540 
Kap. III. 
English summary. 
Although only a limited territory of Jutland has been examined 
I am of opinion that the species of Simulium observed can be 
divided into two biological groups: river-forms and cold-water- 
forms. The results from Haslum and Rebbild — two lo- 
calities far distant from each other — seem to confirm this. 
The number of generation varies in the different species. Tem¬ 
perature and locality are in that respect important factors. 
The river-forms (rivulet-forms): 5. argyreatum, S. equinum and 
5*. ornatum are found in jeveral generations in our larger slow- 
flowing rivulets and brooks; they are at the same time our most 
dangerous Simulium. The higher the temperature, the shorter the 
duration of the egg-, larva- atid pupa-phases. 
But 5. ornatum is peculiar in also appearing in cold springs and 
in forest brooks. 
It is a transition-form between the rheophile and the limnophile 
fauna, and while in rivulets and large brooks it is rich in individuals 
with several generations, and appears dangerous, it is in cold water 
destitute of individuals and has only one or two generations. 
As regards the cold-water-forms: 5*. latipes and 5*. costatum, cir- 
cumstances are somewhat different. Here we generally find two 
generations (spring and autumn), sometimes only one generation, 
but the Imago-stage is certainly of much greater duration than 
that of the river-forms. 
All the Simulium, examined by me, winter in the larva-stage 
with stagnation of growth during the coldest time. 
Several species live in the same water-area, and larvaø and 
pupæ of two or more species are found mixed with each other on 
one and the same stone or plant. The eggs of the river-forms are 
laid on the surface on submerse plants, most frequently on the under 
side of the leaves — rarely deeper. Also on stalks of grass, 
from the bank dipping into the running water, eggs can be found. 
Most eggs are envelopped in a mass of jelly, which is, however, 
more or less dense. The egg stage is comparatively short, probably 
about one week. 
The cold-water-forms oviposit on the bottom of the brook under 
