James Rennie, Insect Architecture, édition augmentée par J.G. Wood, Londres, Bell & Daldy, 1869, p. 202-207.
CADDIS-WORMS AND CARPENTER-CATERPILLARS
There is a very interesting class of grubs which live under water, where they construct for themselves moveable tents of various materials as their habits direct them, or as the substances they require can be conveniently procured. Among the materials used by these singular grubs, well-known to fishermen by the name of caddis-worms, and to naturalists as the lavae of the four-winged flies in the order of Trichoptera of Kirby and Spence, we may mention sand, stones, shells, wood, and leaves, which are skilfully joined and strongly cemented. One of these grubs forms a pretty case of leaves glued together longitudinally, but leaving an aperture sufficiently large for the inhabitant to put out its head and shoulders when it wishes to look about for food.
(Figure. Leaf Nest of Caddis-Worm pg 202)
Another employs pieces of reed cut into convenient lengths, or of grass, straw, wood, &c., carefully joining and cementing each piece to its fellow as the work proceeds; and he frequently finishes the whole by adding a broad piece longer
(Figure. Reed Nest of Caddis-Worm pg 202)
than the rest to shade his door-way overhead, so that he may not be seen from above. A more laborious structure is reared by the rub of a beautiful caddis-fly (Phryganea), which weaves together a group of the leaves of aquatic plants into a roundish ball, and in the interior of this forms a cell for its abode. The following figure from Roesel will give a more precise notion of this structure than a lengthened description.
(Figure. pg 203)
Another of these aquatic architects makes choice of the tiny shells of young fresh-water mussels and snails (Planorbis), to form a moveable grotto; and as these little shells are for the most part inhabited, he keeps the poor animals close
(Figure. Shell Nest of Caddis-Worms pg 203)
prisoners and drags them without mercy along with him. These grotto-building grubs are by no means uncommon in ponds; and in chalk districts, such as the country about Woolwich and Gravesend, they are very abundant.
One of the most surprising instances of their skill occurs in the structures of which small stones are the principal material. The problem is to make a tube about the width of the hollow of a wheat-straw or crow-quill, and equally smooth and uniform. Now the materials being small stones full of angles and irregularities, the difficulty of performing this problem will appear to be considerable, if not insurmountable: yet the little architects, by patiently examining their stones and turning them round on every side, never fail to accomplish their plans. This, however, is only part
(Figure. Stone Nest of Caddis-Worm pg 204)
of the problem, which is complicated with another condition, and which we have not found recorded by former observers, namely, that under-surface shall be flat and smooth, without any projecting angles which might impede its progress when dragged along the bottom of the rivulet where it resides. The selection of the stones, indeed, may be accounted for, from this species living in streams where, but for the weight of its house, it would to a certainty be wept away. For this purpose, it is probable that the grub makes choice of larger stones than it might otherwise want; and therefore also it is that we frequently find a case composed of very small stones and sand, to which, when nearly
(Figure. Sand Nest balanced with a Stone pg 204)
finished, a large stone is added by way of a ballast. In other instances, when the materials are found to possess too great specific gravity, a bit of light wood, or a hollow straw, is added to buoy up the case.
(Figure. Nest of Caddis-Worm balanced with Straws pg 204)
It is worthy of remark, that the cement, used in all these cases, is superior to pozzolana * in standing water, in which it is indissoluble. The grubs themselves are also admirably adapted for their mode of life, the portion of their bodies which is always enclosed in the case being soft like a meal-worm, or garden-caterpillar, while the head and shoulders, which are for the most part projected beyond the door-way in search of food, are firm, hard, and consequently less liable to injury than the protected portion, should it chance to be exposed.
We have repeatedly tried experiments with the inhabitants of those aquatic tents, to ascertain their mode of building. We have deprived them of their little houses, and furnished them with materials for constructing new ones, watching their proceedings from their laying the first stone or shell of the structure. They work at the commencement in a very clumsy manner, attaching a great number of chips to whatever materials may be within their reach with loose threads of silk, and many of these they never use at all in their perfect building. They act, indeed, much like an unskilful workman trying his hand before committing himself upon an intended work of difficult execution. Their main intention is, however, to have abundance of materials within reach: for after their dwelling is fairly begun, they shut themselves up in it, and do not again protrude more than half of their body to procure materials; and even when they have dragged a stone, a shell, or a chip of reed within building reach, they have often to reject it as unfit. (J.R.)
[We have here some examples of the latter kind of nest, i.e., those habitations which are made of stones and shells. Beginning at the upper left-hand figure, we find one that is made of moderately-sized stones cemented together in a way that reminds the observer of the manner in which a builder forms irregular stones into a wall. Next to it is another, in which the stones are larger and narrower, and are arranged much as some of the caddis-worms arrange pieces of stick and straw.
In the second, and on the left-hand side, is a very long and simple tube, made of a grass stem, and balanced by three little sticks attached to its centre. The next figure represents a number of sand-tubes attached to each other. These are built up laboriously of single particles of sand, and are remarkable for their peculiar horn-like shape, the tube having the same regular curve as the horn of an ox or antelope, and tapering gradually from the base to the top. A somewhat similar tube, but of larger size, is shown in the right-hand figure.
(Figure pg 206)
Any one who wishes to see one of these creatures rebuild its house can do so by carefully removing it from its tube, and supplying it with fresh material. Very great care must be taken in the removal, as the grub is easily damaged, and it holds so tightly to the tube with a pair of pincers at the end of its body, that I must rather be coaxed than driven out.
If desirable, they can be made to build their new houses of most singular materials. A lady, Miss Smee, was very successful in a series of experiments which she made with these insects, forcing them to make tubes of different colours and patterns, by supplying them with coloured sand, pieces of stained glass of various hues, gold dust, and similar materials. Although there was scarcely any material which they would not use, they seemed to consider a certain amount of angularity as essential, and rejected any object, such as a bead, of which the surface was perfectly rounded, while they would accept the same, if it were broken or indented.
When the caddis-grub has ceased from feeding, and is about to pass into the perfect stage, it spins over the mouth of the tube a strong silken web. This web is made in quite a pretty pattern, and being woven with rather wide meshes, it allows the water to flow through the tube while it prevents any aquatic foes from penetrating and destroying the pupa.
The remaining figures of the illustration represent tubes, around which are built a quantity of small shells. Generally, stones are mixed with the shells; but in some cases, shells seem to be almost the only material.