SCORPIONS, SPIDERS AND CRABS 261 



the females fight their rivals for the possession of the 

 males, which are, in these species, far less numerous than 

 the females. 



Among the lower Crustacea, such as the " Fairy- 

 shrimps," " Brine-shrimps," the " Water-fleas," and the 

 " Copepoda," which play so important a part in furnishing 

 food for many of the fishes which in turn feed us, secondary 

 sexual characters of an extremely interesting kind are 

 met with. These, however, are never such as appeal to 

 the eye, for the vision in these creatures is but feebly 

 developed. Scent, as is usual where sight is defective, 

 plays an important part in enabling the sexes to discover 

 one another. Selection here secures success only to 

 such as have the proper odour and the most sensitive 

 organs of smell. In these creatures, as with the butter- 

 flies and moths, the odour emanating from the female 

 is most powerful, while the sense of smell is most developed 

 in the male. One of the most striking illustrations of 

 these facts is furnished by that very beautiful species 

 Leptodora hyalina — a veritable giant among these small 

 Crustacea — wherein the antennae of the male are produced 

 into enormously elongated comb-like structures, the 

 teeth of the comb being formed by deHcate olfactory 

 filaments. In the female these antennae are extremely 

 short and their olfactory filaments are limited to a small 

 terminal tuft to the antennae, answering to the larger 

 tuft at the base of the comb of the male. 



To the majority of species, however, delicate odours 

 seem to make little or no appeal, since excessive develop- 

 ment of the olfactory apparatus, such as is seen in the 

 aberrant Water-flea (JLeptodora\ is rare. This is perhaps 

 explained by the fact that Leptodora is a species which 

 does not herd together in vast numbers, hence, probably, 



