EXCRETORY AND REPRODUCTIVE SYSTEMS. 317 



But the apparent body cavity in which the organs lie, and in which 

 the blood circulates, is well developed in Insects. It includes, inter alia, 

 a peculiar fatty tissue, which seems to be a store of reserve material, 

 which is especially large in young insects before metamorphosis, and is 

 also interesting as one of the seats of "phosphorescence " in those insects 

 which glow. 



Excretory System. 



Although no structures certainly homologous with neph-~~ 

 ridia have yet been demonstrated in Insects, the excretory 

 system is well developed. From the hind gut (p^Ctodaeum), 

 and therefore of ectodermic origin, arise fine tubes, or in 

 some cases solid threads, which extend 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 inferred from 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 disadvantageous^ conspicuous in the 

 course of natural selection. There may be truth in both 

 views, but both require to be supplemented by the con- 

 sideration, in part accepted by Wallace, that the " secondary 

 sexual characters" of both sexes are the natural and necessary 

 expressions of their respectively dominant constitutions. 



