W. Furneaux, Life in Ponds and Streams, Londres, Longmans, Green & Co., 1904, pp. 288-294.
If we touch one of the insects, or pick it up by holding the case between the finger and thumb, the protruding segments are immediately retracted within the tube, which now appears quite as devoid of life as when first we saw it. The insects referred to are the larvae of Caddis Flies ; and we gather a number of them, selecting as many different kinds of cases as we can, and place them in boxes with scraps of pondweeds that we may be able to observer them in confinement.The require plenty of room in confinement, especially the larger ones ; and they should have a liberal supply of soil and vegetation, for they are mostly vegetable feeders, either devouring fresh green plants, or the decaying vegetable matter that is constantly accumulating on the bed.
They climb well on the submerged vegetation ; and, considering the size of some of the cases, and the heavy materials, such as stones and shells, of which they are wholly or partly composed, they seem to get along remarkable well. We must remember, however, that objects weigh less in water than they do out of it- that objects, when submerged, lose a weight equal to that of their own volume of water, and therefore the work peformed by a Caddis larva is not so great as it appears to be.
Let us nowexamine the different kinds of cases made by these larvae. Here is one, small, conical, and slightly curved (fig. 235, b), very regular in form, though not perfectly smooth, and reminding us of the marine shell know as the « elephant’s tusk ». On examination with a lens, we find that it is composed entirely of little grains of sand, cemented together with great regularity, and note with surprise that the larva selected grains of a size, just as if it were much concerned about the regular and finished appearance of its domicile. The case is open at both ends, as are all the others ; but the larger end only will admit of the protrusion of the inmate’s head and limbs.
It will be interesting now the examine the case internally ; but in order to do this we must first remove the larva. It may be supposed that this is easily accomplished by waiting till the front segments are exposed, and then quickly seizing the creature and withdrawing it by a slight pull. But this will not do ; for although te larva is not attached to its case as a snail is to its shell, yet the last segment of the abdomen is provided a pair of hooks, the free ends of which are directed forwards, and the larva holds on so effectually by means of these that i twill often allow itself to be torn in two rather than submi to forcible eviction.
But there is a safer, less cruel, and far more effectual way of taking possession of the home. Take a very fine stem, or a stiff bristle, and attack the tenant from the rear. I must admit, that this is a very inglorious procedure, but it answers our present purpose ; and the larva, not being acquainted with this form of attack, hastily quits its tube.
We will not concern ourselves with the peculiariries of the larva itself, but, for the present, examine the case. On splitting it open, we find that it is perfectly smooth within, for it lined with a substance that looks like a very thin and delicate paper. It is really a silken lining, spun by the larva just in the same way as ceratin caterpillars spin their cocoons ; It leaves the body in a fluid condition, and, while soft, is of a glutinous nature, so that foreign substances are easily attached to it. It also hardens under water, thus making the case rigid and strong.
It will of course occur to the reader that one of two alternatives is necessarily open the larva as its body increases in size. Either it must quit the case now too small to contain it, and construct another of larger size ; or it must enlarge the old one. There are a few species that adopt the former alternative, but the majority of them ressort to the latter. They add to the larger end as occasion requires, and even cut off the superfluous portion at the small end, which is now too small to accommodate the terminal segments of the abdomen.
But, it may be asked can the larva perform the latter feat without coming completely out of its home ? Such a procedure would be extremely dangerous to the insert. Caddis larvae are deemed a luxury by fishes and a host of other carnivorous aquatics, as anglers know well, these larvae being in great request as bait ; and it would not be long before the unprotected grub would be devored by some hungry foe.
They answer is this. The case, although not very spacious within, is generally large enough to allow the insect to double and reverse its body, and so the work may be accomplished from within, the creature being perfectly protected as it perform its task.
It may be mentionned here that each of the different species of Caddises constructs a case after its own peculiar style, and generally shows a decided preference for certain materials of construction.
This being the case, it will naturally occur to the reader that each species must have a preference for a particular kind of locality where the necessary materials are to be found. Thus, the case just described, which is constructed of particles of fine sand, is made by a larva ( Sericostoma multiguttatum) that invariably inhabits streams the currents of which are sufficiently strong to prevent the formation of a deposit of mud, and yet not sufficiently rapid currents, in which fine sand or light debris could not accumulate, and these utilise the fragments of a stony or gravelly bed for the construction of their habitation, and even sometimes attach their cases to large stones that they themselves may withstand the force of the stream.
