CABBAGES AND TURNIPS 183 



veloped Ichneumon is a small four-winged fly, with 

 piercing ovipositor, or egg-shoot, and long antennae, 

 which vibrate rapidly, as if to gain from the sur- 

 rounding air some intimation of the neighbourhood 

 of their prey. 



There is, in this as in other like cases, a peculiar 

 relation between the abundance of the caterpillars 

 and the abundance of the parasites, which is only 

 fully brought out by long-continued observation. Let 

 us take as the first of a series of years one in which 

 the caterpillars are plentiful and the Ichneumons few. 

 This state of things favours the increase of the 

 Ichneumons. The caterpillars become infested in 

 large proportion, few pupae yield Butterflies, many 

 yield Ichneumons. When things come to a climax 

 the Ichneumons are extraordinarily plentiful, but 

 their victims fewer than usual. Then the Ichneumons 

 suffer from mutual competition, and many die without 

 propagating their kind. As their numbers decline, 

 the numbers of the caterpillars increase. So the 

 cycle comes round time after time, the maximum of 

 the Ichneumons lagging behind the maximum (and 

 in some cases nearly coinciding with the minimum) of 

 the caterpillars. 



CABBAGES AND TURNIPS. 



My cabbage-plot is, I must admit, ridiculously small, 

 but it gives me plenty of opportunity for observation. 

 The difficulties of the young plants in dry spring 

 weather, and their rapid growth in a wet June are 

 among the little events of our year. The way in 



