The life cycle and some other biological details of the fresh-water snail Physa fontinalis (L.) (continued)

The life cycle and some other biological details of the fresh-water pulmonate Physa fontinalis (L.) were studied in the field during some years. The main locality was at Botshol, a polder about 15 km south of Amsterdam. There, as well as on some other spots, two generations of Physa per annum occurred: a spring generation from early in May till the middle of August and a wintering generation from the close of June until the end of May of the next year. The principal differences between the two generations are compiled in the table on the next page. The beginning of oviposition by the wintering generation is determined by external factors, mainly the temperature. The time of oviposition by the spring generation depends on internal factors, viz. sexual maturity. It has been established that there is a significant positive correlation between the number of eggs per capsule and the size of the animals. No clear preference for particular substrates was observed. Abnormal eggs were rarely present in the field; eggs lacking an egg-cell were only produced by the wintering generation, and presumably may be looked upon as a sign of decay. All the adult animals of both generations die after the breeding season. Nearly all eggs hatch, but among the very young snails mortality is high, at least 90%. In the winter months, when the temperature is 6° C. or lower for 4—5 months, growth practically ceases. The presence of eggs of Haliplidae in the egg-capsules in spring was considered to be fortuitous. Some of them could be identified as Haliplus ruficollis (de Geer). Possible stimuli for the oviposition under natural conditions are discussed.