50 In this stage tlie larvæ have ahvays preserved their power of movement. Some of the mites were loosened from their hold and brought into a jar with waterplants, others into a jar with ^vater but without piants; the larvæ were all able to swim but fastened themselves when occasion was given them as soon as possible on waterplants. Those from 16 VIII had already 20 'VIII broken out of the sixlegged larva-skin and now swam as nymphs round in the water; the old skin is fastened to the plant and with a lens it can be observed that they are fastened by means of the rostrum. Also those larvæ which have had no occasion to fasten themselves to waterplants are able of encystment ; but as far as I have seen many of them never give nymphs being covered with Mucoraceæ. It seems as if the Zijgopteridæ most probably are infested in July; from the first day in September I have never found Z;/(yo/;- teridcv with larvæ. I have in the foregoing pages tried to gather all what we know at the present time with regard to the metamorphosis of the Hy- drachnids. Here and there I have been able to add some new^ facts to what was hitherto known. At the first glance it seems Strange enough that our knowledge of the metamorphosis of these charming little creatures is so trifling as it really is. By a closer consideration this will be easily understood as these studies claim so much intense reflection, a great outer apparatus, and unlimited time for explorations. The difficulty is not only to keep the mites and their larvæ alive; the most troublesome is that the explorer also must keep the hosts of the larvæ alive. It is necessary to hibernate Dytiscs and Hydrocores in aquaria and keep Odonata, Perlidæ, Culicidæ and Chironomidæ living for weeks and often for months. Every entomologis! knows that this is a very difficult thing. It is necessary constantly to be in contact with nature itself and at every moment change the basis of observation from nature to the laboratory or vice versa ; try to catch hold of a thread here and pursue it further there. The explorations may be carried on over more than one year. The first year the investigator nearly always
51 comes too late to get the cornmencement of the observed facts. Because we never understood how difficult these observations really are it is also natura! enough that our kno^^iedge in this field is so restricted as it is. — In this difficulty I beg the reader to see the best apology for the publication of this little paper. Lasily I ^vish to bring my assistent Mag. L. Pedersen my heartiest thanks for his good help with regard to the determination of some of the Hydrachnids and for a systematical survey of the paper. The same thank I beg to render to Mr. Engelhart, pre- sident of the Entomological society of Copenhagen for his kind help with regard to the language. Mainly because I regard this little paper only as a prelimi- nary one for a much more extensive paper dealing with the whole Danish Fauna of Hydrachnids their anatomy and biology being under preparation from this laboratory I have not acompanied my paper with illustrations hoping that they will find a place in the main publication. Freshwater-biological Laborator\% Hillerød 20 III 1918. List of litterature. 1824. Audouin, J. V. Note sur une nouvelle espéce d'Achlysie. Ann. des sci. nat. 2. p. 497. 1885. Ball, T. Drawings and Notes of Larval Form of Hydrachnid found on Dytiscus marginalis. Journ. of Mier. and Nat. Sci. 1827. Baer, C. E. v. Beitråge zur Kenntnis der niederen Tiere. Nova Acta. phys. med. Acad. Caes. Leop.-Carol. Nat. p. 523. 1888-1889. Barrois, Th. Note sur la dispersion des Hydrachnides. Revue biol. du Nord de la France. 1. 1848. Van Beneden. P. Y. Recherches sur l'histoire naturelle et le dé- veloppement de VAtax ypsiliphora. Mém. de l'Acad. Roy. des Sci. de Belgique. 24. p. 1. 1869. Bess el s, E. Bemerkungen iiber die in unseren Najaden schma- rotzenden Atax-kntn. Jahresb. f. vaterl. Naturk. Wiirtt. 25. p. 146.