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- Bramble-bees and Others - 10/47 -
lines, hardly perceptible to the most penetrating lens, show themselves in transverse circles. These are the first signs of segmentation. A contraction appears in the front hyaline part, marking the head. An extremely thin opaque thread runs down either side. This is the cord of tracheae communicating between one breathing-hole and another. At last, the segments show distinctly, with their lateral pads. The grub is born.
At first, one would think that there was no hatching in the proper sense of the word--that is to say, no bursting and casting of a wrapper. The most minute attention is necessary to show that appearances are deceptive and that actually a fine membrane is thrown off from front to back. This infinitesimal shred is the shell of the egg.
The grub is born. Fixed by its base, it curves into an arc and bends its head, until now held erect, down to the red mass. The meal begins. Soon a yellow cord occupying the front two-thirds of the body proclaims that the digestive apparatus is swelling out with food. For a fortnight, consume your provender in peace, my child; then spin your cocoon: you are now safe from the Tachina! Shall you be safe from the Anthrax' sucker later on? Alack!
CHAPTER 3. THE DISTRIBUTION OF THE SEXES.
Does the insect know beforehand the sex of the egg which it is about to lay? When examining the stock of food in the cells just now, we began to suspect that it does, for each little heap of provisions is carefully proportioned to the needs at one time of a male and at another of a female. What we have to do is to turn this suspicion into a certainty demonstrated by experiment. And first let us find out how the sexes are arranged.
It is not possible to ascertain the chronological order of a laying, except by going to suitably-chosen species. Digging up the burrows of Cerceris-, Bembex- or Philanthus-wasps will never tell us that this grub has taken precedence of that in point of time nor enable us to decide whether one cocoon in a colony belongs to the same family as another. To compile a register of births is absolutely impossible here. Fortunately there are a few species in which we do not find this difficulty: these are the Bees who keep to one gallery and build their cells in storeys. Among the number are the different inhabitants of the bramble-stumps, notably the Three-pronged Osmiae, who form an excellent subject for observation, partly because they are of imposing-size--bigger than any other bramble-dwellers in my neighbourhood--partly because they are so plentiful.
Let us briefly recall the Osmia's habits. Amid the tangle of a hedge, a bramble-stalk is selected, still standing, but a mere withered stump. In this the insect digs a more or less deep tunnel, an easy piece of work owing to the abundance of soft pith. Provisions are heaped up right at the bottom of the tunnel and an egg is laid on the surface of the food: that is the first-born of the family. At a height of some twelve millimetres (About half an inch.--Translator's Note.), a partition is fixed, formed of bramble saw-dust and of a green paste obtained by masticating particles of the leaves of some plant that has not yet been identified. This gives a second storey, which in its turn receives provisions and an egg, the second in order of primogeniture. And so it goes on, storey by storey, until the cylinder is full. Then a thick plug of the same green material of which the partitions are formed closes the home and keeps out marauders.
In this common cradle, the chronological order of births is perfectly clear. The first-born of the family is at the bottom of the series; the last-born is at the top, near the closed door. The others follow from bottom to top in the same order in which they followed in point of time. The laying is numbered automatically; each cocoon tells us its respective age by the place which it occupies.
To know the sexes, we must wait for the month of June. But it would be unwise to postpone our investigations until that period. Osmia- nests are not so common that we can hope to pick one up each time that we go out with that object; besides, if we wait for the hatching-period before examining the brambles, it may happen that the order has been disturbed through some insects' having tried to make their escape as soon as possible after bursting their cocoons; it may happen that the male Osmiae, who are more forward than the females, are already gone. I therefore set to work a long time beforehand and devote my leisure in winter to these investigations.
The bramble-sticks are split and the cocoons taken out one by one and methodically transferred to glass tubes, of approximately the same diameter as the native cylinder. These cocoons are arranged one on top of the other in exactly the same order that they occupied in the bramble; they are separated from one another by a cotton plug, an insuperable obstacle to the future insect. There is thus no fear that the contents of the cells may become mixed or transposed; and I am saved the trouble of keeping a laborious watch. Each insect can hatch at its own time, in my presence or not: I am sure of always finding it in its place, in its proper order, held fast fore and aft by the cotton barrier. A cork or sorghum-pith partition would not fulfil the same purpose: the insect would perforate it and the register of births would be muddled by changes of position. Any reader wishing to undertake similar investigations will excuse these practical details, which may facilitate his work.
