Bramble-Bees and Others Part 1

Bramble-bees and Others.

by J. Henri Fabre.

CHAPTER 1. BRAMBLE-DWELLERS.

The peasant, as he trims his hedge, whose riotous tangle threatens to encroach upon the road, cuts the trailing stems of the bramble a foot or two from the ground and leaves the root-stock, which soon dries up.

These bramble-stumps, sheltered and protected by the th.o.r.n.y brushwood, are in great demand among a host of Hymenoptera who have families to settle. The stump, when dry, offers to any one that knows how to use it a hygienic dwelling, where there is no fear of damp from the sap; its soft and abundant pith lends itself to easy work; and the top offers a weak spot which makes it possible for the insect to reach the vein of least resistance at once, without cutting away through the hard ligneous wall. To many, therefore, of the Bee and Wasp tribe, whether honey-gatherers or hunters, one of these dry stalks is a valuable discovery when its diameter matches the size of its would-be inhabitants; and it is also an interesting subject of study to the entomologist who, in the winter, pruning-shears in hand, can gather in the hedgerows a f.a.ggot rich in small industrial wonders. Visiting the bramble-bushes has long been one of my favourite pastimes during the enforced leisure of the wintertime; and it is seldom but some new discovery, some unexpected fact, makes up to me for my torn fingers.

My list, which is still far from being complete, already numbers nearly thirty species of bramble-dwellers in the neighbourhood of my house; other observers, more a.s.siduous than I, exploring another region and one covering a wider range, have counted as many as fifty. I give at foot an inventory of the species which I have noted.

(Bramble-dwelling insects in the neighbourhood of Serignan [Vaucluse]:

1. MELLIFEROUS HYMENOPTERA.

Osmia tridentata, DUF. and PER.

Osmia detrita, PEREZ.

Anthidium scapulare, LATR.

Heriades rubicola, PEREZ.

Prosopis confusa, SCHENCK.

Ceratina chalcites, GERM.

Ceratina albilabris, FAB.

Ceratina callosa, FAB.

Ceratina coerulea, VILLERS.

2. HUNTING HYMENOPTERA.

Solenius vagus, FAB. (provisions, Diptera).

Solenius lapidarius, LEP. (provisions, Spiders?).

Cemonus unicolor, PANZ. (provisions, Plant-lice).

Psen atratus (provisions, Black Plant-lice).

Tripoxylon figulus, LIN. (provisions, Spiders).

A Pompilus, unknown (provisions, Spiders).

Odynerus delphinalis, GIRAUD.

3. PARASITICAL HYMENOPTERA.

A Leucopsis, unknown (parasite of Anthidium scapulare).

A small Scoliid, unknown (parasite of Solenius vagus).

Omalus auratus (parasite of various bramble-dwellers).

Cryptus bimaculatus, GRAV. (parasite of Osmia detrita).

Cryptus gyrator, DUF. (parasite of Tripoxylon figulus).

Ephialtes divinator, ROSSI (parasite of Cemonus unicolor).

Ephialtes mediator, GRAV. (parasite of Psen atratus).

Foenus pyrenaicus, GUERIN.

Euritoma rubicola, J. GIRAUD (parasite of Osmia detrita).

4. COLEOPTERA.

Zonitis mutica, FAB. (parasite of Osmia tridentata).

Most of these insects have been submitted to a learned expert, Professor Jean Perez, of Bordeaux. I take this opportunity of renewing my thanks for his kindness in identifying them for me.--Author's Note.)

They include members of very diverse corporations. Some, more industrious and equipped with better tools, remove the pith from the dry stem and thus obtain a vertical cylindrical gallery, the length of which may be nearly a cubit. This sheath is next divided, by part.i.tions, into more or less numerous storeys, each of which forms the cell of a larva.

