Videnskabelige meddelelser

Videnskabelige meddelelser


on the host but have not been able to get a hole in the chitin,

this being too hard for them at the point of attachment.

As mentioned above all my Dytiscs with larvæ from my aquaria

starved to death in November. During the winter I often caught

Dytiscs with larvæ of Hydrachnids. That the Hydrachna species

hibernate on the hosts is a matter of faet. In the months March-

April the size of the sacks is very different, some very large ones

protruding amongst many which are very small.

The most characteristic facts in the development of H. Willam-

soni is as foUows.

1. The larvæ are parasites probably upon the species of the

genus Dytiscus. 2. Many parts of the chitin are too hard for the

larvæ and most probably many of them starve out. 3. Though I have

examined abt. twenty Dytiscs with Hydrachna larvæ from Hjortesø

I have never found a single specimen with larvæ in the Elythral

room on the abdomen beneath the wingcovers, that spot from which

according to the common opinion we always hatch the nymphs of

N. geographica.

H. geographica O. F. M.

As stated above, the large Dytiscs are infested on the dorsal

side of the abdomen with some large unsymmetrical red sacks

from which Piersig (1900, p. 442) mentions that he has hatched

the nymph of H. geographica, the largest of the European water

niites. The mite is rare everywhere also in our country. In the

neighbourhood of Hillerød I have taken it 5— 6 times.

Remarkably enough I have during my study of the Dytiscidcv

ofien caught specimens with „geographica" sacks upon the abdomen;

I have had more than twenty such specimens; the number of

sacks upon each host is, when the sacks are large, usually no

more than three—four. Two or three is the most frequent number

but once I have found a specimen with 7 large nymphs. The

sacks are placed partly on the first abdominal segment, partly along

the sides in the soft skin. Dates on which I have taken Dytiscs

with sacks are the 21 /VI, 5 VII, 7 /VIII, 21 VIII, 25 VIII, 28/ VIII,

6/TX, 18/X, 20/XII. In some localities the sacks are by no means

rare, in others I have examined 20—30 specimens and not found

a single sack.

Often I have brought Dytiscs with sacks into my aquaria;

Vidensk. Medd. fra Dansk iiaturh. Foren. Rd. 70. 3

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