Nymphal sexual dimorphism in the sheep tick Ixodes ricinus (Acari: Ixodidae).

Unfed nymphs of Ixodes ricinus (L.) can be divided into two morphological groups according to the length of idiosoma, scutum, hypostome and palpal segment III, and the number of dorsal alloscutal setae. Specimens of greater body dimensions and more numerous dorsal alloscutal setae moulted predominantly into females. The frequency of different nymphal length categories in field-collected ticks followed a normal distribution. The length of unfed nymphs correlates well with the length (r = 0.7248 +/- 0.0711, P < 0.001) and weight (r = 0.6519 +/- 0.0782, P < 0.001) of engorged nymphs, however, it varies in ticks of different origin. In field-collected ticks, freshly engorged female nymphs were 2.30-2.94 mm long, male nymphs 2.14-2.46 mm long. Feeding period (P < 0.05) and premoulting period (P < 0.001) were significantly longer in female nymphs both in field-collected and laboratory-derived I. ricinus. The engorgement weight was found to be the best criterion for differentiation of male and female nymphs of ixodid ticks. In field-collected nymphs engorged on BALB/c mice, 98.6% of females moulted from nymphs weighting more than 3.60 mg, while in laboratory-derived ticks, 98.4% of females emerged from nymphs of 3.42 mg body mass or more.