We do not often come upon complete series, comprising the whole laying, from the first-born to the youngest. As a rule, we find part of a laying, in which the number of cocoons varies greatly, sometimes falling as low as two, or even one. The mother has not deemed it advisable to confide her whole family to a single bramble-stump; in order to make the exit less toilsome, or else for reasons which escape me, she has left the first home and elected to make a second home, perhaps a third or more.
We also find series with breaks in them. Sometimes, in cells distributed at random, the egg has not developed and the provisions have remained untouched, but mildewed; sometimes, the larva has died before spinning its cocoon, or after spinning it. Lastly, there are parasites, such as the Unarmed Zonitis (Zonitis mutica, one of the Oil-beetles.--Translator's Note.) and the Spotted Sapyga (A Digger- wasp.--Translator's Note.), who interrupt the series by substituting themselves for the original occupant. All these disturbing factors make it necessary to examine a large number of nests of the Three- pronged Osmia, if we would obtain a definite result.
I have been studying the bramble-dwellers for seven or eight years and I could not say how many strings of cocoons have passed through my hands. During a recent winter, in view particularly of the distribution of the sexes, I collected some forty of this Osmia's nests, transferred their contents into glass tubes and made a careful summary of the sexes. I give some of my results. The figures start in their order from the bottom of the tunnel dug in the bramble and proceed upwards to the orifice. The figure 1 therefore denotes the first-born of the series, the oldest in date; the highest figure denotes the last-born. The letter M, placed under the corresponding figure, represents the male and the letter F the female sex.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 F F M F M F M M F F F F M F M
This is the longest series that I have ever been able to procure. It is also complete, inasmuch as it comprises the entire laying of the Osmia. My statement requires explaining, otherwise it would seem impossible to know whether a mother whose acts one has not watched, nay more, whom one has never seen, has or has not finished laying her eggs. The bramble-stump under consideration leaves a free space of nearly four inches above the continuous string of cocoons. Beyond it, at the actual orifice, is the terminal stopper, the thick plug which closes the entrance to the gallery. In this empty portion of the tunnel there is ample accommodation for numerous cocoons. The fact that the mother has not made use of it proves that her ovaries were exhausted; for it is exceedingly unlikely that she has abandoned first-rate lodgings to go laboriously digging a new gallery elsewhere and there continue her laying.
You may say that, if the unoccupied space marks the end of the laying, nothing tells us that the beginning is actually at the bottom of the cul-de-sac, at the other end of the tunnel. You may also say that the laying is done in shifts, separated by intervals of rest. The space left empty in the channel would mean that one of these shifts was finished and not that there were no more eggs ripe for hatching. In answer to these very plausible explanations, I will say that, the sum of my observations--and they have been extremely numerous--is that the total number of eggs laid not only by the Osmiae but by a host of other Bees fluctuates round about fifteen.
Besides, when we consider that the active life of these insects lasts hardly a month; when we remember that this period of activity is disturbed by dark, rainy or very windy days, during which all work is suspended; when lastly we ascertain, as I have done ad nauseam in the case of the Three-horned Osmia, the time required for building and victualling a cell, it becomes obvious that the total laying must be kept within narrow bounds and that the mother has no time to lose if she wishes to get fifteen cells satisfactorily built in three or four weeks interrupted by compulsory rests. I shall give some facts later which will dispel your doubts, if any remain.
I assume, therefore, that a number of eggs bordering on fifteen represents the entire family of an Osmia, as it does of many other Bees.
Let us consult some other complete series. Here are two:
1 2 3 4 5 6 7 8 9 10 11 12 13 F F M F M F M F F F F M F F M F F F M F F M F M
In both cases, the laying is taken as complete, for the same reasons as above.
We will end with some series that appear to me incomplete, in view of the small number of cells and the absence of any free space above the pile of cocoons:
1 2 3 4 5 6 7 8 M M F M M M M M M M F M F M M M F M F F M M M M M F M F F F F M M M F M
These examples are more than sufficient. It is quite evident that the distribution of the sexes is not governed by any rule. All that I can say on consulting the whole of my notes, which contain a good many instances of complete layings--most of them, unfortunately, spoilt
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