268 



ARTHROPODS, 



into the apparent body-cavity. Their number varies from 

 two (in some Lepidoptera for instance) to one hundred and 

 fifty (in the bee). They twine about the organs in the 

 abdominal cavity, and their excretory significance is proved 

 by the fact that they contain uric acid. 



Reproductive System. — Among Insects the sexes are always 

 separate and often different in appearance. The males are 

 more active, smaller, and more brightly coloured than the 

 females. Darwin referred the greater decorativeness 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 plainness of females to 

 the elimination of the disadvantageously conspicuous in the 

 course of natural selection. There is apparently truth in 

 both views, and also in a third theory, in part accepted by 

 Wallace, that the " secondary sexual characters " of both 

 sexes are the natural and necessary expressions of their 

 respectively dominant constitutions. 



Reproductive Organs. 



Male. 



The paired Testes usually consist of 

 many small tubes. 



Two ducts (vasa deferentia), conducting 

 spermatozoa (perhaps in part com- 

 parable to nephridia). 



An unpaired terminal and ejaculatory 

 duct, paired and with two apertures 

 in Ephemerids only ; sometimes 

 formed by a union of the vasa defer- 

 entia, sometimes by an external inva- 

 gination meeting the vasa deferentia. 



From_ the vasa deferentia or from the 

 ejaculatory duct, a paired or unpaired 

 seminal vesicle for storing sper- 

 matozoa. 



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 consist of 

 many small tubes (ovarioles). 



Two ducts (oviducts), conducting the ova 

 (perhaps in part comparable to ne- 

 phridia). 



An unpaired terminal region or vagina, 

 paired, and with two apertures in 

 Ephemerids ; usually formed from 

 anexternal invagination meeting the 

 united ends of the oviducts. 



From the vagina, a receptaculum seminis 

 for storing spermatozoa received from 

 a male during copulation. 



Various accessory glands, e.g., those 

 which secrete the material surround- 

 ing- the eggs. _ 



Sometimes a special bursa copulatrix in 

 the vagfina. 



Often external hard pieces, e.g.. ovi- 

 positor. 



