GROWTH AND LIFE HISTORIES 



79 



made to relate them to the environmental conditions in which the 

 animals live. 



An annual cycle may be made up of several generations, none of 

 which survives for more than a fraction of a year; it may be per- 

 formed by individuals which live a little more than a year, or it 

 may involve individuals which survive for several years, but have 

 regular annual movements and breeding periods. Variation in the 

 details of such cycles can be immense; only a few examples can be 

 studied here. 



Fig. 37. Phyllosoma larva of Jassus (Decapoda Reptantia, 

 Palinura). 



The annual cycle of Daphnia magna in a temperate climate is a 

 good example of the first of the three main types. The population 

 overwinters as resting eggs, or as females which grow slowly and 

 reproduce very little. In the spring the resting eggs hatch, and the 

 females begin to reproduce more rapidly as the temperature of the 

 water rises and small algae increase to provide more food. As we 



