The reeds invaded are laid horizontally, a condition on which the Bees likewise insist, if only to shelter from the rain the house-door, plugged with pervious materials, such as mud, cotton, or round, leafy disks. Their inner diameter attains an average of two-fifths of an inch. The length occupied by the cells varies greatly. Sometimes the Odynerus takes possession only of that fragment of the interval between two knots which the stroke of the pruning-knife has left free, a fragment longer or shorter according to the chances of the cutting. In that case, a small number of cells is enough to fill the available space. But generally, if the stump be too short and not worth the trouble of working, the insect bores through the partition at the end and thus adds a complete internode to the vestibule with the open entrance. In a lodging of this kind, some eight inches long, the number of chambers will amount to fourteen or fifteen.
In thus enlarging the house by removing a floor, the Odynerus displays two separate talents, the plasterer’s and the carpenter’s. [[188]]Her knack for wood-working, moreover, is extremely useful in another circumstance, as we shall see. The Three-horned Osmia, also an enthusiastic partitioner of reeds, does not employ this means of obtaining a spacious lodging at small cost. I find that she always leaves the first party-wall intact, building the row of cells against it, however short the section may be. To make an opening in a slight barrier is not one of her methods. She could do it if she wished; for to gnaw through the ceiling of the cell on hatching and then through the general door of the nest is a more difficult job. She possesses in her mandibles a tool powerful enough for the purpose; but she is not aware that a splendid gallery lies beyond the obstacle. How did the Odynerus learn, if she did not know from the beginning, what the Osmia, with her greater experience of the reed, does not know?
Apart from the ingenious device of breaking down the party-wall in order to enlarge the premises, the Odynerus is the Osmia’s equal as a plasterer and partition-builder. The results of the two industries resemble each other so closely that we should easily confuse them if we merely examined the structure. We find in both [[189]]cases, at irregular intervals, the same partitions, the same round disks of fine earth, of mud gathered wet on the brink of an irrigation-ditch or stream. Judging from the appearance of the materials, I imagine that the Odynerus has fetched her clay from the banks of the neighbouring torrent, the Aygues.
Identity of construction is maintained even in details which I had at first regarded as a feat peculiar to the Osmia. Let us recall her compartment-building secret. If the reed be of middling diameter, the cell is first stocked with provisions and next bounded in front with a partition run up then and there, without any pause in its construction. If the reed, without being excessively wide, be of a certain thickness, the Osmia, before stowing away the victuals, gets to work on the front partition, providing it with an opening at the side, a sort of service-hatch, through which the honey is more easily discharged and the egg more easily placed in position. Well, this secret of the service-hatch, which was revealed to me by the glass tube, is as well-known to the Odynerus as to the Osmia. She, too, in the bigger reeds, finds it to her advantage to close the larder in front before [[190]]bringing the game; she shuts the cell with a door provided with a sort of wicket, through which the victualling and the laying are done. When everything is finished inside, a plug of mortar closes the hatch.
I did not of course see the Odynerus working at her partition with its wicket-door, as I saw the Osmia performing in my glass tubes; but the work itself speaks quite plainly of the method followed. In the centre of the partitions in the medium reeds there is nothing in particular to be seen; in the centre of the partitions in the larger reeds there is a circular aperture, afterwards filled with a plug, which always differs from the rest of the partition by projecting inwards and sometimes differs in colour. The thing is obvious: the small partitions are made in one spell, whereas the work on the larger ones is interrupted and then resumed.
As we see, it would be pretty difficult to distinguish the Odynerus’ nest from the Osmia’s, if our enquiries were confined to the cells. One characteristic, however, and not the least curious enables an attentive eye to tell the owner without opening the reed. The Osmia closes her dwelling with a thick plug of earth similar in nature to that employed [[191]]for the partitions. The Odynerus, it goes without saying, does not neglect this means of defence: she, too, makes a solid stopper; but to the unsophisticated method of the Osmia she adds the resources of a more highly-finished art. Over her earthen stopper, a thing liable to be spoilt by frost and damp, she spreads, on the outside, a good thick layer of a composition of clay and chopped-up woody fibres. It matches the red wax with which we seal the corks of our bottles.
These fibres, which resemble the remains of a coarse tow retted by long exposure to the air, I should be inclined to look upon as taken from reeds spoilt by the rain and bleached by the sun. The Odynerus planes them off in shavings, which she afterwards crumbles by chewing them. This is how the Common Wasps and the Polistes work on soft dead wood, when gathering the raw material for their brown paper. But the reed-dweller, who has no intention of employing her scrapings for paper-making, does not cut up these fibrous particles anything like so finely. She contents herself with breaking them up and unravelling them a little. Mixed with thick mud, the same as that of the partitions and the final plug, [[192]]they make an excellent loam, which is far less liable to go to pieces than unmixed clay would be. The efficacy of this ingenious stucco is evident. After some months of exposure to the inclemencies of the weather, the Osmia’s door, made of earth only, is very much dilapidated, whereas the Odynerus’ door, covered on the outside with a layer of fibrous composition, remains intact. Let us credit the Odynerus with inventing and patenting the loam covering and proceed.
After the nest, the victuals. One sort of game alone is served to the Odynerus’ family: this is the larva of the Poplar Leaf-beetle (Chrysomela populi, Lina p.), a larva which, in company with the adult insect, ravages the poplar-leaves at the end of spring. Consulted merely by our taste, the Odynerus’ game is anything but enticing in shape and still less in smell. It is a plump, thickset grub, with a bare, flesh-white skin covered with several lines of glossy black dots. The abdomen, in particular, has thirteen rows of these black spots, namely, four on the top, three on each side and three underneath. The four dorsal rows vary in structure: the two in the middle consist of plain black specks; those [[193]]on either side consist of little pimples, each shaped like a truncated cone with a minute opening at the top. One of these cones rises on the right and left of each abdominal segment, except the last two; there is also one on the right and one on the left of the metathorax and mesothorax. These two are larger than the others. There are nine pairs of perforated pimples in all.
If we tease the creature, we see welling up from the bottom of these several little craters an opalescent liquid, which runs and spreads all over the larva. It has a strong smell of bitter almonds, or rather of nitrobenzene, commonly known as essence of mirbane, a powerful and most repulsive smell. The discharge of this substance is a means of defence. We have only to tickle the insect with a straw or to grip one of its legs with the tweezers and the eighteen scent-bottles at once begin to work. Whoso handles the grub will find his fingers stink and will throw away the noisome perfumer in disgust. If the Chrysomela-larva’s object in placing nine pairs of nitrobenzene-stills on its back was to repel man, it has, I admit, thoroughly succeeded.
But man is the least of its enemies. Far more formidable is the Odynerus, who [[194]]seizes the scented creature by the skin of the neck and, despite its sprays of perfume, dispatches it with a few stings. This was the bandit against whom, above all, it should have defended itself; and the poor grub has not been happily inspired in this respect. Considering the huntress’ exclusive taste for this sort of game, we must presume that the Chrysomela’s drug-shop possesses a delicious aroma in the Odynerus’ opinion. The defensive secretion becomes a deadly bait. Even so with other means of protection: each advantage invariably has some corresponding disadvantage.