The purpose of this study was to estimate the number of siblings in each Japanese family with multiple birth children using two sets of census data, from 1990 and 1995. The mean number of siblings for singletons was 2.3 in 1990 (2.4 in 1995), 2.9 (2.9) for one set of twins, 4.6 (4.5) for two sets of twins, and 3.5 (3.3) for one set of triplets. For birth order of multiples, the highest mean sibling number was 4.4 for one set of twins (the middle), 7.5 for two sets of twins (the middle-middle), and 4.3 for one set of triplets (the last). The mean sibling number was slightly higher for like-sexed twins and triplets than in unlike-sexed twins and triplets. The mean sibling number was highest in the Okinawa District, among nine districts (0.4 for singletons, 0.6 for one set of twins, 1.0 for two sets of twins and 0.6 for triplets). The current study of the number of siblings in each family with multiple birth children will help provide data for designing programs to help support multiple-birth families. The method of using census data on multiple births might result in a system to analyze nationwide data on multiple birth children if there is no national registry of multiple births after live births in any country.

Zygosity of triplet births in Japan was estimated by Allen's equation with the assumption that the rate of dizygotic (DZ) triplets reflects that of twins. Whereas the DZ triplet rate increased during the period from 1975 to 1994, reflecting the increase in the DZ twinning rate in the period, monozygotic (MZ) triplet rates remained constant from 1975 (28 per million births) to 1994 (23). The trizygotic (TZ) triplet rate gradually increased from 1975 (18) up to 1985 (29), and rapidly increased thereafter to 1994 (202). The higher TZ triplet rate since 1986 is most likely attributed to the higher proportion of mothers treated with ovulation-inducing hormones and partially to in-vitro fertilization in Japan. As for maternal age, MZ triplet rates remained nearly constant for all the maternal age groups except the youngest and the oldest ones. On the other hand, TZ triplet rates increased up to the age group of 30-34 years and decreased thereafter in almost every year. The TZ rate in the age group of 30-34 years slowly increased from 1975 to 1988 (63 per million births) and rapidly increased thereafter (314 in 1994). The TZ rate was statistically significantly higher in the period 1986-1994 than in the period 1975-1985 in each of the nine districts in Japan. Geographical variations in the TZ rates in the latest period have drastically changed from those during the period from 1955-1959 and in 1974.

The monozygotic (MZ) twinning rate in Japan had remained nearly constant from 1975 (3.74 per 1,000 births) to 1994 (4.23), whereas the dizygotic (DZ) twinning rate had remained nearly constant from 1975 (1.86) to 1986 (2.27), and had gradually increased up to 1994 (3.89). The higher DZ twinning rate since 1987 has been attributed to the higher proportion of mothers treated with ovulation-inducing hormones and partially attributed to in-vitro fertilisation in Japan. As for maternal age, MZ twinning rates have remained nearly constant for maternal age groups except the youngest and the oldest age groups. On the other hand, DZ twinning rates increased up to the 35-39 years of age group and decrease thereafter. In 1994, the twinning rate was higher in DZ twins than in MZ twins for maternal age groups of 30-34 years and of 35-39 years. As for geographical variations in twinning rates, DZ rates statistically significantly increased with the year during the period from 1986 to 1994 in 31 out of 47 prefectures. In 1994, twinning rates in 21 out of 47 prefectures were higher in the DZ than the MZ rate, and the DZ rate is equal to the MZ rate in two prefectures. Geographical variations in twinning rates by zygosity in 1994 drastically changed from those during the period from 1955-1959 and in 1974.

The band-legged ground cricket group comprises a pair of very closely related species; Dianemobius fascipes occurs widely over tropical and subtropical regions of Southeast Asia, while D. nigrofasciatus is restricted to temperate regions. A similar situation is found in the lawn ground cricket group; the tropical/subtropical species D. taprobanensis is replaced by D. mikado in temperate regions. In both southern species, tropical strains were almost homodynamic, but they produced a few delayed eggs independently of photoperiod and a variable proportion of macropters in response to photoperiod; subtropical strains showed clearer responses. Both northern species controlled egg diapause, nymphal development and wing form by various photoperiodic responses. The two species groups showed parallel geographical profiles of size variation, and the adults and eggs were smaller in the southern than in the northern species. In the absence of strong climatic selection for larger growth rate, size increase in tropical regions would be counteracted by predation pressure. The optimal adult size thus seems to be smaller in tropical than in temperate regions, being determined by the geographically variable balance between counteracting selecting forces.