly, curved her abdomen under and stabbed the ventral face of
the cephalothorax. All her motions were deliberate, and after the
operation she delayed a moment before picking it up by a leg and flying
off. We often found raspberry stems which had been filled with spiders
by this wasp, but we do not know the length of time required for the
development of the egg, nor how long the larva eats before pupation.
The cocoon is very different in appearance from those of rubrocinctum
and albopilosum, being exceedingly long, slender, and almost white,
instead of short, wide, and brown. The perfect insects come out in
September, and the last cocoon formed is the first one to hatch. This
was also true of the cocoons of rubrocinctum formed in straws.
Years ago, when we found that many of the orb–weavers laid enormous
numbers of eggs (A. cophinaria from 500 to 2000), we wondered what
became of the thousands of spiderlings. An acquaintance with
Trypoxylon has shown us their fate, and has given us an illustration of
how closely the two groups are related. To make a very modest estimate
there must have been twenty wasps at work in our straw–stack. During
the six weeks which make the busiest part of their working season each
of these must have stored, at the very least, thirty cells, putting
an average of ten spiders into a cell. It may then be considered
certain that the straw–stack, with its working surface of twelve by
twenty feet, was the mausoleum of six thousand spiders, and it is very
probable that twice as many were interred within its depths. It must
be remembered, too, that before the spiders have grown large enough to
be interesting to rubrocinctum, bidentatum has had her turn at them,
and that those that are allowed to grow too large for rubrocinctum are
preyed upon grade after grade, first by albopilosum and finally by
Pelopæus, Pompilus, and other genera.
The wasps of this genus lose their interest in family affairs about the
second week in August, though after this time they may still be seen
taking their well–earned holiday on the blossoms of the aster and the
golden–rod.