Videnskabelige meddelelser

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Videnskabelige meddelelser

26

Geer (1783) regards them also as eggs but is surprised that an

organism in the eggstage is able to suck food and grow at the

expense of the host. E. v. Baér (1827, p. 590) is the first who

shows that the pearshaped bodies are larvæ; but as late as 1834

Burmeister (p. 138) maintains the older view. Audouin (1824,

p. 497) regards the parasites as a separate genus of Hydrachnids

which he describes as Achlysia.

In the same year (1824) Duges publishes his fundamental

paper on Hydrachnids; on the pages 165—171 he gives the most

thorough description of the development of one of the Hydrachnids

hitherto published. He is the first to point out that Hydrachno

glnhohis Hermann lays its eggs in the spongy tissue of water-

plants {Polninogeton). The female pierces a hole by means of the

rostrum and pushes the eggs into it. These eggs lack the protect-

ing layer, which Hydrachnid eggs commonly have. The egglaying

takes place in May and six weeks later the larvæ are hatched.

These larvæ are described and Duges shows that, transferred

to aquatic insects {Dytiscidæ, Ranatra, Hydrophilus piceus) thev

are transformed into nymphs here; the posterior part of the body

is prolonged, feet and palps are drawn in. but the capitulum is

constantly in connection with the host, and through the mouth the

larva gets its nourishment from the tissues of the insects. The

larva hibernates upon the host; later on it is possible to see the

eight legs of the nymph through the skin of the larva; at last the

nymph leaves the sack and swims away. After having lived for

some weeks it hangs itseif up upon the leaves of water piants,

piercing the rostrum into the plant, attaching itseif also by means

of the palpi ; then it passes a new nymph stage from which the

fullgrown Hydrachna finally hatches.

It is from this really classic picture that all the later Hydrach-

nologists have drawn their information. As far as I know no one

after Duges has examined the development in nature and he is

the only one who has seen the egglaying process; still it is pos-

sible that Piersig also has done so (1900, p. 459). Remarkably

enough the lucid description of Duges has been doubted by

Koenike (1895, p 230 and later on 1896. p. 242). He maintains

that the females lay their eggs directly on the water insects. What

he advances as arguments for his supposition is of no scientific