The Hydrachnellae of the Oudemans collection

On several occasions the Hydrachnellae of the Oudemans Collection have been the subject of a publication. In 1898 Oudemans himself published a "List of Dutch Acari, 6th part, Hydrachnellae Latr., 1802" in which nineteen indigenous species were enumerated. Afterwards Romijn (1916) dealt with 'Oudemans' Hydracarina", thereby increasing the number of Dutch species to 26, although he paid attention to the adult mites only. Finally there appeared a catalogue of the Oudemans Collection, prepared by Buitendijk (1945), in which the Hydrachnellae are listed on pp. 343-347. Because Buitendijk (1945) considered her catalogue a preliminary one, it is evident that reliable conclusions on the material were still to be awaited. This justifies the appearance of the present paper which contains a more definite report on the Hydrachnellae of the collection. It has been impossible to prepare a complete and satisfactory comparison of the above-mentioned lists. Several slides referred to in the cited papers are no more in existence, whilst Oudemans provided some of the remaining slides with new labels (it is unknown whether this happened before Romijn published his list, or afterwards). A number of Romijn's corrections are, however, still present on the labels, as mentioned below. With the exception of some striking and altogether certain facts, I shall generally refer only to the preliminary catalogue (Buitendijk, 1945). The present paper is to be regarded as a supplement to Buitendijk's Catalogue. The number of species proved to be much larger than was originally expected. A great number of slides of larvae is still present. The original drawings, mentioned below up till now all have remained unpublished. When studying the collection it became evident that after 1900