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Many organisms with complex life cycles show considerable variation in size and timing at metamorphosis. Adult males of Megarcyssignata (Plecoptera: Perlodidae) are significantly smaller than females and emerge before females (protandry) from two western Colorado streams. During summer 1992 stoneflies from a trout stream emerged earlier in the season and at larger sizes than those from a colder fishless stream, and size at metamorphosis did not change over the emergence period in either stream. We performed two experiments to determine whether variation in size at metamorphosis affected the fecundity, reproductive success and longevity of individuals of this stonefly species and if total lifetime fecundity was affected by the number of matings. In the first experiment, total lifetime fecundity (eggs oviposited) was determined for adult females held in small plastic cages in the field. Males were removed after one copulation, or pairs were left together for life and allowed to multiply mate. Most copulations occurred in the first few days of the experiment. Females in treatments allowing multiple matings had significantly lower total lifetime fecundity and shorter adult longevity than females that only mated once. Multiple matings also reduced longevity of males. Fecundity increased significantly with female body mass at emergence, but only for females that mated once. While multiple matings eliminated the fecundity advantage of large female body size, number of matings did not affect the significant positive relationship between body mass at metamorphosis and longevity of males or females. In a second experiment designed to determine if body mass at emergence affected male mating success, we placed one large and one small male Megarcys in an observation arena containing one female and recorded which male obtained the first mating. The large and the small male had equal probabilities of copulating with the female. Copulations usually lasted all night, and the unmated male made frequent, but unsuccessful attempts to take over the copulating female. Our data suggest that selection pressures determining body size at metamorphosis may operate independently on males and females, resulting in evolution of sexual size dimorphism, protandry, and mating early in the adult stage. We emphasize the importance of interpreting the fitness consequences of larval growth and development on the timing of and size at metamorphosis in the context of the complete life cycle.

Flume experiments were carried out to examine whether larval blackflies (Simulium ornatum complex, Diptera: Simuliidae) use microhabitats with a highvelocity current to reduce the risk of predation by some of their main predators, viz. larvae of the stoneflies Isoperla grammatica and Diura nanseni (Plecoptera: Perlodidae), and the caddis-fly Rhyacophila nubila (Trichoptera: Rhyacophilidae). We exposed blackfly larvae to four different current velocities and measured their feeding rate using dye particles. The maximum feeding rate was recorded at intermediate velocities (18.8 and 36.2 cm/s), whereas at low (7.3 cm/s) and high (53.3 cm/s) velocities, the feeding rate was reduced. In separate experiments, we investigated the behaviour and attack success of the different predator species. The two perlodids showed a similar hunting behaviour, which was significantly less successful at higher velocities. Drift of the perlodids from the experimental arena resulted in reduced encounter and attack rates, especially in I. grammatica, which had completely lost efficiency at 36.2 cm/s. R. nubila had a slower mode of hunting and was unaffected by current speed within the velocity gradient studied. Drift in Rhyacophila was rare. Observations on the behaviour of blackfly larvae were performed in the same experiments. The larvae showed no apparent ability to sense the presence of the predators except when these disrupted the flow pattern or were in physical contact, which often resulted in aggressive defence, though without effect on the predators. Escape of blackfly larvae by drift did occur, but this was no more common than being captured. In a current velocity gradient, blackfly larvae showed a weak preference for increasing velocities. Thus, at velocities between 7 and 54 cm/s, blackfly larvae appear to select microhabitats with high current velocities, despite a reduction in feeding optimality, thereby easing the predation impact from perlodids, though not from Rhyacophila. The study demonstrates the importance of microhabitat selection by blackfly larvae both for efficient feeding and predator avoidance.

We conducted experiments in replicated circular streams to measure the effect of intraspecific larval density on growth rates, size at emergence, timing of emergence, and fecundity of two species of predatory stoneflies (Megarcys signata and Kogotus modestus, Perlodidae). Early instars of both species showed no significant effect of intraspecific larval density on mean growth rates, despite the observation that in the absence of competitors stoneflies ate on average, significantly more prey (Baetis bicaudatus, Ephemeroptera, Baetidae) than in the presence of competitors. However, larval size of stoneflies held at higher densities (two per chamber) diverged over time, resulting in a greater size variability (coefficient of variation) among Kogotus than in treatments with low densities of stoneflies (one per chamber). The effect of doubling the density of early-instar Megarcys larvae was also asymmetrical, resulting in one larger and one smaller individual. In contrast, doubling the density of last-instar stoneflies whose feeding rates declined significantly prior to emergence had few measurable consequences, except that male Megarcys, which continued to feed throughout the last instar, had lower average feeding rates in high-density than in low-density chambers, and emerged at a significantly smaller mean size. We conclude that competition between early-instar stonefly larvae results in an asymmetry of body sizes, but that competitive effects are reduced as larvae slow or cease feeding before emergence. Since larger females of both stonefly species produced more eggs, the probable cost to females of early-instar larval competition was a reduction in their potential contribution of offspring to the next generation. The cost of attaining a smaller body size for male stoneflies is unknown; but if, as in many other insects, larger males have greater reproductive success, larval competition may increase the opportunity for sexual selection among males. This hypothesis remains to be tested experimentally.

