A comparison of the life cycles of the slugs Deroceras reticulatum (Müller) and Arion intermedius normand on permanent pasture

Abstract

Life cycles of the slugs Deroceras reticulatum and Arion intermedius were compared over a four-year period on permanent pasture near Ovingham, Northumberland. Both species were semelparous, their life cycles taking about one year. D. reticulatum, unlike A. intermedius and other species, had two overlapping generations that laid their eggs in late spring and autumn respectively and also had one instead of two immature phases in its life cycle. The pattern and rates of growth for the immature phases in the life cycle of A. intermedius differed markedly from those described under laboratory conditions. Growth in the infantile phase was delayed in the field by low winter temperatures while second stage growth was usually delayed in late spring by dry conditions. The life cycle of A. intermedius appears to be synchronised by seasonal changes in photoperiod, unlike that of D. reticulatum. Growth in D. reticulatum continued throughout the winter months except under exceptionally cold conditions and was not usually delayed by dry conditions in spring. The size of hermaphrodite gland relative to body weight in D. reticulatum reached a maximum in mature-unmated slugs and then became progressively smaller as the slug approached the post-reproductive stage which was relatively brief. Exceptionally dry conditions in 1962 delayed the development of the hermaphrodite gland, the maximum size reached was significantly reduced and fewer eggs were laid. © 1989 The Malacological Society of London.