Food Habits of Hellgrammite Larvae, Corydalus cornutus (Megaloptera: Corydalidae), in the Brazos River, Texas1

Abstract

Corydalus cornutus L. larvae ingested larvae of Simulium (25.8%). Cheumatopsyche (23.9%), and Hydropsyche (l9.0%) with greatest frequency. Chironomidae larvae and Simulium pupae were ingested at 6.8% frequency, and 17 other aquatic insect food groups were ingested at lower frequencies. There was little diurnal periodicity in feeding, except that Hydropsyche larvae appeared to he ingested at higher rates after dusk. Calculation of Forage Ratios and Ivlev's Electivity Index indicated positive selection or election for baetid mayflies (Isonychia), Hydropsyche and Simulium. Because of abundance of Cheumatopsyche in the environment and a functional response in its predator-prey interaction with C. cornutus, somewhat misleading negative selection was indicated by the indices for Cheumatopsyche. Chironomids were taken at random or were slightly preferred February-April but were highly preferred in May. Simulium larvae were positively selected except in April; Electivity Index for them in May was maximum (1.0), and a high Forage Ratio (33) was determined.