Robert MacLachlan, « On caddis-worms and caddis-flies », West Kent Natural Hist. Microscopical and Photogra. Soc., 1882, p. 41-45.
So soon as the larvae are hatched, they comence to make houses for themselves, in which they pass the rest of their aquatic existence. These houses may be mainly divided into two classes. Fistly, those which are portable, and only become fixed when the larva is abot to assume the pupal condition. Secondly, those which are constantly fixed to stones or other substances.
In the first classe, the form and conditions are very varied. Speaking generally, these houses (or cases) consist of an inner silken tube, to the exterior of which variuos extraneous substances are attached. Some species (Phryanea, Neuronia) form perfectly cylindrical straight tubes of morsels of leaves and other vegetable substances, beautifully aranged in a spiral manner (these are not uncommon in the ponds of our district), and of nearly equal diameter throughout ; and the inmate is able to present its head at either end indiscriminately by turning itself in its case. Of course, it is apparent that the case of a young larva must be much smaller and narrower than that of a full-grown one. Now, with us, when we outgrow our clothes, the tailor is usually applied to for a fresh suit, and the old ones are discarded ; but with Caddis-worms it is ordinarily different ; the creature does not come out of its case and thus expose itself to danger while manufacturing a new house, but it ingeniously cuts off from time to time the end portion that has become too small, and goes on adding simultancously to the other end. This will apply to all the tubular case. A very common form (Anabolia) still occuring bundantly in our litle river Ravensbourne, is a tube of fragments of gravel and sand, and as the larva lives in running water, and might be in danger of being swept away by the current, there are attached to this tube long fragments of small twigs which serve probably as balancers. Some of the most interesting forms are those in which the exterior of the case is ornamented with shells (often containing their living inmates), seeds of the smaller species of the same group. These are mostly constructed by species of Limnophilus, and the same species wil often use a variety of materials according to the consitions in which the larvae are placed. Sometimes comparatively large shells are used, sometimesmyriads of minute shells to the number of several hundred, beautifully arranged in mosaic are wholly employed sometimes wholly the seeds of some particular water plant, sometimes portions of the case wholly of one material and the other portion of another , and finaly, all these substances, combined with sand, gravel, and vegetable matters, are sometimes arranged indiscriminately on one and the same case. These shell cases are especially interesting to the Conchologist, because by collecting such cases he may occasionally procure species of fresh-water shells that he has searched for in vain ; the Caddis-worm is a better collector of seeds may play in part in the distribution of water plants hitherto unsuspected. Caddis-worms no doubt form part of the food of many aquatic birds, as also of those of the snipe family, &. ; it would be impossible for these to pick the larvae out of their cases, and these latter are commonly swallowed with the inmates ; the hard seeds and other substances would probably be passed in an undigested condition by the birds, when perhaps many miles from the locality Where they were swallowed ; hence plants, may be thus distributed. This suggestion requires working up from actual observation, but at present I think there is some force in it. A very common form of theses tubular cases is that in which stoney fragments or sand is exclusively usued ; thse are usually curved and taper a good deal to the tail-end, resembling shells of the genus Dentalium in form. Such cases differing only in size, amount of curvature, coarseness or fineness of materials, &., are constructed by a vast number of different species and even of genera. Very rarely (Setods,&.) the case is formed wholly of hardened silky secretion, with no covering of extraneous matters. One of the most wonderful of British forms of case is that constructed by the genus Molanna. It may be found in parts of the Ravensbourne, where the bottom is sandy, and in ponds on Hayes Common, &. These cases are wholly made of fine sand ; internally there is the usual tube, but the sides are broadly expanded, sot hat the form is long-oval, the upper side slightly convex, the lower slightly concave ; the larva lives on the sandy bottom, from which the case is scarcely distinguishable unless the inmate move, and an additional peculiarity is that the upper or convex side is produced at the mouth-end far beyond the lower side, so as to form a covering for the l arva when feeding at the sandy bottom ; A still more remarkable instance occurs in a small case from Ceylon in which the actual case is merely an ordinary straight tube, but to the mouth-ends is affixed a circular shield, concave beneath, and equally protecting the inmate when its anterior segments are extended in search of food. Some cases (Goëra, &.) are nearly oblong in shape, consisting of a tube formed of fine gravel, to the sides of which are fixed, much larger, stoney fragments ; such are common in the Ravensbourne. Very extraordinary cases are those of Helicopspyche , spread over nearly the whole word, but not occuring in Britain. These are for all the world like small spiral shells formed of sand or gravel, and such is their deceptive ressemblance to shells that they were originally described as such, and even a new genus (Thelidonus) was formed for their reception by Conchologists. Their true nature has long been know, but it is only recently that the perfect insects of the species that construct them have been satisfactorily determined.
Were I to only briefly allude to the many marvellous forms that exist ? I should have to occupy your attention for several evenngs. Hitherte I have alluded to Caddis-worm as peculiarly aquatic.