The benthos of Lake Kitaura, Ibaragi Prefecture was surveyed in August and November of 1964. 1. A shell zone was found in this lake. 2. The shell zone is situated at the depth, not influenced by wave action. 3. At the stations Futaesaku, Kajiyama, the shell zone is composed largely of the shells of Sinotaia histrica. At Uzaki and Tsumaki, where the sea water invades more than the aforesaid stations, the shell zone is composed mainly of the shells of Corbicula japonica.
Experiments were carried out to assess the effects of nymphal and adult densities on the fecundity and other reproductive performances of the adults. A high nymphal population density adversely affects the fecundity and the duration of the life span of the adults by giving rise to adults of lighter weight. By contrast, the deleterious effects of the population density of the adults were induced directly by the mutual interference among the adults, which resulted in the shortening of the life span and decrease in fecundity. It was revealed that the physiological conditions of the females, e.g. reproductivity, is determined not only by the population density during the nymphal and adult stages, but also by the nature of the parent eggs : whether the eggs were deposited in the early or late period during the ovipositional period. The percentage reductions in the daily ovipositional rate due to five times increase in densities of nymphs and adults were 15 per cent and 43 per cent, respectively. Therefore, the effect of the population density of the adult on the fecundity was greater than that of nymphal density.
In this paper, the temperature relationships of the development and the mortality of eggs and larvae of the brown planthopper were studied, when their parents was conditioned with different environmental conditions such as temperature, photoperiod, crowding and quality of food plant. Rearing of the parental generation in this experiment was made under the following four conditions. Case A : the rearing temperature is 27.5℃ and long day photoperiod (16 hours in light and 8 hours in dark per day), high population density (that is 10 pairs (of adult) or 20 larvae per rearing tube), seedling of rice plant as food. Case B : temperature, photoperiod and food are the same as in the case A, low population density (that is 1 pair (of adult) or 1 larva per rearing tube). Case C : temperature, photoperiod and population density are the same as in case A, leaf sheath of rice plant given as food. Case D : 28℃ of rearing temperature, short day photoperiod (that is 8 hours in light and 16 hours in dark per day), population density and food condition are the same as in case D. The incubation periods of the eggs from the female which are deposited by the female (brachypterous) of case B, become longer than that of the eggs produced by the female (macropterous) of case A (the difference can be seen at 90% level of confidence limit). Comparing the incubation periods of the eggs which are deposited by the macropterous female of case A with that of the eggs which are produced by the macropterous female of case C, the incubation periods of the former are longer than that of the latter (the difference can be seen at 80% level of confidence Iimit). There is no clear difference among all cases in the relation between temperature and developmental periods of the larvae produced by the females which were reared under the different conditions mentioned above. The developmental zeros and the total effective temperatures in the egg and each larval stage of progeny are different, when their parent was reared under the different conditions. The mortality during the larval stage is highest when the larvae were reared at 30℃ and lowest at 22.5℃. The mortality of the larvae produced from the parents reared with the rearing condition of case A or B was higher than that of the parents reared with rearing condition of case C or D.
In the present paper, a description is given of the distribution of the freshwater planarians in the Akiyoshi, Omijima Island in the Sea of Japan (Yamaguchi Pref.) and Mt. Sambe (Shimane Pref.) districts in southwest Japan. The Akiyoshi district is characterized by the development of a Karst, so-called Akiyoshi-dai (Akiyoshi limestone plateau), which is about 200 to 400 metres above sea level. There are many limestone caves in this district (the largest one, Akiyoshi-do Cave…also spelled Shuho-do Cave, is located at the southern foot of the Akiyoshi-dai ; cf. KAWAKATSU 1960). The area surveyed is situated in Lat. 34°00′N. to 34°30′N. and Long. 131°00′E. to 131°11′E. The surveys were made in February, March, April and August, 1965. Mt. Sambe, an extinct volcano, is 1126 metres in height and is situated in Lat. 35°08′N. and Long. 132°37′E. The survey of the volcano was made in April, 1958. In these districts, the two species of freshwater planarians, Dugesia japonica ICHIKAWA et KAWAKATSU and Phagocata vivida (IJIMA et KABURAKI), were found. D. japonica, the commonest flatworm in Japan, was found in the rivers, creeks, brooks, brooklets and springs both in the plains and lower mountain districts. This species was also found in many underground streams and pools in the caves. Ph. vivida is rather common in the cold-water biotopes, such as small springs, outlets of caves and pools in some caves. It is an interesting fact that Ph. vivida is very rare in mountain or hill streams in the Akiyoshi district ; this species is found commonly in the cold-water mountain streams in Central and North Japan. The inhabitable water temperature range of D. japonica and Ph. vivida in the Akiyoshi and Mt. Sambe districts is as follows : D. japonica (4.3〜27.0℃) ; Ph. vivida (4.0〜21.5℃). The type of vertical distribution in these districts is J-JV (J : D. japonica ; V : Ph. vivida). The new records on the distribution of the freshwater planarians in southwest Japan is also described. Phagocata kawakatsui OKUGAWA is evidently common in the vicinity of Takamatsu City, Kagawa Prefecture in Shikoku (type locality : Minoda, Asahi-cho in Kameoka City in Kyoto Pref. ; also found at Saga-tenryuji-cho in Kyoto City and in Matsumoto City in Nagano Pref. in Central Japan).