How insects behave 



Fig. 62. A wingless mutillid wasp, 

 Order Hymenoptera-Aculeata 



appear from time to time, especially if living conditions become un- 

 favourable, and then fly to other plants, where fresh colonies are estab- 

 lished. Males and egg-laying females appear in the autumn. The over- 

 wintering eggs are usually laid on the twigs of woody plants, where the 

 young larvae emerging in the spring can feed on the opening buds. 



Insects tend to gather in communities wherever an abundance of 

 food brings them together. 'Plagues' of caterpillars are an example of 

 how the coincidence of several favourable factors can bring about a 

 sudden increase of population. One factor is the weather, particularly 

 that of the preceding winter. Over- wintering inseas are biologically 

 adjusted to tolerate a certain amount of cold, and even of frost, and often 

 cannot develop properly without a resting-period, or diapause, in cold 

 weather. A mild, open winter keeps them too active, uses up their store 

 of energy-giving food too soon, and exposes them to the attacks of birds. 

 On the other hand, a late, cold spring means delay in starting the season 

 of growth and reproduction. In most years one factor will be favourable, 

 others not; but occasionally there is a year in which all the factors are 

 in the insect's favour, and a big population is the result. 



An early season is most helpful to insects that have several generations 

 during the summer. If the population increases fourfold at each genera- 

 tion, then a season that permits one extra generation to be fitted in will 

 give a final population four times as big as usual. This may account for 



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