58 THE DURATION OF LIFE. 



on land and in water. In the water, fish, frogs, newts, 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 freshwater 

 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 condition, 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 inequality 

 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 ' complementary 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 



