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The number of nymphal instars of Nysius huttoni White was investigated at five constant temperatures (15, 20, 25, 30 and 3590.58C), one fluctuating (laboratory room conditions) temperature and three photoperiods (16-h, 12-h, and 8-h photophases) in the laboratory. Nymphs passed through four to six instars before they emerged as adults, individuals with five instars formed over 90% of the population, whereas those with four or six instars formed 4.2Á5.8 and 1.6Á2.9% respectively. Two nymphs were each recorded to pass through three and seven instars before emergence as adults. Both temperature and photoperiod affect the number of nymphal instars. Insects with four instars were more frequent at lower temperatures and those with six instars tended to be most frequent at higher temperatures. An 8-h short-day photoperiod resulted in more 4-and 6-instar types than other photoperiods. There was no obvious difference in the duration of nymphal stages of individuals with a maximum four, five, or six instars (P!0.05, t-test). This is the first report to show variation in the number of instars in a species of Lygaeidae. The results indicate a potential increase in survival of this adult-overwintering species, as more autumn generation nymphs in the field could reach the adult stage.

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