DEPARTMENT OF NATURAL HISTORY.

INSECT ARCHITECTURE-CADDIS-WORMS.

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 caddisworms, and to naturalists as the larva of the four-winged flies in the order Trichoptera, 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. Another employs pieces of

Leaf Nest of Caddis-Worm.

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 than

Reed Nest of Caddis-Worm.

the rest to shade his door-way over-head, 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. The following figure from Roesel will give a more precise notion of this structure than a lengthened description.

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 prisoners, and drags them without

Shell Nest of Caddis-Worm.

mercy along with him. These grotto-building grubs are by no means uncommon in ponds; and in chalk districts.

One of the most surprising instances of their skill occurs in the structures of which small stones are the prin cipal 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 materials being small stones full of angles and irregularities, the

Stone Nest of Caddis-Worm.

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 of the problem, which is complicated with another condition, and which we have not found recorded by former observers, 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

Sand Nest balanced with a Stone.

added by way of 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.

Nest of Caddis-Worm balanced with Straws.

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 in

* A cement prepared of volcanic earth or lava. VOL. II.

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habitants 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 materi als 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.

VEGETABLE SUBSTANCES-THE CORK TREE.

The Cork Oak is not so large a tree as the common oak. There are several varieties: a broad leaved and a narrow leaved, which are evergreens; beside other varieties which shed their leaves. The broad-leaved evergreen is, however, the most common, and it is the one from which the cork of commerce is chiefly obtained. It is mentioned by Theophrastus, Pliny, and some other ancient naturalists as being well known in the days of the Greeks and Romans, the latter of whom used it for a variety of purposes, and among the rest for the stopping of bottles. They used it for floats to their nets and fishing tackle; for buoys to their anchors; and when Camillus was sent to the Capitol, through the Tiber, during the siege by the Gauls, he had a life-preserver of cork under his dress.

The Cork Oak is indigenous, or at least abundant, in Portugal, Spain, part of the south of France, and Italy; on the opposite coast of the Mediterranean, and the Levant. Spain and Portugal supply the greater portion of the cork which is consumed in Europe. The cork

is the bark which the tree pushes outwards, as is common to all trees; but here the outer bark is of a larger quantity, and is more speedily renewed. When removed, there is a liber, or inner bark, below it, and from this the cork is re-produced in the course of a few years,while the tree is said to live longer, and grow more vigorously, than if the cork were not removed. The

first time that the cork is taken off, is when the tree is about fifteen years old. That crop is thin, hard, full of fissures, and consequently of little value; and the se cond, which is removed about ten years after, is also of an inferior quality. After this, the operation is repeated once in eight or ten years, the produce being greater in quantity, and superior in quality, each successive time. According to Duhamel, a cork tree thus barked will live a hundred and fifty years. The months of July and Augus are those which are chosen for removing the cork.