in Life of

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Notes on the Life Cycle of Rhyacophila nubila ^(Zett.) (Trichoptera) in a North ^Swedish River'

UI,F KARLSTROM

I)epartment of Ecological Zoology, University of Umei, 5-901 87 Umefl, Sweden

Abstract

Klnr,srn0n, U. Notes on the life cycle of Rhgacophila nubila (Zett.\ (Trichoptera) in ^a

Nortlr ^Swedish river. Ent. Tidskr. 97:39-42,

1976.

The larval development of Rhgacophila nu'

bila ^(Zett.) was followed 1973-75 in a small

river in northern Sweden. The species has 5 in- stars in the river. The pattern of larval devel-

opment is very complex. No definite statemenl can be made, but it is assumed that R. nubila

is univoltine with two ^groups emerging at ^dif- ferent times. No clear separation exists between the groups, hence the flight period is protracted and all larval stages can be found simultane- ously during the summer.

Introduction

One of the dominant ^features of running rvaters is the continuous transport of fine particulate organic matter ^(FPOM). ^This ^con- stitutes the food base for several important bottom invertebrates, e.g. simuliids and net-

spinning trichopterans. One successful pre- dator being able to exploit the important food-chain FPOM-Simuliidae is the ^genus

Rhgacophila, Trichoptera (Schwoerbel 1971, Ezenwa 1974, Karlstrdm unpubl.). N{any spe- cies of Rhgacophila are also abundant in fast- flowing streams (Percival & Whitehead 1929, Schwoerbel op. cit., Thut 1969).

The genus Rhgacophila is large. In Europa there are at least 72 species (Botosaneanu 1967) and in North America about 100 spe-

cies (Wiggins ¹⁹⁶⁶⁾ . In northern Sweden 3 species occur, with R. nubila (Zett.) as the

dominating one. Despite its importance the lifecycle of R. nubila has not yetbeenworked out (Ulfstrand 1968). The species has a pro- tracted flight-period, extending up to 4 months

(Tobias 1969, Giithberg 1970, 1973).

Other species within the genus Rhgacophila have also been found to have a complex life- cycle (Thut 1969, Hynes 1970:295).

The aim of this ^paper is to contribute ^to the understanding of the lifecycle of R. nubila.

Investigation area

The river Ricklein, 48 km long, has its rise in Iake Bygdetrisket (29 kms) and enters the Both- nian Bay approximately 50 km north of Umei (64o5'N) . It is a small river, with stones and boulders in the rapids, surrounded mainly by forests. Further information can be found ⁱⁿ Karlstr<im (1973), Giithberg & Karlstriim (1975).

Methods

The material was taken with a semi-quanti- tative bottom fauna sampling device, using colo- nization baskets filled with stones from the river (Karlstriim 1974 a, b) . The samples were screen-

ed through a net with a mesh-size of 250 pm.

Head capsule measurements were made with an adjustable eyepiece micrometer.

Results and discussion

In the river R. ^nubila ^appears ^to ^have ⁵ ^in- stars, easily separated (Fig. 1). This ^pheno- menon has also been reported for other spe- cies of Rhgacophila (Thut 1969).

During the summer all instars can be found and the instar ^frequency distribution shows

I _Report _from _the _Ricklei _Field Station No 65.

E″ ι .rじ asλ ″ .ク・ 1976・ 1-2

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40 ULF KARLSTROM

ENelH DF HEAD CApsuLE:ヽ uNIT5

/f ｀

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n349

30 WIDTH Oヽ HEAD CApsuLE IN UNlTS

'I UNtT.O.04mn

Fig' 1. Length and width of ^the head capsule of. Rhgacophila nubila (Zett.). The material is from Storlingforsen rapids, Rickleln river Aug. 18, 1973.

ヽ１

、

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no distinct peak. In the late autumn the distribution pattern changes giving a peak of small instars (Fig. 2). A successive change

is brought about during the winter, and in the spring larger instars dominate. No eggs Ent. Tidskr.97 ^. 1976' ¹ -2

seem to hatch during the winter, and, thus,

it ^appears that each autumn a cohort starts to grow and in the early summer this cohort concludes its larval development.

The instar frequency distribution shon's

ヽ、、

、、

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／ ′ ノ

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generally an unilnodal pattern. During the weeks 29--361973,there is a slight tendency towards a bilnodal pattern.But it is not pOs―

sible to state definitely whether the species has one or two generations per year. It is, however, not likely that a large species like R.nabfrα would be able to develop a second generation during the sunllner. In the lite―

rature the univoltine lifecycle seems to be the donlinating One u′ ithin the Rぬ yacο

ρ

^hflα

・

Ulfstrand(1968)assumed that R.凛

“

bビ

ra

has a two‐ year cycle in the riverヽアindelalven.

Part of his data suppOrt the assumptiOn,but he only occasionally found l st instar larvae.

Ulfstrand used the total body length tO char―

acterize the larval stage. This method is, however,nOt without lilnitations,which Ulf―

strand also adnlits. The data fronl the river Ricklean hardly pernlit a two‐ year cycle.

Thut(1969)repOrted that fOr R.

υθ

rrura

ヽ

lilne all instars were found at practically every sampling date and that the emergence pattern

、

vas diffuse. He believes the species to be univOltine.

It is reasonable to assume that the pro‐

tracted flight‐ periOd of R.nIIbfrα is accompa―

nied with cOpulation, ovipositiOn and hatch‐

ing Of eggs during the whole vegetatiOn periOd. Individuals emerging early in the

summer will give rise to next year's first spe―

cimens. These are fonowed later in the sum‐

mer by a second, more numerous group,

giving birth tO the autumnal peak of l st in―

star larvae.But there is nO definite separation

bet、 veen the two grOups,

、Ⅳ

hich results in a 10ng, unbrOken flight_periOd and silnultan‐

eous occurrence of larvae of allinstars during the summer.Smith(1968)also fOund that a species of R■ IJacο

ρ■ビ^Iα

had two peaks of

emergence,one in early spring,and the other in early fall.He assumed that the two groups were of the same generation.

Acknowledgements

l want to acknowledge the skilful assistance of Karin Bystr6m in the laboratory work.I also

RHYACOPHILA NUBILA IN A NORTH Sヽ VEDISH RIVER ₄₁

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YR WEEK

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n=1刀

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目ヨ n.51

義 n,・

串 n=ロ

還

n.聡

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目 __ n・ 255

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Fig. 2. Instar frequency distribution of Rhgaco-

phila nubikt (Zett.) from Storlingforsen rapids, RickleAn river 1973-75. Frequencies are ^given in ^per cent.

1973

引

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1師

Eπ ι .r,asヵ″

97

ゴ ,76・ ′ -2

聡

柁

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42 ULF KARLSTROM

appreciate the discussions with coneagues in the

Ricklea group, and especially suggestiOns from Anders Gёthberg have been valuable.This study was done within the frame of the river Ricklein project, supported by the Swedish Natural Sci‐

ence Research Council(NFR).

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en presenta―

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¹⁰⁸

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^―一

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――

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_一

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