P. iridipennis was observed to oviposit into the stems of Cyperus involucratus by making long slits through which up to 90 eggs were packed in a single row, 0.5 mm below the surface and at 60° to the long axis of the stem. In contrast, when ovipositing into the leaflike bracts of the same plant, females inserted each egg through a separate slit in a manner resembling that used by other calopterygids. Females more commonly oviposited into bracts than into stems, and bracts were found to contain up to 9 times more eggs mm"3 than stems. When offered the subcrescentic leaves of Typha domingensis as abnormal oviposition sites, eggs were laid using either of the two modes, and it is suggested that stems and bracts of i C. involucratus are normally distinguished by tactile receptors on the legs. The stems of Cyperus contain many more sclerenchyma strands than the bracts, and the force required to drive in a pin by a set amount was 2.38 times greater than that needed for bracts. The unusual mode of oviposition used by P. iridipennis when ovipositing into stems may therefore be an adaptation for penetrating tough plant tissues.