42 In small lakes and ponds also another .-l/aa"-species occurs which I have hitherto supposed also to be Å.crassipes but which most prob- ably is A. figiiralis Koch; more thorough observations are wanting. It is rather peculiar that the Hydrachnids have been able to give some contingent to the pelagic region of the lakes and that this contingent has been restricted to one or perhaps two species of the same genus. The knowledge of the pelagic mode of life of Atax crassipes is perhaps older than 1 have indicated in earlier papers. Already Haller (1882, p. 76) says that the species has „eine nåchtliche, pelagische Lebensweise". According to an indication by Haller, Paves i is supposed to have been the first who has observed the pelagic manner of living. Asper has made the same observation in the lakes of Switzerland. The knowledge of the phenomenon goes perhaps back to a much earlier time. C la- paré de (1868, p. 471) says namely that „Alax crassipes im fliessenden Wasser bei Genf sehr haufig ist " Much later Atax crassipes was regarded as a member of the pelagic society by Seligo and myself (1904, p. 203); as such it is now reported from very many lakes under different latitudes and of greatly vary- ing size. None of the planctologists have had any knowledge of these older Communications by Haller and Claparéde. On the other hånd some of the Hydrachnologists have fully misapprehended the mode of living of the mite. Thon (1901. p. 128) says that it is common in the littoral region in November it is said to seek to the bottom where it hibernates. As it is without swimming brushes the mite is only able to creep on the bottom. „Seine Be- wegungen sind langsam. schaukeind, der Korper erhebt sich an den hohen Fiissen wie bei den Opilionen." Musselius (1914, p. 70) says: „Er wird sehr schnell mijde und sinkt zu Boden, wo er langsam mit nach vom gerichteien Palpen und erstes Bein- par umherkriecht." These statements show clearly how much we are able to misunderstand the morphology and biology of a living animal when it is not studied where it lives in nature itself. The want of luxuriant swimming organs is in concordance with so many other pelagic animals; the relatively powerful legbows with the large thorns is the floating apparatus of the animal which diminishes the falling velocity and has the effect that, when it only now and then in a peculiar idie manner moves the legs, it can keep in the same layer of water.
43 In deeper lakes it seems that its home is the middle layers of water; by means of nets with wide meshes I have from 20 to 25 m in Furesø got a peculiar plancton consisting of Bythotrephes longimanns, Leptodora hijalina, Atax crassipes and some larvæ of Corethra pliimicornis. It is most common over the submerged meadows of Characeæ, Elodea a. a. in the large bay Store Kalven in Furesø. Further investigations may show whether a periodicity can be pointed out. It seems as if there are seasons when all the mites caught are nymphs, but more thorough studies are desirable. The metamorphosis of the mite is very remarkable. As stated above Koenike (1890, p. 138) showed that the nymph which Claparéde (1868, p. 471) supposed to be that of Atax crassipes really belonged to a new species ^4. aciilealus Koen. Wolcott (1899, p. 209) and Kelly (1899, p. 414) mention Atax crassipes as parasite in mussels. From Europe a long series of observations shows that A. cras- sipes at all events can accomplish its metamorphosis in Spongilla colonies. Twenty five years ago I found in Teglgaardsø, Hillerød, some fine colonies of Spongilla, in which numerous black mites were parasites. They were determined as Alax crassipes. They were found in all stages as sixlegged larvæ, in the first pupa stage, as newly hatched nymphs and in the second pupa stage. The co- lonies were brought to the zoological museum at Copenhagen. In the following years I have cut hundreds of Spongilla-colonies through, but I never again saw a single colony with mites. Piersig (1900, p. 56) indicates that Alax crassipes in all probability in the larva stages is parasitic in Spongilla-colonies but is free swimming as adult. So ar (1906, p. 365) reports that he in England (Sutton Broad) found A. crassipes in great numbers in Spongilla and „Die figures a piece with mites. In : Koenike (1909, p. 96) mentions : Suszwasserfauna Deutschlands" „Imagines und Nymphe freilebend, die Larve bei Spongilla schmarotzend". Micoletzky (1912, p. 42!) reports that he found Atax crassipes in Spongilla lacuslris in Niedertrummersee in Salzburg; they were present partly in young stages, partly as imagines. Musselius (1914, p. 63) communicates that 16 VIII he found a Spongilla lacnstris with numerous eggs from which the sixlegged larval stage of Alax cras- sipes was hatched a few days later.