Features of the adaptablity to the density was compared for two species of bean weevils, C. chinensis L. and C. rhodesianus Pic (Species name is not yet verified by taxonomist). Why the types of reproduction curve are different for these two species was analyzed experimentally (Utida 1959a). The experiment was conducted under the condition of 30℃, 75 per cent R.H. and the 16hr. photo-period of illumination. Numbers of deposited eggs per bean and numbers of hatched eggs per bean were counted at two weeks after the introduction of the adults. The numbers of adults emerged from a single bean was also counted after all the adults died. The rise in mortality towards low densities was particularly remarkable in C. rhodesianus than in C. chinensis at any life stages. Though the single larva of C. chinensis can develope in a bean, that of C. rhodesianus was subjected to high mortality at such situation and the best survival rate was obtained at the density of more than 4 larvae per bean. In the low densities the adult of C. chinensis could still mate, but that of C. rhodesianus seemed to mate with difficulty. Therefore, the percentage of un-fertilized eggs in C. rhodesianus increased in low densities. On the other hand, in C. chinensis the death by the trampling was more important. Random-distribution of the egg was observed at a higher density in C. chinensis has ability to distinguish beans on which the eggs were deposited and undeposited than that of C. rhodesianus. This difference gave some effects on the distribution of the leavae hatched from a bean. Though the relation between the adult density and the number of deposited eggs per bean was almost the same for both species through the whole range of parent densities, the relation of the adult density to the numeber of hatched eggs per bean was different for both species. These facts seem to give an explanation why the population growth is slower in C. rhodesianus than in C. chinensis. It was shown that the heighest number of adults which is able to emerge from a bean is limited by the size of bean. Number of adults was regulated by the larval density in each bean. When number of hatched eggs per bean was 9,number of adults emerged from a bean became to be at certain level which is limited by the size of bean. When number of hatched eggs exceeded 9 per bean, number of adults emerged per bean did not increase in C. rhodesianus but continued to increase in C. chinensis. On the basis of these assumptions, the processes responsible to the difference in types of reproduction curves were discussed. The results mentioned above suggest that C. chinensis is opposite in many aspects of its ecology to C. rhodesianus. Significance of adaptation in ecological characters in the natural regulation of animal numbers was discussed briefly in relation to Lack's associated adaptation(Lack 1965).