348 PHYLUM ARTHROPODA. 
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 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 comparab’e to neph- 
ridia). 
An unpaired terminal and ejacula- 
tory duct, paired and with two 
apertures in Ephemeridsonly ; 
sometimes formed by aunion 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 
ofmanysmall 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 ofthe oviducts. 
Near or from the vagina, opens 
a receptaculum seminis for 
storing spermatozoa received 
froma male during copulation. 
Various accessory glands, ¢.g. those 
which secrete the material sur- 
rounding the eggs. 
Sometimesa special bursacopula- 
trix in the vagina. 
Often external hard pieces, e.g., 
ovipositor. 
