The working bee which, being an imperfect female, may not be impregnated, will, however, give birth to parthenogenetic offspring, such offspring always being male. In the case of Aphides, the sex depends on the conditions of nutrition. During the summer months while food is plentiful and nutritious, females are parthenogenetically produced, but with the return of autumn and the attendant scarcity of food, together with the low temperature, only males are brought forth. In seasons in which food is abundant, Cladocera and Aphides lose the power to copulate; they nevertheless multiply parthenogenetically at a marvellous rate of increase, giving birth to generation after generation of parthenogenetic females, so long as the environment remains favourable, but giving birth, as soon as the conditions of life become less favourable, to males and to females which require fertilization.[26] [26] Prof. W. K. Brooks, _Pop. Science Monthly_, vol. xxvi., p. 327. It is stated also that if caterpillars are shut up and starved before entering the chrysalis stage, the butterflies which make their appearance are males, while the highly nourished caterpillars are sure to come out females. In the case of moths unnutritious food produces only males. Experiments show that when tadpoles are left to themselves the average number of females is about fifty-seven in the hundred, but that under favourable conditions the percentage of females is greatly increased. The following is the result of one series of observations by Yung. In the first brood, by feeding one set with beef, the percentage of females was raised from fifty-four to seventy-eight; in the second, with fish, the percentage rose from sixty-one to eighty-one, which in the third set, when the nutritious flesh of frogs was supplied, only eight males were produced to ninety-two females.[27] [27] Geddes and Thomson, _The Evolution of Sex_, 1890, p. 42. It is stated that although scarcity of food is an important factor in determining the appearance of males, temperature also plays an important part in their production. Kurg having found a few males in midsummer in pools which were nearly dried up was induced to attempt their artificial production. So successful was he, that “he obtained the males of forty species, in all of which the males had previously been unknown.” He proved that any unfavourable change in the water causes the production of males, which appear as it dries up, as its chemical constitution changes, when it acquires an unfavourable temperature, or, in general, when there is a decrease in prosperity.