Others, less well-endowed with strength and implements, avail themselves of the old galleries of other insects, galleries that have been abandoned after serving as a home for their builder's family. Their only work is to make some slight repairs in the ruined tenement, to clear the channel of its lumber, such as the remains of coc.o.o.ns and the litter of shattered ceilings, and lastly to build new part.i.tions, either with a plaster made of clay or with a concrete formed of pith-sc.r.a.pings cemented with a drop of saliva.

You can tell these borrowed dwellings by the unequal size of the storeys. When the worker has herself bored the channel, she economizes her s.p.a.ce: she knows how costly it is. The cells, in that case, are all alike, the proper size for the tenant, neither too large nor too small.

In this box, which has cost weeks of labour, the insect has to house the largest possible number of larvae, while allotting the necessary amount of room to each. Method in the superposition of the floors and economy of s.p.a.ce are here the absolute rule.

But there is evidence of waste when the insect makes use of a bramble hollowed by another. This is the case with Tripoxylon figulus. To obtain the store-rooms wherein to deposit her scanty stock of Spiders, she divides her borrowed cylinder into very unequal cells, by means of slender clay part.i.tions. Some are a centimetre (.39 inch.--Translator's Note.) deep, the proper size for the insect; others are as much as two inches. These s.p.a.cious rooms, out of all proportion to the occupier, reveal the reckless extravagance of a casual proprietress whose t.i.tle-deeds have cost her nothing.

But, whether they be the original builders or labourers touching up the work of others, they all alike have their parasites, who const.i.tute the third cla.s.s of bramble-dwellers. These have neither galleries to excavate nor victuals to provide; they lay their egg in a strange cell; and their grub feeds either on the provisions of the lawful owner's larva or on that larva itself.

At the head of this population, as regards both the finish and the magnitude of the structure, stands the Three-p.r.o.nged Osmia (Osmia tridentata, DUF. and PER.), to whom this chapter shall be specially devoted. Her gallery, which has the diameter of a lead pencil, sometimes descends to a depth of twenty inches. It is at first almost exactly cylindrical; but, in the course of the victualling, changes occur which modify it slightly at geometrically determined distances. The work of boring possesses no great interest. In the month of July, we see the insect, perched on a bramble-stump, attack the pith and dig itself a well. When this is deep enough, the Osmia goes down, tears off a few particles of pith and comes up again to fling her load outside. This monotonous labour continues until the Bee deems the gallery long enough, or until, as often happens, she finds herself stopped by an impa.s.sable knot.

Next comes the ration of honey, the laying of the egg and the part.i.tioning, the last a delicate operation to which the insect proceeds by degrees from the base to the top. At the bottom of the gallery, a pile of honey is placed and an egg laid upon the pile; then a part.i.tion is built to separate this cell from the next, for each larva must have its special chamber, about a centimetre and a half (.58 inch.--Translator's Note.) long, having no communication with the chambers adjoining. The materials employed for this part.i.tion are bramble-sawdust, glued into a paste with the insects' saliva. Whence are these materials obtained? Does the Osmia go outside, to gather on the ground the rubbish which she flung out when boring the cylinder? On the contrary, she is frugal of her time and has better things to do than to pick up the scattered particles from the soil. The channel, as I said, is at first uniform in size, almost cylindrical; its sides still retain a thin coating of pith, forming the reserves which the Osmia, as a provident builder, has economized wherewith to construct the part.i.tions.

So she sc.r.a.pes away with her mandibles, keeping within a certain radius, a radius that corresponds with the dimensions of the cell which she is going to build next; moreover, she conducts her work in such a way as to hollow out more in the middle and leave the two ends contracted. In this manner, the cylindrical channel of the start is succeeded, in the worked portion, by an ovoid cavity flattened at both ends, a s.p.a.ce resembling a little barrel. This s.p.a.ce will form the second cell.

