Different life history strategies in lake-outlet blackflies (Diptera: Simuliidae)

The life histories of Simulium noelleri Fried. and S. truncatum Ldstr. were studied at a lake-outlet site in Finland. A detailed study was made of the second summer generation of both species and this consisted of a single, synchronized cohort in S. noelleri, and overlapped cohorts in S. truncatum. Larvae of the two species also show differences in their habitat preference: those of S. noelleri predominate on the sluice which marks the beginning of the river, and S. truncatum larvae just downstream from the sluice. In winter, the latter site was dominated by immature individuals of two other species, S. erythrocephalum de Geer and S. equinum L., and these were found downstream from the two ‘lake-outlet’ species during the summer. Adult females of S. noelleri are able to mature a first batch of eggs without taking a blood meal but those of S. truncatum need to feed on blood before eggs can be fully developed. The consequences of these different reproductive strategies on the ecology of individuals of the two species is discussed.