Growth and life-cycle of invertebrates from Danish springs

Abstract

1. The material primarily originates from a thorough study of springs in Himmerland (Jutland) an was collected during thirteen collections in the course of fourteen months. Measurements of the water temperature in the collecting localities were taken at the same time. Measurements of the animals were made with the help of an Edinger Projector. 2. Baetis rhodani has two generations yearly, a long winter generation and a short summer generation. It is possible that these are composed of two generations each. The growth has a stoppage in winter. 3. Brachyptera risi has only one generation per year, which, however, is accomplished in a very short time during winter. There is no marked growth-stoppage in winter. 4. Nemoura (Nemurella) picteti has one generation yearly, stretched over most of the year and with a distinct growth-stoppage in winter. 5. Leuctra hippopus has two generations per year, a longlasting winter generation and a short summer generation. It is not improbable that the material consists of two species, the winter generation being Leuctra hippopus and the summer generation Leuctra fusca. 6. Agapetus fuscipes has one generation yearly. In winter the growth is retarded, or there is perhaps even a stoppage. 7. It has not been possible to draw growth-curves for Crunoecia irrorata, but the histograms show that the species has one generation per year. It must be assumed that there is a growth-stoppage in winter. 8. Pericoma blandula (?) (determination uncertain). The species has one generation per year, with a long period in winter without evident growth. 9. Simulium ornatum has a biennial life-cycle with three generations, having flying times in spring, early summer and autumn. Growth-stoppage takes place in winter. 10. Ancylus fluviatilis takes two years to reach full maturity. Two generations occur simultaneously. The growth stops in late summer and the growth-stoppage is lasting the entire winter. 11. It is not possible from the measurements made to ascertain the growth conditions and life-cycle of Asellus aquaticus. The reason for this is probably that there are several simultaneous generations, and at the same time, the species has a long breeding period. 12. It is not possible to explain the animals' growth-rate by means of temperature conditions in the various localities where the animals were collected. 13. It appears that the nourishment conditions for certain of the species under discussion can offer an explanation of the growth-rate — at the same time, it is difficult to find a correlation for other species. 14. The stony bottom appears to provide a more favourable environment than the other biotopes examined, as the species living on stony bottom seem to complete their growth in a shorter period than species from other biotopes. 15. The same species do not necessarily have the same year-cycle and growth-rate in different localities.