As for the rubbish, it is utilized on the spot for the lid or cover that serves as a ceiling for one cell and a floor for the next. Our own master-builders could not contrive more successfully to make the best use of their labourers' time. On the floor thus obtained, a second ration of honey is placed; and an egg is laid on the surface of the paste. Lastly, at the upper end of the little barrel, a part.i.tion is built with the sc.r.a.pings obtained in the course of the final work on the third cell, which itself is shaped like a flattened ovoid. And so the work goes on, cell upon cell, each supplying the materials for the part.i.tion separating it from the one below. On reaching the end of the cylinder, the Osmia closes up the case with a thick layer of the same mortar. Then that bramble-stump is done with; the Bee will not return to it. If her ovaries are not yet exhausted, other dry stems will be exploited in the same fas.h.i.+on.

The number of cells varies greatly, according to the qualities of the stalk. If the bramble-stump be long, regular and smooth, we may count as many as fifteen: that, at least, is the highest figure which my observations have supplied. To obtain a good idea of the internal distribution, we must split the stalk lengthwise, in the winter, when the provisions have long been consumed and when the larvae are wrapped in their coc.o.o.ns. We then see that, at regular intervals, the case becomes slightly narrower; and in each of the necks thus formed a circular disk is fixed, a part.i.tion one or two millimetres thick.

(.039 to.079 inch.--Translator's Note.) The rooms separated by these part.i.tions form so many little barrels or kegs, each compactly filled with a reddish, transparent coc.o.o.n, through which the larva shows, bent into a fish-hook. The whole suggests a string of rough, oval amber beads, touching at their amputated ends.

In this string of coc.o.o.ns, which is the oldest, which the youngest? The oldest is obviously the bottom one, the one whose cell was the first built; the youngest is the one at the top of the row, the one in the cell last built. The oldest of the larvae starts the pile, down at the bottom of the gallery; the latest arrival ends it at the top; and those in between follow upon one another, according to age, from base to apex.

Let us next observe that there is no room in the shaft for two Osmiae at a time on the same level, for each coc.o.o.n fills up the storey, the keg that belongs to it, without leaving any vacant s.p.a.ce; let us also remark that, when they attain the stage of perfection, the Osmiae must all emerge from the shaft by the only orifice which the bramble-stem boasts, the orifice at the top. There is here but one obstacle, easy to overcome: a plug of glued pith, of which the insect's mandibles make short work. Down below, the stalk offers no ready outlet; besides, it is prolonged underground indefinitely by the roots. Everywhere else is the ligneous fence, generally too hard and thick to break through. It is inevitable therefore that all the Osmiae, when the time comes to quit their dwelling, should go out by the top; and, as the narrowness of the shaft bars the pa.s.sage of the preceding insect as long as the next insect, the one above it, remains in position, the removal must begin at the top, extend from cell to cell and end at the bottom. Consequently, the order of exit is the converse to the order of birth: the younger Osmiae leave the nest first, their elders leave it last.

The oldest, that is to say, the bottom one, was the first to finish her supply of honey and to spin her coc.o.o.n. Taking precedence of all her sisters in the whole series of her actions, she was the first to burst her silken bag and to destroy the ceiling that closes her room: at least, that is what the logic of the situation takes for granted. In her anxiety to get out, how will she set about her release? The way is blocked by the nearest coc.o.o.ns, as yet intact. To clear herself a pa.s.sage through the string of those coc.o.o.ns would mean to exterminate the remainder of the brood; the deliverance of one would mean the destruction of all the rest. Insects are notoriously obstinate in their actions and unscrupulous in their methods. If the Bee at the bottom of the shaft wants to leave her lodging, will she spare those who bar her road?

