The females of certain insects may be fertile without coupling, and then produce eggs which have not been subjected to spermatozoal fecundation. When this phenomenon' is accidental, as happens sometimes with the mulberry silkworm moth, the eggs develop only rarely and with difficulty, but when it becomes normal the result is quite different. Note, for example, the Adoxus vitis of the vine, where the males are almost unknown and seem to play no part, and the insect then gives birth to a series of generations in Which the non-fecundated females produce only females until a final generation which contains both females and males.
Whatever form it may take, reproduction without spermatozoa is called agamic, or partheno-genetic. It is quite different from the sexual reproduction characterized by the fusion of the egg with the spermatic cell, which is the rule with almost all animals. In such forms as the phylloxeras, the Chermes, the plant-lice (and certain phytophagic Hymenoptera of the family Cyni-pidce, and of the sawflies), where the last of the agamic generations produces individuals of both sexes, these individuals couple and their fertilized eggs produce females exclusively. Thus we have a cycle, or a sexual generation intercalated regularly between one or more agamic generations.