348 



PHYLUM ARTHROPODA. 



Reproductive system. Among Insects the sexes ar< 

 always separate and often different in appearance. The 

 males are more active, smaller, and more brightly coloured 

 than the females. Darwin referred the greater decorative- 

 ness of the males to the sexual selection exercised by the 

 females. The handsomer variations succeeded in courtship 

 better than their rivals. Wallace referred the greater plain- 

 ness of females to the elimination of the disadvantageously 

 conspicuous in the course of natural selection. There may 

 be truth in both views, but both require to be supplemented 

 by the consideration, in part accepted by Wallace, that the 

 " secondary sexual characters " of both sexes are the natural 

 and necessary expressions of their respectively dominant 

 constitutions. 



The organs consist of: 



MALE. 



FEMALE. 



The paired testes, usually formed 

 of many small tubes. 



Two ducts (vasa deferentia) con- 

 ducting spermatozoa (perhaps 

 in part comparable to neph- 

 ridia). 



An unpaired terminal and ejacula- 

 tory duct, paired and with two 

 apertures in Ephemeridsonly ; 

 sometimes formed by a union of 

 the vasa deferentia, sometimes 

 by an external invagination 

 meeting the vasa deferentia. 



From the vasa deferentia or from 

 the ejaculatory duct, opens a 

 paired or unpaired seminal 

 vesicle for spermatozoa. 



Various accessory glands, whose 

 secretion sometimes unites the 

 spermatozoa into packets or 

 spermatophores. 



Sometimes a copulatory penis. 



Often external hard pieces. 



The paired ovaries, usually formed 

 of manysmall tubes(ovarioles). 



Two ducts (oviducts) conducting 

 the ova (perhaps in part com- 

 parable to nephridia). 



An unpaired terminal region or 

 vagina, paired and with two 

 apertures in Ephemerids ; 

 usually formed from an ex- 

 ternal invagination meeting 

 the united ends of the oviducts. 



Near or from the vagina, opens 

 a receptaculum seminis for 

 storing spermatozoa received 

 from a male during copulation. 



Various accessory glands,!?.^, those 

 which secrete the material sur- 

 rounding the eggs. 



Sometimes a special bursacopula- 



trix in the vagina. 

 Often external hard pieces, e.g., 



ovipositor. 



