A taxonomic revision of the genus Xanthocnemis (Odonata: Coenagrionidae) and an investigation of the larval behaviour of Xanthocnemis zealandica.

Abstract

The genus Xanthocnemis (Odonata: Coenagrionidae) is endemic to New Zealand and includes 4 described species. Of these X. tuanuii is confined to the Chatham Islands, X. sobrina in Northland forests and X. sinclairi is described for the first time from Whitcombe Pass in the headwaters of the Rakaia River. Xanthocnemis zealandica (McLachlan) is the commonest and most widely distributed species, occurring on the 3 main islands of New Zealand in many still and some running water habitats. In the present study, larval behaviour was examined extensively in laboratory aquaria with particular emphasis on interactions with conspecifics and the use of defended, territorial sites. Field studies were used to complement laboratory work on feeding. X. zealandica larvae have an extensive repertoire of agonistic displays 25 of which are identified and described. All are associated with site defence. Larvae adopt a sedentary behaviour pattern from the earliest free living stage and many displays are well developed by instar 5. Late instar larvae select particular types of sites on stems with a preference for those of 4-7 mm diameter observed in experimental work. They often remain on a particular site for many weeks. This was consistent with what was known of their behaviour in the field. It was found that the perches occupied probably served the larvae primarily as refuges from predators rather than as 'fishing sites'. Behaviour on sites was insensitive to feeding regimes, i.e. different prey type and density. The sites were effective refuges against some invertebrate predators including larval and adult Dytiscidae. It is considered that the 'sit and wait' tactics of X. zealandica are mainly associated with predator avoidance. The sedentary and cryptic behaviour of X. zealandica made experimental investigation of the predatory behaviour of larvae impracticable. However, the opportunity was taken to examine the varied predatory behaviour of Hemianax papuensis (Aeshnidae) which was more amenable to study. This demonstrated unequivocally the presence of predatory versatility in odonate larvae.