The Effect of Temperature on the Life-Cycle of Four Species of Lake-Dwelling Triclads

Abstract

Cocoon production, incuba- tion and development to maturation have been studied in Polycelis nigra, P. tenuis, Dugesia lugubris and Dendrocoelum lacteum at selected temperatures over a range of 1.5[degree] to 25[degree]C. D. lacteum can complete its life-cycle from 3.5 to 15[degree]C, D. lugubris from 10[degree] to 23 [degree]C and the Polycelis species from 3.5[degree] to 20[degree]C; P. nigra is rather more tolerant of cold and less tolerant of cold and less tolerant of heat than P. tenuis. Failure to breed at low temperatures was mainly due to retardation of the testes; both testes and ovaries were adversely affected at high temperatures. Other blocks to completion of the cycle also operated such as failure in copulation and inability to produce cocoons. These laboratory data agreed with the evidence of breeding in nature. The influence of temperature on the distribution of these species in nature is discussed.