Do damselfly larvae recognize and differentially respond to distinct categories of macroinvertebrates

Zygopteran larvae normally encounter other aquatic macroinvertebrates that are predators, competitors, and prey and should therefore demonstrate varied responses when faced with different categories of opponent. In a laboratory experiment individual final-instarIschnura posita (Hagen) larvae were observed in interactions with six categories of invertebrate opponents. The opponent categories were a nonconspecific damselfly and a small crayfish, which represented threatening opponents because they were larger than theI. posita subject larva, and a mayfly and a stonefly, which constituted nonthreatening opponents because they were smaller than the subject larva. The levels of threat posed by conspecific larvae of final and penultimate instar were inferred by comparison to the other opponent categories. Multivariate analysis showedI. posita's response differed between the two larger opponents, but responses were statistically indistinguishable between the two smaller opponents. Larvae retreated, moved around the stalk, and struck their opponents with their lamellae more often in the presence of a crayfish than the nonconspecific zygopteran. In contrast, they assumed an S-bend posture frequently with the zygopteran. Responses toward final-instar conspecifics differed from responses toward the larger opponents. Effectively, larvae wagged their abdomens only in the presence of final-instar conspecifics and retreated and moved around the stalk less frequently in these trials. Responses toward the smaller conspecifics differed from the responses to the small opponents. Larvae struck penultimate-instar conspecifics with their lamellae more frequently than the other small opponents. Our results suggest that larval zygopteran behaviors (such as S-bend and SCS) that have previously been described as intraspecific displays are of a more general nature and used toward a variety of opponents, whereas wag is unique to intraspecific interactions inI. posita.