[Dynamic change of population structure of ].

To understand the successive dynamic change of population structure of during a one-year period, so as to provide the evidence for snail control. A river beach and a ditch infested with snails were selected and longitudinally investigated in the midmonth during one year. The snail survey indices included the survival status, gender, number of whorls, length and width of shell, and gonad development status (measured by the color depth of gonad and the length ratio of gonad to liver), and the monthly snail eggs in the soil were collected and counted simultaneously. In addition, the temperature and humidity of the soil and the daily data of air temperature and precipitation were measured or collected during the study period (every month). Both survival rate of snails and live snail density at the two environments were positively correlated with the temperatures of air and soil. With a slight bimodal distribution, the snail survival rate peaked from May to June, and in September. The living snail densities got the highest level in July and September in the river beach, and from April to May in the ditch. The regression equations of snail length (L) and width (W) were L = 2.355 + 1.678W ( = 2 989.43, < 0.01) and L = 0.478 + 2.091W ( = 2.989.43, < 0.01), respectively. The snails were the ones with 4.07-11.81 mm in the length (8.98 ± 0.92) mm in the river beach, and the snails were the ones with 3.63-9.92 mm in the length (7.03 ± 0.82) mm in the ditch. The main snails were the ones with five to eight whorls of shell in the river beach and four to seven whorls of shell in the ditch. The proportions of snails with less than or equal to five whorls (in the river beach) and four (in the ditch) were the highest in May and September, about 20%. The ratios of male and female snails were 1.66 in the river beach and 1.22 in the ditch, respectively. The gonad development status of male and female snails was basically synchronous and had a bimodal abundance period - from April to May and September to October. The numbers of snail eggs in the soil among months were significantly different, reaching the highest in June in the river beach (100.8/0.1 m), and May in the ditch (82.5/ 0.1 m). The principal periods of breeding and alternation of generations of snails are April-May and SeptemberOctober every year, which should also be the optimal time for mollusciciding in schistosomiasis susceptible zones.