Poor fool! Your victim went out under your nose and you never saw it. The worm has sunk to the bottom and taken refuge in the mysteries of the rock work. If things were happening in the large expanse of a pond, it is clear that, with their system of expeditious removals, most of the lodgers would escape scot-free. Fleeing to a distance and recovering from the sharp alarm, they would build themselves a new scabbard and all would be over until the next attack, which would be baffled afresh by the selfsame trick. In my narrow trough, things take a more tragic turn. When the sheaths are done for, when the caddis worms that are too slow in making off have been eaten up, the Water beetles return to the rockery at the bottom. Here, sooner or later, there are lamentable happenings. The naked fugitives are discovered and, succulent morsels that they are, are forthwith torn to pieces and devoured. Within twenty-four hours, not one of my band of caddis worms is left alive. In order to continue my studies, I had to lodge the water beetles elsewhere. Under natural conditions, the caddis worm has its persecutors, the most formidable of whom appears to be the Water beetle. When we consider that, to thwart the brigand's attacks, it has invented the idea of quitting its scabbard with all speed, its tactics are certainly most appropriate; but, in that case, an exceptional condition becomes obligatory, namely, the capacity for recommencing the work. This most unusual gift of recommencing it possesses in a high measure. I am ready to see its origin in the persecutions of the Dytiscus and other pirates. Necessity is the mother of industry. Certain caddis worms, of the Sericostoma and Leptocerus species, clothe themselves in grains of sand and do not leave the bed of the stream. On a clear bottom, swept by the current, they walk about from one bank of verdure to the other and do not think of coming to the surface to float and sail in the sunlight. The collectors of sticks and shells are more highly privileged. They can remain on the level of the water indefinitely, with no other support than their skiff, can rest in unsubmersible flotillas and can even shift their place by working the rudder. To what do they owe this privilege? Are we to look upon the bundle of sticks as a sort of raft whose density is less than that of the water? Can the shells, which are always empty and able to contain a few bubbles of air in their spiral, he floats? Can the big joists, which break in so ugly a fashion the none too great regularity of the work, serve to buoy up the over-heavy raft? In short, is the caddis worm versed in the laws of equilibrium and does it choose its pieces, now lighter and now heavier as the case may be, so as to constitute a whole that is capable of floating? The following facts are a refutation of any such hydrostatic calculations in the animal.