A COMPARISON OF THE LIFE CYCLES OF THE SLUGS DEROCERAS RETICULATUM (MULLER) AND ARION INTERMEDIUS NORMAND ON PERMANENT PASTURE

Life cycles of the slugs Deroceras reticulatum and Arion intermediuswere compared over a four-year period on permanent pasture nearOvingham, Northumberland. Both species were semelparous, theirlife cycles taking about one year. D. reticulatum, unlike A.intermedius and other species, had two overlapping generationsthat laid their eggs in late spring and autumn respectivelyand also had one instead of two immature phases in its lifecycle. The pattern and rates of growth for the immature phasesin the life cycle of A. intermedius differed markedly from thosedescribed under laboratory conditions. Growth in the infantilephase was delayed in the field by low winter temperatures whilesecond stage growth was usually delayed in late spring by dryconditions. The life cycle of A. intermedius appears to be synchronisedby seasonal changes in photoperiod, unlike that of D. reticulatum.Growth in D. reticulatum continued throughout the winter monthsexcept under exceptionally cold conditions and was not usuallydelayed by dry conditions in spring. The size of hermaphroditegland relative to body weight in D. reticulatum reached a maximumin mature-unmated slugs and then became progressively smalleras the slug approached the post-reproductive stage which wasrelatively brief. Exceptionally dry conditions in 1962 delayedthe development of the hermaphrodite gland, the maximum sizereached was significantly reduced and fewer eggs were laid. (Received 17 September 1987; accepted 2 January 1988)