Abstract:
Aspects of the nymphal habitat, food, feeding, life history and growth are studied, along with emergence and adult behaviour. Systematic notes on the genus Ichthybotus are also included.
Nymphs inhabit stable lentic environments in New Zealand native forest regions. Although able to burrow into substrates between -4.5 and 4.5 phi, a preference was shown for substrates from -2.5 to -3.5 phi. Nymphs are not common below areas of farmland runoff.
Nymphs feed predominantly on organic detritus (FPOM). The structure of their mouthparts suggests that they are not filter feeders. Nymphs can be classified according to Cummins (1973) as fine particle detritovores ("collectors").
Nymphs undergo a two, sometimes three year life cycle. Growth occurs mainly during the warmer summer months of the first year. The Palmen body method was used to determine the number of instars required for maturity. Nymphs may reach full body size after one year of growth, requiring between 11-14 instars. The second year is concerned mainly with adult tissue development, requiring between 4 and 7 instars. Measurements of final instar exuviae over two emergence seasons reveal a decrease in the size of emerging adults as the emergence season progresses. This is thought to be a result of size/temperature thresholds acting on the growing nymphs, causing variation in the timing of the onset of tissue maturation. The decrease in size is proportionately greater in females than males, and may be related to mating success.
The emergence period is 2.5 months long, lasting from late November to the end of January. Emergence occurs at nightfall. Light intensity and possibly photoperiod are the cues which activate daily and seasonal emergence patterns respectively, with water temperature and dissolved oxygen concentration possibly exerting modifying influences on the magnitude of emergence. Swarming, copulation and oviposition occur at dusk, with the level of light intensity being the factor controlling its onset and cessation.
Ichthybotus does not conform to the pattern of ecological flexibility, with respect to habitat requirements, food requirements, and life history patterns, which predominates among New Zealand's freshwater insect fauna.
Investigations of type and alcohol preserved specimens of I. hudsoni and I. bicolor show that the morphological differences described by Tillyard (1923) are largely incorrect. The two species can be differentiated by hindwing colouration. They are separated geographically by Cook Strait.
