8 f)hili(ia' from nature is almost incredible. The larvæ of Hijdro- philidiv are rather difficult to Hnd and apart from 2— ?> species all are very small ; they are little known and most of them re- garded as rare. The „Wasserkaferlarven" can by Piers i g really only signify larvæ of Dytiscs; on the other hånd from Blunck's and my own observations 1 am inclined to maintain that these larvæ as carriers of larvæ of mites in nature itself only play a very in- conspicuous role. I suppose that Piersig has only been interested in getting larvæ and nymphs for description ; he has then got the Hydrach- nids to lay eggs in aquaria, has hatched the larvæ and for their further development furnished them with the most casual material of aquatic insects he could get. The unhappy larvæ have made a virtue of necessity and really developed into nymphs. But from this result Piersig is by no means permitted to present the mat- ter as if the larvæ of Hydrachnids in nature are parasitic upon all aquatic insects indiscriminately and in this respect by no means are tied. Perez besides (1904, p. 263) has maintained quite a similar opinion Piersig goes so far that on p. 17 he indicates Krendowsky's correct observation that the larva of .4r/7?
used for my studies, being infested with larvæ oF mites. During the preparation of my papers relating to the biology of these in- sects I very often found remarks relating to larvæ of mites; as far as I know these numerous scattered observations have been almost unknown to Hydrachnologists. In the foUowing pages I wil) try to gather what, for a period of abt. 10 years, I have cited from this litterature. It seems that the larvæ of Hydrachnids live by no means in- discriminately as parasites upon the various aérial insect groups. Firstly in one or another way the insects must have connection with water. Larvæ of Hydrachnids have never been found upon Lepidoptera. Hymenoplera and truly terrestrial Coleoptera. But also several of those insects which live as larvæ in fresh water seem only exceptionally to carry larvæ of mites. This is the case with Ephemeridæ and Phryc/anidæ. As mentioned later on it is rather probable that the many moults as larva and the peculiar subimago stage prevent the Hydrachnids from using the Epheme- ridæ as hosts for their larvæ; still Soar indicates that he has found them on Ephemera larvæ. (1901, p. 66). On the imagines of Trichoptera Musselius (1914, p. 66) has seen them and Ulmer (1912, p. 117) has, peculiarly enough, found them on Trichoptera in amber. They are certainly by no means common upon Trichop- tera; neither Sil tal a nor Ulmer mention them from recent msects; and I have never seen them myself. Otherwise they are said to be very common on the large foreign Perlidæ (see p. 37), well known from Odonaia (see p.46) and from Diptera (see p. 23 and 35) especially Xemocera. From the litterature it seems as if they only rarely occur upon Brachycera ; this is according to my view not correct; it is a very common thing to find red pearshaped bodies on flies living near ponds and lakes; only they have never been studied. Of the Xemocera they occur especially upon Ciili- cidæ and Chironomidæ. These are mentioned by Bruyant(1900 p. 132). Dyé (1905 p. 5) and Sergents (1904, p. 110). Further by Mankowski (1905, p. 277). Blanchard (1905, p. 134). Giles (1902, p. 151) but especially by Howard, Dyar and Knab (1912, p. 172— 175). In recent years the Hydrachnids as tormentors of the swarms of gnats have occupied naturalists a good deal, as some of them have hoped in them to find allies in the