INSECT SOCIETIES 



will eat. The solitary bees and wasps, which lay still fewer 

 eggs than most, look after them even more. Although no 

 solitary ants or termites are known, we can be certain that 

 the ants had solitary, wasp-like ancestors, to whose habits 

 the colony-founding queen of most species reverts. The past 

 history of the termites is less well understood, and it is 

 possible that co-operation between males and females, rather 

 as in the bark beetles, may have existed before they were 

 really social. 



The reduction in the egg-number of nest-making insects is 

 a redistribution of physiological energy. If the female is to 

 live longer and to work hard at making a nest, it is impossible 

 for her to produce large numbers of eggs at the same time. 

 It may be in a sense an accident of the past history of a 

 species which determines whether it is capable of balancing a 

 reduction in eggs by taking better care of them. 



Nevertheless, some maternal care has appeared again and 

 again in the course x)f evolution. Nest building occurs in vari- 

 ous fish, such as the stickleback. Here the eggs are guarded 

 by the male and the number laid by the female is very small. 

 The land vertebrates, especially birds and mammals, nearly 

 always produce fewer young than any solitary bee or wasp, 

 but show a similar or higher degree of care for them. Most 

 species restrict their breeding to a definite season of the year. 

 Even in tropical countries there are usually seasonal differ- 

 ences in rainfall or in the quality of the light, and the breeding 

 of most animals is adjusted to the seasons. Thus one very 

 common restriction on reproduction is to confine it to a 

 relatively short breeding season. This not only slows down 

 multiplication but adjusts it to the most favourable time of 

 the year. 



Many birds, such as our common finches, aggregate into 

 flocks in the autumn and winter, and separate into pairs only 



N 193 



