Seasonal regulation in Pyrrhosoma nymphula (Sulzer) (Zygoptera: Coenagrionidae) 2. Effect of photoperiod on larval development in spring and summer

In the laboratory, larvae of the Western Palaearctic P. nymphula were exposed to conditions of temperature and food conducive to uninterrupted development, and to one of three, constant photoperiods; 13.5 h (equivalent to either equinox). 18 h (14 May and 30 July) and 19.5 h (5 June and 8 July), and their responses were interpreted in the light of seasonal development in the field. Synchronised entry to F-0 (the final instar) in late summer is caused by the summer arrest in F-l (the penultimate instar) that is induced by exposure in F-l, but not in F-2 or F-3, to 19.5 h, but not to 13.5 h. The refractoriness of F-l declines during summer so that the response to 19.5 h changes abruptly during late August (16-16.5 h; maximum daily increment of change of 5 min/day), when F-0 is entered promptly. Termination of emergence in summer is caused by the long-day arrest that prevents F-l larvae from entering F-0 during the second half of May (18 h or more). Photoperiod does not evidently affect the mean duration of F-2 or F-3, or the onset and rate of metamorphosis in spring. Persistent exposure to 19.5 h during their summer arrest causes about 25% of F-l larvae to undergo a supernumerary moult, still within F-l; such individuals can be distinguished from normal F-l (after the supernumerary moult) by their larger wing-sheaths; Supernumerary moulting occurs also in the field. Remaining unknown are; the responses of F-0 which prevent emergence (i.e. the onset of metamorphosis) in autumn; and the role, if any, of changing photoperiods in regulating rate of development or sensitivity to absolute photoperiod.