Selective feeding of third- and fourth - instar larvae of Chaoborus flavicans in the field

Zooplankton population dynamics were examined in Lake Klostersee (Bavaria, Southern Germany) from March to September, with special emphasis on third- and fourth-instar larvae of the phantom midge Chaoborus flavicans (Diptera) and their potential zooplankton prey. The samples were taken at night, during the feeding period of the larvae which show a diel vertical migration behaviour. I used an electivity index to compare the crop contents of third- and fourth-instar larvae to the density of their zooplankton prey to see whether the larvae fed selectively on any of them. Regardless of the prey frequencies, Chaoborus always showed negative electivity towards Ceriodaphnia, copepods, and rotifers, but positive electivities towards Bosmina. Electivities shifted from negative to positive during the study period for Daphnia, Diaphanosoma, and copepod nauplii and the reverse for Chydorus. Selective predation by the midge larvae could probably be explained by differential vulnerabilities of the prey due to induced anti-predator defenses.