Behavior of focal individuals of two potentially competing sympatric stonefly species, Megarcys signata and Kogotus modestus (Perlodidae), was videotaped in flow-through plexiglass arenas placed in the East River, Gunnison County, Colorado. Focal individuals were observed alone and in pairs with conspecifics and allospecifics at four prey (Baetis bicaudatus, Baetidae, Ephemeroptera) densities to determine whether competitors and prey resource levels affected prey capture rates. Presence of conspecific or allospecific competitors reduced stonefly prey capture rates, especially for Kogotus, the smaller of the two species, due to a significant decline in predator-prey encounter rates with competitors present. This competitive effect was not observed at the lowest and highest prey densities due to very low or very high predator-prey encounter rates, respectively. Thus, interference affected feeding rates only at intermediate prey densities. Competitors had no effect on the probability of attacks per prey encounter or capture success per attack. Within each stonefly species the effects of intra-and interspecific interference on feeding rates were similar, even though behavioral responses by both stoneflies to interspecific encounters were more frequent than to encounters with conspecifics. Kogotus showed the highest levels of response to encounters with other stoneflies, maintaining those high levels of response to Megarcys over all prey densities. Further, male Kogotus, which are the smaller sex, responded more frequently to competitive interactions than did females. These data are consistent with the hypothesis that interspecific interference was asymmetrical with Megarcys, the larger species, being the superior competitor.

Laboratory feeding experiments using Hesperoperla pacifica (Banks), Perlidae, and Megarcys signata (Hagen), Perlodidae, as predators and Baetis tricaudatus Dodds and Ephemerella altana Allen as prey indicate a strong effect of prey morphology and mobility and predator hunger on prey selection by stoneflies. Knowledge of both dietary composition and feeding behavior was necessary to fully understand prey selection by these stoneflies. Fasted stoneflies presented with live prey ate more E. altana while satiated stoneflies ate approximately equal numbers of the two mayfly species. This pattern of dietary composition was the result of a reduction of attack frequency on the slower swimming E. altana with predator satiation and a continued high attack rate on B. tricaudatus regardless of recent feeding history. In contrast, fasted H. pacifica fed fresh frozen mayflies ate more B. tricaudatus indicating the importance of differences in prey mobility in controlling dietary composition. The high degree of similarity in patterns of feeding and mechanisms underlying those patterns for H. pacifica and M. signata suggest that they may be using similar “rules” for choosing mayfly prey and we suggest that mayfly prey are ranked by stoneflies on the basis of handling times. A general mechanistic model for stoneflies feeding on mayflies is presented and discussed.

Playback experiments conducted in a Rocky Mountain, USA, stream determined whether predatory stonefly nymphs (Kogotus modestus; Plecoptera: PerlodiMae) used hydrodynamic cues to discriminate prey species from nonprey species. In the laboratory we recorded pressure wave patterns associated with swimming escape behavior of Baetis bicaudatus (Baetidae), the favored mayfly prey species, and those of a nonprey mayfly, Ephemerella infrequens (Ephemerellidae). We video taped the responses of 24-h starved Kogotus to Baetis playbacks, Ephemerella playbacks or no playbacks made by oscillating (or not) live mayflies (Ephemerella) or clear plastic models placed within in situ flow-through observation boxes. The probability of attacks per encounter with Baetis playbacks was highest and independent of the model type used, but Kogotus also showed an unexpected high probability of attacks per encounter when Ephemerella playbacks were made through live Ephemerella. Thus, Kogotus discriminated between Baetis and Ephemerella swimming patterns but only when playbacks were made through the plastic model. Kogotus never attacked motionless mayflies or motionless plastic models. We allowed some Kogotus to successfully capture one small Baetis immediately before playbacks, which resulted in a much higher probability of attacks per encounter with Baetis playbacks on either model and a heightened discrimination of prey versus nonprey playbacks. The probability of attacks per encounter by Kogotus with live Baetis swimming under similar experimental conditions was strikingly similar to its response to Baetis playbacks made by oscillating the plastic model after a successful capture. Order of playback presentation (Baetis first or Ephemerella first) did not influence predatory responses to mayfly swimming patterns. This study is the first to document the use of hydrodynamic cues by stream-dwelling predators for discrimination of prey from nonprey and provides a mechanism to explain selective predation by stoneflies on Baetis in nature.