Prey Size Selection by Larvae of the Damselfly, Ischnura elegans (Odonata)

Abstract

(1) A method is described using faecal pellet analysis whereby the dry weights of individual prey items captured in the field can be determined. (2) Entomostracan prey were the most important numerically to Ischnura, though insect larvae, particularly chironomids, were relatively more important in biomass terms. (3) For those prey groups studied in detail, the minimum prey size consumed by a wide range of instars of Ischnura elegans (van der Linden) was very similar, while maximum prey size increased logarithmically with instar number. (4) When data from all prey groups were pooled, linear relationships were revealed between the logarithms of both mean and maximum prey sizes consumed and instar number. (5) Maximum prey sizes for instars 8 to 12 of Ischnura predicted from laboratory experiments with a single prey species, Daphnia magna Straus, were close to those actually found for a wide range of the natural prey of Ischnura in the field. (6) It is tentatively suggested that no prey size selection occurs by Ischnura larvae within the size range that they are physically capable of dealing with. This contention is supported by laboratory data from a single prey species experiment and a 'no selection' model.