PAEASITE OF CABBAGE BUTTERFLIES. 163 



It will be noticed that these Ichneumon Flies are 

 all very much alike in shape, and that their wings are 

 much veined. 



The Parasite Wasp-flies of the two other families, 

 Avhich are most important to us {CJialcidiike and 

 Proctotrupidce) , are for the most part very small, 

 with almost, or quite, veinless wings. 



The Pteromalus imparum (Fig. 127, magnified, one 

 of the Chalcididce) is one of the kinds which destroy 

 chrysalids of the Large and Small Cabbage Butterflies. 

 This very small Fly is stated to lay its eggs on the 

 outside of the chrysalis, as soon as it has cast off its 

 caterpillar skin, and is still soft and tender. The 

 maggots from these eggs soon hatch and eat into the 

 ■chrysalis, and sometimes as many as from two 

 hundred to three hundred live thus in one chrysalis, 

 where they change to the perfect Parasite Fly, and 

 come out soon in summer to 

 continue the useful work. In 

 winter some remain in the 

 chrysalis. 



The use of these Pteromali 

 is well shown by their work 

 in North America, in checking 

 increase of Cabbage Butterfly. 

 /"^% 127- -Parasite of Tj^^ g^^^jj -^yj^-^^ Cabbage 



■chrysalis of Cabbage i3ut- t> n n i -j. 



teriiics, magnified. Butterfly made its appearance 



(or at least was first observed 

 there) at Quebec about 1859, and gradually spread and 

 caused much damage, but for several years no Para- 

 site appeared to check it. In 1871 — that is, about 

 twelve years after the appearance of the Butterfly 

 — it was announced that the Parasite we are now 

 speaking of had appeared : the British Parasite had 

 followed the British Butterfly, and was doing its 

 appointed work. In 1875 it (the Parasite) had be- 

 come quite common in the State of New York ; and 

 later on it was observed to be still on the increase. 

 The great benefit gained by natural helpers of this 