The difficulty is great, obviously; it seems insuperable. Thereupon we become suspicious: we begin to wonder if the emergence from the coc.o.o.n, that is to say, the hatching, really takes place in the order of primogeniture. Might it not be--by a very singular exception, it is true, but one which is necessary in such circ.u.mstances--that the youngest of the Osmiae bursts her coc.o.o.n first and the oldest last; in short, that the hatching proceeds from one chamber to the next in the inverse direction to that which the age of the occupants would lead us to presume? In that case, the whole difficulty would be removed: each Osmia, as she rent her silken prison, would find a clear road in front of her, the Osmiae nearer the outlet having gone out before her. But is this really how things happen? Our theories very often do not agree with the insect's practice; even where our reasoning seems most logical, we should be more prudent to see what happens before venturing on any positive statements. Leon Dufour was not so prudent when he, the first in the field, took this little problem in hand. He describes to us the habits of an Odynerus (Odynerus rubicola, DUF.) who piles up clay cells in the shaft of a dry bramble-stalk; and, full of enthusiasm for his industrious Wasp, he goes on to say:

'Picture a string of eight cement sh.e.l.ls, placed end to end and closely wedged inside a wooden sheath. The lowest was undeniably made first and consequently contains the first-laid egg, which, according to rules, should give birth to the first winged insect. How do you imagine that the larva in that first sh.e.l.l was bidden to waive its right of primogeniture and only to complete its metamorphosis after all its juniors? What are the conditions brought into play to produce a result apparently so contrary to the laws of nature? Humble yourself in the presence of the reality and confess your ignorance, rather than attempt to hide your embarra.s.sment under vain explanations!

'If the first egg laid by the busy mother were destined to be the first-born of the Odyneri, that one, in order to see the light immediately after achieving wings, would have had the option either of breaking through the double walls of his prison or of perforating, from bottom to top, the seven sh.e.l.ls ahead of him, in order to emerge through the truncate end of the bramble-stem. Now nature, while refusing any way of escape laterally, was also bound to veto any direct invasion, the brutal gimlet-work which would inevitably have sacrificed seven members of one family for the safety of an only son. Nature is as ingenious in design as she is fertile in resource, and she must have foreseen and forestalled every difficulty. She decided that the last-built cradle should yield the first-born child; that this one should clear the road for his next oldest brother, the second for the third and so on. And this is the order in which the birth of our Odyneri of the Brambles actually takes place.'

Yes, my revered master, I will admit without hesitation that the bramble-dwellers leave their sheath in the converse order to that of their ages: the youngest first, the oldest last; if not invariably, at least very often. But does the hatching, by which I mean the emergence from the coc.o.o.n, take place in the same order? Does the evolution of the elder wait upon that of the younger, so that each may give those who would bar his pa.s.sage time to effect their deliverance and to leave the road clear? I very much fear that logic has carried your deductions beyond the bounds of reality. Rationally speaking, my dear sir, nothing could be more accurate than your inferences; and yet we must forgo the theory of the strange inversion which you suggest. None of the Bramble-bees with whom I have experimented behaves after that fas.h.i.+on.

I know nothing personal about Odynerus rubicola, who appears to be a stranger in my district; but, as the method of leaving must be almost the same when the habitation is exactly similar, it is enough, I think, to experiment with some of the bramble-dwellers in order to learn the history of the rest.

My studies will, by preference, bear upon the Three-p.r.o.nged Osmia, who lends herself more readily to laboratory experiments, both because she is stronger and because the same stalk will contain a goodly number of her cells. The first fact to be ascertained is the order of hatching.

I take a gla.s.s tube, closed at one end, open at the other and of a diameter similar to that of the Osmia's tunnel. In this I place, one above the other, exactly in their natural order, the ten coc.o.o.ns, or thereabouts, which I extract from a stump of bramble. The operation is performed in winter. The larvae, at that time, have long been enveloped in their silken case. To separate the coc.o.o.ns from one another, I employ artificial part.i.tions consisting of little round disks of sorghum, or Indian millet, about half a centimetre thick. (About one-fifth of an inch.--Translator's Note.) This is a white pith, divested of its fibrous wrapper and easy for the Osmia's mandibles to attack. My diaphragms are much thicker than the natural part.i.tions; this is an advantage, as we shall see. In any case, I could not well use thinner ones, for these disks must be able to withstand the pressure of the rammer which places them in position in the tube. On the other hand, the experiment showed me that the Osmia makes short work of the material when it is a case of drilling a hole through it.

