In some of the hymenopterous insects, the bees for example, it has been discovered that the sex of the embryo is determined by the entrance, or lack of entrance, of the spermatozoon. In the honey-bee all the fertilized eggs produce females and the unfertilized eggs males. The same relation is probably true also in the case of ants and of wasps. In the saw-flies, the conditions are very remarkable. Sharp gives the following account of some of these forms:[35]—“It is a rule in this family that males are very much less numerous than females, and there are some species in which no males have been discovered. This would not be of itself evidence of the occurrence of parthenogenesis, but this has been placed beyond doubt by taking females bred in confinement, obtaining unfertilized eggs from them, and rearing the larvæ produced from the eggs. This has been done by numerous observers with curious results. In many cases the parthenogenetic progeny, or a portion of it, dies without attaining full maturity. This may or may not be due to constitutional weakness, arising from the parthenogenetic state. Cameron, who has made extensive observations on this subject, thinks that the parthenogenesis does involve constitutional weakness, fewer of the parthenogenetic young reaching maturity. This, he suggests, may be compensated for—when the parthenogenetic progeny is all of the female sex—by the fact that all those that grow up are producers of eggs. In many cases the parthenogenetic young of Tenthredinidæ are of the male sex, and sometimes the abnormal progeny is of both sexes. In the case of one species—the common currant-fly, _Nematus ribesii_—the parthenogenetic progeny is nearly, but not quite always, entirely of the male sex; this has been ascertained again and again, and it is impossible to suggest in these cases any advantage to the species to compensate for constitutional parthenogenetic weakness. On the whole, it appears most probable that the parthenogenesis, and the special sex produced by it, whether male or female, are due to physiological conditions of which we know little, and that the species continues in spite of the parthenogenesis rather than profits by it. It is worthy of remark that one of the species in which parthenogenesis with the production of males occurs—_Nematus ribesii_—is perhaps the most abundant of saw-flies.” Footnote 35: “The Cambridge Natural History,” Vol. V, “Insects,” by David Sharp.