John Barlow Burton, Lectures on Entomology, Londres, Simpkin & Marshall, 1837.
There is a very interesting class of Grubs which live under the 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 larvae of four-winged flies, we may mention sandstones, 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. Another employs pieces of reed cut into convenient lengths, 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 than the rest to shade his doorway overhead, so that he may not be seen from above. A more laborious structure is reared by the Grub 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. Another of these aquatic architects makes choice of the tiny shells of young fresh water muscles 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 prisoners, and drags them without mercy along with him. These grotto buildings are by no means uncommon in ponds and in chalk districts, such as the country about Woolwich and Gravesend. 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 a crow quill, and equally smooth and uniform. Now, the material 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 insects, by patiently examining their stones, and turning them round on every side, never fail to accomplish their plan. This, however, is only part of the problem which is complicated with another condition, namely, that the 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 swept 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 finished, a large stone is added by way of ballast. In other instances, when the material is found to possess too great specific gravity, a bit of light wood or a hollow straw is added to buoy up the case. It is worthy of remark, that the cement used in all these cases is superior to pozzolana (a cement prepared of volcanic earth or lava), in standing water, which is indissoluble. The Grubs themselves are also admirably adapted for their mode of life, the portion of their bodies, which is always enclosed in a case, being soft like a Meal Worm or 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, says Rennie, 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 the 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. The main intention is, however, to have abundance of materials within their 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 for use.