Surface Activity and Growth of Larvae of Nebria Brevicollis (F.) (Coleoptera, Carabidae)

To supplement our knowledge of the growth and the exceptionally high surface activity of the larvae of N. brevicollis (Coleoptera, Carabidae) field and laboratory experiments were carried out. A positive correlation was found between the size of the young adults and the amount of food the larvae got during their development. In each instar larvae need to increase in weight at least 2.6 times before they will moult. Under highly favourable conditions this increase in weight can be up to 3.5 times the starting weight. However, in the field this occurs only rarely, as the size of beetles bred from field sampled larvae and of field sampled beetles was below that of beetles bred under laboratory conditions with food in excess. Apparently, local field conditions (especially the food level) are suboptimal. This was also demonstrated by weighing-experiments : larvae released in an enclosure decreased in weight, althought the density was low. Capture-recapture experiments showed that third instar larvae during their instar can cover some tens of meters; first and second instar larvae up to 10-15 m (estimated minimum values). There are indications that third instar larvae show two types of movement: directed walk and random walk. The normal locomotory activities often lead to dispersal out of the preferred habitat (woodland), and it is shown that they may result in exchange with other subpopulations. As N. brevicollis is virtually semelparous, population survival is highly dependent on the chances that the winter active larvae meet with suitable habitats. In this life strategy with a high surface activity of the larvae, exceptional among carabids, the encounters of food patches apparently compensate for the higher risk of predation, regarding the widespread and abundant occurence of the species, in spite of generally suboptimal field conditions.