L. Forró - É. Murai szerk.: Miscellanea Zoologica Hungarica 6. 1991 (Budapest, 1991)

Szlávecz, K.: The terrestrial isopod fauna of the Hortobágy National Park

ratio is remarkably constant in 1974 and 1975 (0.79 and 0.78, respectively) for the latter species, it varies greatly for the other two. P. collicola is especially interesting, where out of the 173 individuals (collected in 1975) 143 were males, and only 30 females were caught. Table 2. Population data of the three most abundant isopod species in Újszentmargita. Data for 1974 and 1975 are combined. (N: number of individuals, M: male, F: female) A. vulgare P. collicola T. rathkii N Adults Juveniles Sex ratio Females in M% F% % (M/F) % of adults 1973 31.2 39.6 29.2 0.79 56.0 639 46.5 43.7 9.9 1.06 48.4 308 46.1 47.1 6.8 0.98 50.5 HOWARD (1940) found that in stable A. vulgare populations female are in excess (about 71 %), and BEYER (1957) also reported higher percentage of females. On the other hand, SORENSEN & BURKETT (1977) and PARIS & PITELKA (1962) found that sex ratio varied considerably throughout the year. The latter authors concluded that the sex ratio is equal in the young, but males become dominant in older age classes. Sex ratio data for T. rathkii are not less contradictory. In MCQUEEN'S study (1976) the values showed great fluctuation, whereas SNIDER & SHADDY (1980) reported a more constant sex ratio, around 60% females. BEYER (1957) also found female dominance (about 70%) in his study. He points out that different collecting methods may yield different sex ratios. To my knowledge no data are available in the literature for P. collicola. In another faunistic survey (SZLÁVECZ 1988) I found that the P. collicola population consisted of 66 % females, which is different from the values reported here. Sex ratio data are difficult to interpret (MCQUEEN 1976) especially if pitfall traps are used. Behavioural differences (eg. different activity patterns, degree of aggregation) or different mortality can result in different ratios (PARIS & PITELKA 1962, MCQUEEN 1976). The values can be different among different populations of the same species (HORNUNG 1988) or, even within a population in different microhabitats (HOWARD 1980). In our case perhaps sample size was also too small to obtain a more consistent value. The three species, although found in all examined plant communities at Újszentmargita, were not equally distributed among them (Fig. 1). Armadillidium vulgare was most abundant in the relic oak forest in each of the samples. MILLER & CAMERON (1987) found the same tendency, when comparing A. vulgare populations of an oak forest and the neighbouring grassland in Texas. Porcellium collicola also seems to prefer this habitat or its edge most of the time. The traps in the meadow never caught more than five individuals, and isopods were caught in the reeds only once. Trächelipus rathkii was the least common in the forest, it occurred primarily in more open habitats and in the transitional area between the oak forest and the meadow. The abundance of this species, however, was generally low. LOKSA (1973) reported similar results in his study.