NORSK ENTOMOLOGISK TIDSSKRIFT - Norsk entomologisk forening

116 Albert Lillehammer
number. The flow of water in these tributaries decreases during
the summer months. They accordngly have an annual flow
resembling that in Storelva and Fl~gstadbekken and, therefore,
are more to be compared with these. A largish number of A.
sulcicollis was found in Storelva; according to Brinck, this is
also eucoenic in southern rivers.
The third of the species which dominate in the two rivers, is
L. fusca. According to Brinck, it is general in all watercourses
where it can develop in the course of the season.
Nymphs of the two Amphinemura species as well as the
Leuctra species obtain their principal sustenance from higher
plant and moss tissue, besides detritus (undetermined plant
matter) (Brinck 1949). The dominance of A. borealis and L.
fusca daI; therefore bh connected with a rich supply of foliage
remnants deposited on the dry areas in the Autumn where they
serve as food for Plecoptera when the same areas become flooded
during the summer. Along both the Suldalsligen and Storelva
there are thick foliiferous woods on the slopes down to the river
course. The decreasing water level and, consequently, the higher
temperature along Storelva during the summer months, can be
one of the reasons for the large number of the more southern
species A. sulcicollis and for the early emergence of A. borealis.
Fl~gstadbekken differs markedly from the two rivers in that
there is no marked dorninence by any species and that only one
species of the carnivorous forms, namely, I. grammatica, has
been found. The common A. sulcicollis, is eucoenic in southern
streams ; A. borealis, also common, is eucoenic in northern streams.
The marked contribution made by Plecoptera in the watercourses
investigated can for a large part be due to the specieal
temperature conditions found along the rivers of West Norway
where the snow-melted water has but a short way to the sea and,
therefore, hardly time to become warmed up so very much.
Accordingly, during the first half-year a low mean temperature
is found - something which is favourable for most Plecoptera.
As an example it may be mentioned that the average temperature
for Suldalsligen, first half year, 1. January - 30. June, was
3.5"C whereas in the second half year, 1. July-31. December
the average was 9.3"C, a difference of almost 6°C.
Because of possible sources of error while making the collections
(Lillehammer 1965) it can happen that some very rare
species of Plecoptera in watercourses, have not been included;
should this be the case, it would apply, most likely, to Flogstadbekken
and perhaps Storelva. I would assume, however, that the
possibility of this having happened is small both because the investigations
are all the year round and because the areas investigated
and the nymph material are very large.