I.] APPENDIX. 59 



ducks and other water-fowl, and on land many birds, the 

 hedgehog, toads, etc., largely depend upon them for food. 



If the principles developed in this essay apply to the fresh- 

 water Mollusca, we must then infer that snails which maintain 

 the mature condition — the capability of reproduction — for one 

 year, are in this state more exposed to destruction from the 

 attacks of enemies than those species which remain sexually 

 mature for two or three years, or that the latter suffer from a 

 greater proportional loss of eggs and young. 



Note 6. Unequal Length of Life in the two Sexes. 



This inequality is frequently found among insects. The 

 males of the remarkable little parasites infesting bees, the 

 Strepsiptera, only live for two to three hours in the mature con- 

 dition, while the wingless, maggot-like, female lives eight days : 

 in this case, therefore, the female lives sixty-four times as long 

 as the male. The explanation of these relations is obvious ; a 

 long life for the male would be useless to the species, while the 

 relatively long life of the female is a necessity for the species, 

 inasmuch as she is viviparous, and must nourish her young 

 until their birth. 



Again, the male of Phylloxera vastatrix lives for a much 

 shorter period than the female, and is devoid of proboscis and 

 stomach, and takes no food : it fertilizes the female as soon as 

 the last skin has been shed and then dies. 



Insects are not the only animals among which we find in- 

 equality in the length of life of the two sexes. Very little 

 attention has been hitherto directed to this matter, and we 

 therefore possess little or no accurate information as to the 

 duration of life in the sexes, but in some cases we can draw 

 inferences either from anatomical structure or from the mode 

 of development. Thus, male Rotifers never possess mouth, 

 stomach, or intestine, they cannot take food, and without doubt 

 live much shorter lives than the females, which are provided 

 with a complete alimentary canal. Again, the dwarf males of 

 many parasitic Copepods — low Crustacea — and the 'comple- 

 mentary males' of Cirrhipedes (or barnacles) are devoid of 

 stomach, and must live for a much shorter time than the females ; 

 and the male Entoniscidae (a family of which the species are 

 endo-parasitic in the larger Crustacea), although they can feed, 



