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Schaum, H., on the elytra of Dytiscus and Hydroporus. Scherzer and Schwarz, measurements of savages. Schelver, on dragon-flies. Schiodte, on the stridulation of Heterocerus. Schlegel, F. von, on the complexity of the languages of uncivilised peoples. Schlegel, Prof., on Tanysiptera. Schleicher, Prof, on the origin of language.
The greatest care must be taken not to introduce, or allow to intrude, any water beetles or the larger carnivorous aquatic larvæ of insects, into the rearing boxes. I have known cases where the larvæ of the Dytiscus marginalis, the largest of our carnivorous water beetles, have destroyed almost all the fry in a rearing pond.
The other will feed not only on dead animal substances, but on living creatures, and is equipped with sharp teeth, which work like a saw. One of these kept in an aquarium fastened on to and slowly devoured a small frog confined in the same vessel. The large dytiscus beetle is the great enemy of small fish.
They live as parasites on various water insects, such as Dytiscus, Nepa and Hydrometra, and when mature live free in the water, though Von Baer observed an adult Hydrachna concharum living parasitically on the gills of the fresh-water mussel, Anodon. The species are of minute size.
We have here a curious and inexplicable case of dimorphism, for some of the females of four European species of Dytiscus, and of certain species of Hydroporus, have their elytra smooth; and no intermediate gradations between the sulcated or punctured, and the quite smooth elytra have been observed. See Dr. H. Schaum, as quoted in the 'Zoologist, vols. v.-vi. 1847-48, p. 1896.
While this ferocious enucleation continues with the object of reaching the dainty morsel contained within, the caddis worm, close pressed, appears at the mouth of the sheath, slips out and quickly decamps under the eyes of the Dytiscus, who appears to notice nothing. I have said before that the trade of killing can dispense with intelligence.
The great carnivorous water-beetle, the dytiscus, after catching and eating other creatures all day, with two-minute intervals to come up, poke the tips of its wings out of the water and jam some air against its spiracles, before descending once more to its subaqueous hunting-grounds, will rise by night from the surface of the Thames, lift again those horny wing-cases, unfold a broad and beautiful pair of gauzy wings, and whirl off on a visit of love and adventure to some distant pond, on to which it descends like a bullet from the air above.
This illustration is taken from a photograph of a specimen lent to me by Mr. F. M. Halford, and both the fish and the larva were alive when they were caught. Unfortunately the trout is a little shrivelled, and the legs of the Dytiscus have been broken. D. marginalis lays its eggs in the stems of rushes. The larva, when hatched, makes its way out, and proceeds to lead a predatory life.
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.
One of these was put into a bowl with a dytiscus beetle, which, "pouncing upon him like a hawk upon an unsuspecting lark, drove its scythe-like horny jaws right into the back of the poor little fish.
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