Insect Architecture

by James Rennie

A represents a part of an espalier prop, tunnelled in several places by the violet Carpenter-Bee: the stick is split, and shows the nests and passages by which they are approached. B, a portion of the prop, half the natural size. C, a piece of thin stick, pierced by the Carpenter-Bee, and split, to show the nests. D, perspective view of one of the partitions. E, Carpenter-Bee (Xylocopa violacea). F, Teeth of the Carpenter-Bee, greatly magnified; a, the upper side; b, the lower side.

The tunnel in the wood, however, is only one part of the work; for the little architect has afterwards to divide the whole into cells, somewhat less than an inch in depth. It is necessary, for the proper growth of her progeny, that each should be separated from the other, and be provided with adequate food. She knows, most exactly, the quantity of food which each grub will require during its growth; and she therefore does not hesitate to cut it off from any additional supply. In constructing her cells, she does not employ clay, like the bee which we have mentioned above, but the sawdust, if we may call it so, which she has collected in gnawing out the gallery. It would not, therefore, have suited her design to scatter this about, as our carpenter-bee did. The violet-bee, on the contrary, collects her gnawings into a little store-heap for future use, at a short distance from her nest. She proceeds thus:—At the bottom of her excavation she deposits an egg, and over it fills a space nearly an inch high with the pollen of flowers, made into a paste with honey. She then covers this over with a ceiling composed of cemented sawdust, which also serves for the floor of the next chamber above it. For this purpose she cements round a wall a ring of wood-chips taken from her store-heap; and within this ring forms another, gradually contracting the diameter till she has constructed a circular plate, about the thickness of a crown-piece, and of considerable hardness. This plate of course exhibits concentric circles, somewhat similar to the annual circles in the cross section of a tree. In the same manner she proceeds till she has completed ten or twelve cells; and then she closes the main entrance with a barrier of similar materials.

Let us compare the progress of this little joiner with a human artisan—one who has been long practised in his trade, and has the most perfect and complicated tools for his assistance. The bee has learnt nothing by practice; she makes her nest but once in her life, but it is then as complete and finished as if she had made a thousand. She has no pattern before her—but the Architect of all things has impressed a plan upon her mind, which she can realize without scale or compasses. Her two sharp teeth are the only tools with which she is provided for her laborious work; and yet she bores a tunnel, twelve times the length of her own body, with greater ease than the workman who bores into the earth for water, with his apparatus of augurs adapted to every soil. Her tunnel is clean and regular; she leaves no chips at the bottom, for she is provident of her materials. Further, she has an exquisite piece of joinery to perform when her ruder labour is accomplished. The patient bee works her rings from the circumference to the centre, and she produces a shelf, united with such care with her natural glue, that a number of fragments are as solid as one piece.

The violet carpenter-bee, as may be expected, occupies several weeks in these complicated labours; and during that period she is gradually depositing her eggs, each of which is successively to become a grub, a pupa, and a perfect bee. It is obvious, therefore, as she does not lay all her eggs in the same place—as each is separated from the other by a laborious process—that the egg which is first laid will be the earliest hatched; and that the first perfect insect, being older than its fellows in the same tunnel, will strive to make its escape sooner, and so on of the rest. The careful mother provides for this contingency. She makes a lateral opening at the bottom of the cells; for the teeth of the young bees would not be strong enough to pierce the outer wood, though they can remove the cemented rings of sawdust in the interior. Réaumur observed these holes, in several cases; and he further noticed another external opening opposite to the middle cell, which he supposed was formed, in the first instance, to shorten the distance for the removal of the fragments of wood in the lower half of the building.

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That bees of similar habits, if not the same species as the violet-bee, are indigenous to this country, is proved by Grew, who mentions, in his ‘Rarities of Gresham College,’ having found a series of such cells in the middle of the pith of an elder branch, in which they were placed lengthwise, one after another, with a thin boundary between each. As he does not, however, tell us that he was acquainted with the insect which constructed these, it might as probably be allied to the Ceratina albilabris, of which Spinola has given so interesting an account in the ‘Annales du Muséum d’Histoire Naturelle’ (x. 236). This noble and learned naturalist tells us, that one evening he perceived a female ceratina alight on the branch of a bramble, partly withered, and of which the extremity had been broken; and, after resting a moment, suddenly disappear. On detaching the branch, he found that it was perforated, and that the insect was in the very act of excavating a nidus for her eggs. He forthwith gathered a bundle of branches, both of the bramble and the wild-rose, similarly perforated, and took them home to examine them at leisure. Upon inspection, he found that the nests were furnished like those of the same tribe, with balls of pollen kneaded with honey, as a provision for the grubs.

The female ceratina selects a branch of the bramble or wild-rose which has been accidentally broken, and digs into the pith only, leaving the wood and bark untouched. Her mandibles, indeed, are not adapted for gnawing wood; and, accordingly, he found instances in which she could not finish her nest in branches of the wild-rose, where the pith was not of sufficient diameter.

The insect usually makes her perforation a foot in depth, and divides this into eight, nine, or even twelve cells, each about five lines long, and separated by partitions formed by the gnawings of the pith, cemented by honey, or some similar glutinous fluid, much in the same manner with the Xylocopa violacea, which we have already described.

[This species is probably Ceratina cærulea, as the second species, C. albilabris, seems to have little claim to be considered as a British insect. It is plentiful in spots where it resides, but is very local. It can best be found by collecting all the specimens of bramble branches that have holes bored into the pith.