The Distribution and Growth of Prosimulium Larvae (Diptera: Simuliidae) in Hill Streams in Northern England

The present work describes certain features of the altitudinal distribution and growth of the larvae of Prosimulium hirtipes Fries and P. infiatum Davies in hill streams in northern England. An explanation of the facts is presented, but it is recognized that since the information obtained is incomplete it must be regarded as no more than a working hypothesis. The annual cycle of P. hirtipes (sens. lat.) is already well known. O'Kane (1926), working in New Hampshire, U.S.A., showed that larvae appeared in streams in the autumn and became fully grown by early spring when pupation occurred, adults emerging in April-May. Since eggs laid by these adults in May-June did not hatch until the autumn the species is apparently univoltine. Later workers (Twinn 1936, D. M. Davies 1950, Rubtzov 1956) have confirmed these observations. However, Rothfels (1956) has since shown that P. hirtipes in eastern Canada consists of at least two sibling species and L. Davies (1957) has produced anatomical evidence that the northern European P. hirtipes Fries as re-defined by Edwards (1915) and Puri (1925) is specifically distinct from the north American forms hitherto grouped under this name. The insect dealt with in this paper is the northern European form as found breeding in hill streams in Britain. Larvae of this species have been considered to be cold-water stenotherms, an idea based on their restriction to the colder months and to streams supposed to be relatively cold (Rubtzov 1940, Dorier & Freychet 1946, Grenier 1949). To test this hypothesis laboratory experiments were carried out on the effects of higher temperatures, and field observations were made on their seasonal development and on water temperatures in their natural environment.