The inner wall of the case

James L. Drummond, Letters to a Young Naturalist on the Study of Nature and Natural Theology, Londres, Longman, 1832, p. 37-38.

The generic name of the perfect insect is Phryganea ; and the larvae are well know from their practice of forming cases of extraneous bodies, in which they enclose and protect themselves. You have often in streamlets and pools, observed bits of straw as it were, or fragments of wood moving at the bottom : these are the cases I allude to…/.. I may remark that we have examples of other animal being partly enveloped in a strong crust, or coat of mail, while the rest of the body is covered merely by a soft skin ; such are the Hermit-crabs- one species of which, cancer Bernhardus, or Bernard the Hermit, is common on many of our shores, and is vulgularly considered as a young lobster. To protect the naked part of his body, he takes possession of some empty shell, and retreat into it, tail foremost, and there is secure from the attacks of ennemies which otherwise might annoy or destroy him. As the crab, grows in size, his house, of course, becomes too small, and he has then to search for a larger ; which being found he leaves the old, and takes possession of the new dwelling, till he is again obliged to make another flitting to gain further accommodation.
The larva of the phryganea has a different mode of proceeding. One might suppose it next to an impossibility that an animal resident in water could spin a thread, and that a strong one, or that it could form for itself an envelope not unlike a mantle of silk ; yet such is the case : and there are animals living in water which can spin threads more durable and strong perhaps than is done by any on land, the silkworm not excepted. You know the common muscle ; it spins threads of remarkable strength,- and why ?- that it may moor itself for the rocks and other substances, and thereby save itself from being cast ashore by the violence of the waves ; and of these cables the animal can spin many hundreds, though it proportions their number to the risk of shipwreck to which it many be exposed. ../… To return to our larva of the phryganea ; it also is a spinner, but for a somewhat different purpose. The muscle moors itself to the rock, but the larva carries the bodies to which it is attached along with it. The naked and unprotected creature forms a case, or habitation, in which it lies as in a citadel, secure from the generality of its ennemies. The inner wall of the case is composed of a substance like silk ; but it must be evident that this of itself would be but a weak defence, and the larva is not of a similar way of thinking with the currier in the besieged town, who insisted that there was nothing for the defence of a city like leather. It attaches to the silken wall an outer battery, either of stone-work, or grass, or dead straws or rushes, or small shell, &c. ; and thus there are nearly as many varieties of the case as there are species of the insect.
In the generality of the structures formed by insects either in the larva of perfect state the greatest exactness and regularity are conspicuous but in the present instance this is not so ; and yet the circumstance is any thing but an imperfection. On the contrary, indeed it evinces, like every thing else, an inscrutable and all-powerful wisdom. The caddis-worm is specifically heavier than water, and therefore, in constructing its case, it is of the utmost importance that it be made neither too light nor to heavy : if the former, it would prove troublesome and inconvenient for the animal to drag along. It has the skill, therefore, to form the case of the exact degree of buoyancy necessary : should it be too heavy, it attaches to it a piece of straw, or wood, or other light substance, to give the necessary levity ; and if too light, it gives the due quantum of ballast by glueing on a stone or shell. Now, this being the fact, we may see at once that the building of the caddis worm must necessity be irregularity is connected with an admirable instinct. The animal knows nothing of physics, it knows nothing of gravitation why a straw should float, and the unerring certainty with which the end wished for is accomplished, we are brought to the inevitable and delightful conclusion, that this is the work of God.
The great object in forming the case is to procure a defence and covering, and the lightness or weight of this we may observe to vary according to situation : thus the caddis worm which inhabit stream generally form the house of gravel, else it would be carried down by the current ; while those found in ditches, ponds, and other still waters, are of lighter substances, and very near the specific gravity of the water. You may find many more particulars respecting these larvae than it is necessary for me to advert to here ; for my object is not to exhaust the subjects on which I write, but rather to stimulate you to enquire for yourself. One more circumstance then, and well shall go to something else.