THEIR RELATION TO EACH OTHER 



123 



forms a larva which develops as the host develops 

 and finally, when the latter is full grown, it is filled 

 with minute, maggot-like grubs ready to form pupa 

 cases which will fill the caterpillar skin so completely 

 that it seems ready to burst. From a single parasite 

 egg, we may thus get fifty 

 or more adults; but they 

 will all be of one sex as 

 determined by the egg 

 originally laid. It will be 

 readily seen what enor- 

 mous reproductive powers 

 some of these minute spec- 

 imens really have, for even 

 if each laid only ten eggs 

 and each egg produced 

 fifty adults, the progeny 

 would still number 500: 

 not at all bad for such 

 small creatures ! 



With such enormous 

 powers of reproduction, it 

 seems surprising that the 

 hosts are not completely 

 exterminated; and yet as 

 we know, they are not. 

 If we collect chrysalids of 



cabbage butterflies in spring, the chances are that out 

 of one hundred we may get ten butterflies and several 

 thousand parasites; more than enough it would seem to 

 overpower the next brood of caterpillars completely. 

 But now, if we collect full-grown caterpillars from the 

 scant lot produced by the few spring butterflies, we are 

 likely to get a butterfly from every caterpillar. From 

 the second brood we are likely to get almost as clean a 



Fig. 6r. — a. Listomastix parasite 

 laying a single egg in the egg of a moth ; 

 b. the full-grown caterpillar with par- 

 asitic cocoons from the single egg. 

 After Marchal. 



