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.

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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 oesophagus. The greater part of this mass, which lies behind the oesophagus, 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.

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.

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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.