Full-text - Norsk entomologisk forening

about two weeks before the peak for P. cillgllla­/IIS, and there is a considerable overlap in flightperiods (Fig. 8). A similar partitioning of flightperiods also occurs when P. la/ipennis and P.cingllla/us fly in the same habitats at Kongsvoll(Fig. 10). This temporal separation was constantduring the years 1973 - 75 for P. latipennis andP. cingula/us, and in 1979 - 81 for P. nigricornisand P. cingula/us.Since high frequencies of animal fragmentswere detected in the gut contents of larvae of P.cingula/us and P. nigricornis, these species mustbe regarded as omnivorous. Lillehammer (1978)reported plant fragments and detritus in the gutcontents of P. cingulatus from Ovre Heimdalsvatn,Jotunheimen. Siltala (I908) and Nielsen(I 942) mentioned plant fragments as food for P.nigricornis. Identifications of larvae of P. cingula/usand P. latipennis earlier than 1980-81must in many cases be regarded with caution,because the first reliable diagnostic charactersfor separating these two species appeared inWallace (1980). Larvae of P. latipennis should,according to the present investigation, be regardedas shredders. The very low percentage ofanimal fragments in their gut contents indicatesthat other invertebrates were not a major fooditem. This is not so for larvae of P. cingula/usand P. nigricornis, where other aquatic invertebratesoccur with such a high frequency in thegut coentents that they are bound to be major• food item. It should also be kept in mind thatthese frequency counts of animal fragments giveminimum frequences. The counts are mostly basedon sclerotized fragments, and if soft bodiedinvertebrates, e.g. oligochaets or soft portions ofaquatic invertebrates, are eaten, they will be difficult,if not impossible, to recognize in the gut\ contents.The differences found in the diet, between P.cingula/us and P. nigricornis on one hand, andP. latipennis on the other, indicate that care shouldbe taken not to state that all species in agenus have similar food habits. However, theview that the genus is a morphological and ecologicalunit, used by Wiggins and Mackay(I 978) as a general premise, may have a highervalidity in the central area of the geographicaldistribution of a genus, because more optimalconditions occur here than at the periphery. Inthe present case, high altitudes are the peripherialand thus marginal areas for Po/amophylaxspecies. A species that is opportunistic in foodhabits is better adapted to complete its life cycleabove the tree line than a shredder. The differencein food habits between P. cingula/lis, P.Iligricomis and P. la/ipennis may be a major factorregulating the distribution of Po/amophylaxspecies at high altitudes. above the tree line.During autumn and early winter at watertemperatures that are constantly close to ooe (