But even in rapid streams there is generally a quantity of rooted vegetation, which is cut up by the mandibles of certain Caddis larvae, and cemented together to form a case. Thus, a specie called Halesus auricollis make its case of fragments of vegetable matter, weighted with stones at the « tail end » and often fixes it to a large stone when the current is a rapid one.
It will be inferred from the above remarks that the nature of the case remains pretty constant for each species. So much is this so, indeed, that many species may be readily identified by a cursory inspection of their habitations.
Some species of the family Phryganeidae make use of pieces of stick or stem, which they cut up into suitable lengths, and arrange them either spirally, or in longer pieces running parallel from end to end. Their cases are often smaller at one end than at the other while the larva is young, but of equal size at both ends at a later stage.
The several species of the genus Limnophilus employ a great variety of materials. Some use sticks and stems, some cut up dead leaves, while others use sand, small stones, and the shells of aquatic moluscs, and even, at times, the elytra of dead water beetles.
Few caddis cases are more interesting than those formed whelly or partially of aquatic shells ; and it is astonishing how easily quite a collection those shells can be made by the aid of the larvae. I have met with ponds where hundred of botom shells, such as those of the genera Sphoerium Pisidium, could be taken in a few minutes by simply sweeping the Caddis larvae from the weeds, and without the slightest occasion for disturbing the mud in which these molluscs lie. And the larvae do not confine themselves to the empty shells of dead molluscs. In fact, on some occasions the cases are made up almost entirely of tenanted shells, the molluscs having to subsist as best they can while being dragged about from place to place by the grub that has appropriated their shells to its own use.
Live species of Planorbis are often captured in the same way by Caddises, and it is often amusing to watch the procedure of the molluscs as they attempt to follow their own inclinations. The molluscs will make an atempt to crawl away at a time when the larva is at rest, but it is not stong enough to move the heavy case ; and just when the snail has succeeded in getting a foothold, and attempts to go its own way, the larva decides on an opposite course , and the poor snail is torn away from the object to which it had attached itself.
In addition to the shells already named, the maller Limnaea, the freshwater limpets (Ancylus), and species of Buthynia and Valvata are all appropriated by Caddis larvae for their cases.
The shells slected are almost invariably fixed with their flattest side turned inwards, and the same applied to the stones and other objects used in the construction of the cases.
Among the other materials employed for this purpose may be mentioned pieces of reeds and rushes, cut into suitable lengths ; the seeds of rushes and other plants ; and also dead leaves. The last named are sometimes cut up into small pieces and arranged spirally, or in longer pieces placed side by side, or are seven used entire…./…
In the first place it may be noticed that a larva, deprived of its home and placed in a glass of water, will frequently wave its abdomen in the manner described, even though there may be no necessity for it underthe circumstances. But this is not all, for it is possible to compel the creature to use transparent substances for the construction of the case, so that the motions of its body may be witnessed under more natural conditions. Turn a larva out of its tube, and place it in a vessel of water containing no solid substances except what you have yousel introduced for building material, and it will soon commence constructing a tube of them.
Many very interesting results may thus be obtained, though I must say that I Have not found all species equally ready to build under circumstances so unnatural to them. The commoner species of Phryganea which inhabit ponds and ditches almost everywhere, and some of the sand-builders of our ponds and rivers are very willing workers under such conditions, and in the course of a few hours will construct cases of almost anything with which they are provided.
If our objects is to obtain amusement only, very pretty and striking results may be produced by supplying the larvae with sands or small glass beads of different colours ; and if they are transferred from one vessel to another during the operation, they may be made to complete their cases with substances of a different nature from those with which they started. But to return to the transparent cases to which we have aboved referred. These I have obtained as follow :
The larvae, after removal from their cases, are placed in a vessel of clean water without any sediment. They are then provided with very thin plates of glass, such as may be easily obtained by heating the end of a piece of glass tubing in a blowpipe flame, blowing out a thin bulb, and then breaking it into little pieces of about an eighth of an inch or more in length. The pieces should not be too small, or the case produced will be too opaque to allow a clear view of the larvae within. In fact, the larger they are the better, providing they are not too cumbersome for the insects to manipulate.
With cases produced in this way the larvae may be watched with case, and the undulations of the abdomen by means of which the water currents are maintained for breathing purpose rendered visible.