502 COMPARATIVE ANATOMY chap. 



by means of fertilised eggs. In the colonial ITymeiwjjtera only males come from 

 unfertilised eggs. In the Aphides, in summer, several generations of partheno- 

 genetically reproducing, viviparous, generally wingless females succeed one another. 

 The last viviparous summer generation, however, produces winged males, and either 

 winged or wingless females, whose fertilised eggs remain through the winter. 

 From these latter the first summer generation of parthenogenetically reproducing 

 females is again produced. The reproductive cycle of Phylloxera is similar, with 

 this distinction only, that all the generations are wingless, except that one out of 

 whose eggs the sexual (male and female) generation is produced. Phylloxera is 

 not viviparous. 



The reproductive arrangements of Chermes are very peculiar ; the males of this 

 fir louse were, till recently, altogether unknown. 



A wingless generation (I.) of Chermes abieiis hibernates on the fir-tree, and in the 

 spring lays unfertilised eggs, out of which a second winged generation (II.) of females 

 is produced. Some of these females migrate from the fir to the larch. From their 

 unfertilised eggs a third wingless generation (III.) of females comes, which winter iu 

 the larch, and in the spring of the second year lay unfertilised eggs, from which a 

 fourth winged generation (IV.) of females comes. These fly back to the fir tree, and 

 from their unfertilised eggs a fifth generation (V. ) of males and females is developed. 

 The sixth generation ( VI. ) which comes from the fertilised eggs of this generation, 

 again corresponds with the first hibernating generation with which we started. 

 Some of the second winged generation (II.) of females, however, remained on the fir- 

 tree. From the unfertilised eggs of these females which remained arises an unwinged 

 generation of females, and these, again, are succeeded by a winged summer generation, 

 and so on. In this second parallel series of generations of Chernies individuals 

 which remained on the fir-tree, two generations of females appear yearly, alternately 

 winged and wingless, both reproducing parthenogenetically. Now it is probable 

 that in this parallel series the generations do not thus reproduce (parthenogenetically) 

 ad infinitmn, but rather that, sooner or later, the parallel series re-enters the original 

 series, so that then a generation of males and females again ajjpears. The different 

 generations differ considerably from each other in form, even apart from the alternat- 

 ing absence or presence of wings. 



The Aphides afford an example of a kind of cyclic reproduction (heterogeny) in 

 which the parthenogenetically reproducing females are viviparous. The unfertilised 

 eggs here develop within the mother body. A similar phenomenon occurs in the 

 Diptera {Cecidomyia) also ; here, however, in the generation of females which repro- 

 duces parthenogenetically, the germarium which corresponds with the ovary becomes 

 mature very early, i.e. in the larval stage. The unfertilised eggs are here developed 

 within the larval body ; thus, in the cycle of reproduction of Cecidomyia, an imaginal 

 generation reproducing sexually by means of fertilised eggs alternates with several 

 generations of parthenogenetically reproducing viviparous larvie. This special kind 

 of heterogeny is called Psedogenesis. In one species of Ghironomus also the pupa 

 may occasionally lay eggs, which develop just in the same way as the fertilised eggs 

 of the imago. 



D. Development of the Myriapoda. 



The embryonic development of the Myriapoda, as far as it is known, does not 

 greatly differ from that of Inseda. Embryonic envelopes, however, do not appear 

 to form. 



AVhen the young Myriapoda are hatched, they are either provided with the 

 definitive number of segments and legs, as is the case in the Scolopendridce and 

 GeopliiUdcc {Chilopoda), or they possess a smaller number, to which the missin" 



