SEXES REPRODUCTION. 339 



of all the great divisions of the Arthropoda which live in air 

 tracheal structures have been developed. Those of Peripatus, 

 the Myriapods and Insects need not be further mentioned here. 

 In the Arachnida tracheae are present in addition to the lung- 

 books, or as the sole respiratory organs. Even the Crustacea 

 furnish us with an incipient tracheal system in the tufts of 

 branching air-tubes found in the abdominal legs of some Isopods 

 (wood-lice) (Fig. 297).* 



The sexes are separate in the great majority of Arthropods, 

 but the Cirripedes, having adopted a sessile habit in the adult 

 stage, are hermaphrodite, and some genera of them present the 

 remarkable phenomenon, discovered by Darwin, of the existence 

 of a minute and short-lived "complementary male " in addi- 

 tion to the hermaphrodite individuals of their species. Certain 

 of the Isopoda, parasites on fish or other Crustacea, are protan- 

 drous hermaphrodites (cf. pp. 484 and 490). 



Parthenogenesis occurs as a constant feature of the life- 

 history in many genera of the Phyllopoda and Ostracoda among 

 the Crustacea, and of the Hymenoptera and Hemiptera among 

 Insects. The number of generations arising in this manner 

 before the recurrence of a sexual phase may be large, as in 

 Aphis, and many genera of the Entomostraca ; in some of the 

 latter indeed (Cypris reptans, Limnadia Hermanni), as well as 

 in some Insects (cf. p. 640), males are unknown. 



Among the Crustacea the parthenogenetic females are usually 

 similar in their general structure to those which are capable 

 of being fertilized, but in Insects the members of the life-cycle 

 are frequently dimorphic, or (Phylloxera) trimorphic, and differ 

 so considerably from one another that, before their life histories 

 were ascertained, they were referred to distinct genera. 



The eggs of Arthropods are usually heavily laden with yolk. 

 In some few Crustacea, in which there is little yolk stored in 

 the egg, the cleavage is complete, and in the pelagic Decapod 

 Leucifer the hypoblast is formed by a normal process of invagina- 

 tion. But in the great majority of Arthropods the course of the 

 segmentation is modified owing to the presence of the yolk. 

 It is generally stored in the central region of the egg and though 

 segmentation may be complete at first the large yolk-laden 

 central spheres often fuse together subsequently ; or the 



* Compare the development of tracheae in the Velellidae, vol. I, p. 142. 



