We may advantageously compare our Beetle with Peripatus, and note the points of agreement and of difference.
Now, if our captive Beetles are to yield us the greatest possible amount of profit, we shall keep them under observation for some time, so as to watch their habits.
In keeping these Beetles we shall not require a large aquarium. A small gathering of aquatic weed will be necessary to keep the water in good condition and the aquarium ready for its tenants.
My interest in these Beetles was quickened by a letter in the Field (Oct. 28, 1893), in which a correspondent at Weybridge asked ‘for information as to what animal or bird bisects so neatly the shells of the Water Snail (Planorbis).’ I thought then, and know now, that the shells were ‘bisected,’ if that is the proper word, by Water Beetles. From that time I have had, and still have, several living in small aquaria, but for a long time was unable to get direct evidence on the subject.
Fig. 16.—Shells of Molluscs broken up by Dytiscus.
(From a photograph by Cherry Kearton.)
Many experiments were tried, and at last these proved successful. Several specimens of Dytiscus[7] were obtained, and put into a small aquarium in which was no other food for them than some snails and other molluscs. The Beetles were carefully watched, and were several times seen trying the snails. In crawling along the inner surface of the glass, Planorbis and Limnaea both protrude the foot to a considerable extent, and pieces were ripped out by the strong mandibles of the Beetles before the shells were actually broken up.
All the shells represented in Fig. 16 were taken from this aquarium, so that there is good evidence as to what creatures broke them up and devoured their inmates. In these, as in the specimen kindly sent me by Mr. Tegetmeier, the Natural History Editor of the Field, the bisection is not complete, though in all cases it is carried far enough to allow of the extraction of the mollusc. The large Limnaea shell in the centre has been attacked, but it seems to have been left when the beetles discovered it was empty. (The empty shell was noted before the Beetles were put into the tank.) Another Limnaea shell is figured, from which the snail has been picked out, and that of a fresh-water mollusc.
After these observations had been recorded in the Field[8], I found that I had been anticipated by about forty years. I picked up, at a bookstall, a copy of G. B. Sowerby’s Popular History of the Aquarium, and there I found that the author had distinctly seen Dytiscus at this kind of work. He says[9]: ‘I have only once witnessed him in the act of seizing an unfortunate Planorbis or Flat-coiled Water Snail. At first, the Dytiscus seemed to be roaming about in quest of something, first under, then over, the leaves of a water-lily. At last, in a rather dark corner, he seemed to perceive suddenly a Planorbis which was browsing upon the stem of a plant just under the shade of a broad leaf. He darted at this, seized it, and then, putting his tail out of water, for the purpose of taking in a fresh supply of air, moved slowly down, bearing the snail with him. He held it by his fore-feet, turning round the coil until the aperture of the shell was opposite his mandibles, then he began nibbling away at the animal. In vain did the poor mollusc try to withdraw within its shelly fortress, for the beetle picked off the edges of the shell bit by bit, so as to expose the body as fast as it was withdrawn. All the way down to the bottom of the tank was this process continued, air-bubbles rising to the top, and bits of broken shell falling, till the beetle with his burden reached a stone near the bottom, where I left him still busy at his work.’