To keep out the light, which would disturb my insects destined to spend their larval life in complete darkness, I cover the tube with a thick paper sheath, easy to remove and replace when the time comes for observation. Lastly, the tubes thus prepared and containing either Osmiae or other bramble-dwellers are hung vertically, with the opening at the top, in a snug corner of my study. Each of these appliances fulfils the natural conditions pretty satisfactorily: the coc.o.o.ns from the same bramble-stick are stacked in the same order which they occupied in the native shaft, the oldest at the bottom of the tube and the youngest close to the orifice; they are isolated by means of part.i.tions; they are placed vertically, head upwards; moreover, my device has the advantage of subst.i.tuting for the opaque wall of the bramble a transparent wall which will enable me to follow the hatching day by day, at any moment which I think opportune.

The male Osmia splits his coc.o.o.n at the end of June and the female at the beginning of July. When this time comes, we must redouble our watch and inspect the tubes several times a day if we would obtain exact statistics of the births. Well, during the six years that I have studied this question, I have seen and seen again, ad nauseam; and I am in a position to declare that there is no order governing the sequence of hatchings, absolutely none. The first coc.o.o.n to burst may be the one at the bottom of the tube, the one at the top, the one in the middle or in any other part, indifferently. The second to be split may adjoin the first or it may be removed from it by a number of s.p.a.ces, either above or below. Sometimes several hatchings occur on the same day, within the same hour, some farther back in the row of cells, some farther forward; and this without any apparent reason for the simultaneity. In short, the hatchings follow upon one another, I will not say haphazard--for each of them has its appointed place in time, determined by impenetrable causes--but at any rate contrary to our calculations, based on this or the other consideration.

Had we not been deceived by our too shallow logic, we might have foreseen this result. The eggs are laid in their respective cells at intervals of a few days, of a few hours. How can this slight difference in age affect the total evolution, which lasts a year? Mathematical accuracy has nothing to do with the case. Each germ, each grub has its individual energy, determined we know not how and varying in each germ or grub. This excess of vitality belongs to the egg before it leaves the ovary. Might it not, at the moment of hatching, be the cause why this or that larva takes precedence of its elders or its juniors, chronology being altogether a secondary consideration? When the hen sits upon her eggs, is the oldest always the first to hatch? In the same way, the oldest larva, lodged in the bottom storey, need not necessarily reach the perfect state first.

A second argument, had we reflected more deeply on the matter, would have shaken our faith in any strict mathematical sequence. The same brood forming the string of coc.o.o.ns in a bramble-stem contains both males and females; and the two s.e.xes are divided in the series indiscriminately. Now it is the rule among the Bees for the males to issue from the coc.o.o.n a little earlier than the females. In the case of the Three-p.r.o.nged Osmia, the male has about a week's start.

Consequently, in a populous gallery, there is always a certain number of males, who are hatched seven or eight days before the females and who are distributed here and there over the series. This would be enough to make any regular hatching-sequence impossible in either direction.

These surmises accord with the facts: the chronological sequence of the cells tells us nothing about the chronological sequence of the hatchings, which take place without any definite order. There is, therefore, no surrender of rights of primogeniture, as Leon Dufour thought: each insect, regardless of the others, bursts its coc.o.o.n when its time comes; and this time is determined by causes which escape our notice and which, no doubt, depend upon the potentialities of the egg itself. It is the case with the other bramble-dwellers which I have subjected to the same test (Osmia detrita, Anthidium scapulare, Solenius vagus, etc.); and it must also be the case with Odynerus rubicola: so the most striking a.n.a.logies inform us. Therefore the singular exception which made such an impression on Dufour's mind is a sheer logical illusion.

An error removed is tantamount to a truth gained; and yet, if it were to end here, the result of my experiment would possess but slight value.