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 materials 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 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 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. [Illustration: Stone Nest of Caddis-Worm.] [Illustration: Sand Nest balanced with a Stone.] [Illustration: Nest of Caddis-Worm balanced with Straws.] It is worthy of remark, that the cement, used in all these cases, is superior to pozzolana[BN] 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.