Beneficial insects. 



333 



their victims. This is especially noticeable in the case of 

 the corn aphis [Aphis avcnce) and of the hop aphis [Phorodon 

 hitimili). Another of the same order infests the destructive 

 ribbon-footed corn fly [Chlorops taeniopns] and sometimes 

 materially diminishes its numbers. 



At the time when so much fear was felt that the Hessian 

 fly would cause great mischief in British cornfields, in 1886, 

 it was discovered that this insect was liable to extensive 

 infestation by one of the ichneumon flies [Semt'otelhis fiigripes). 

 Professor Riley, the late United States Entomologist, stated 

 that this parasite of the Hessian fly was not known in the 

 United States, and he was so impressed by the reports of its 

 action that he accepted Mr. Knock's offer to send him some 

 for distribution in that country. With regard to this fly 

 Professor Riley reported as follows in 1894: — "Parasitised 

 puparia of the Hessian fly were received in large numbers 

 and distributed to various points, and placed in the hands of 

 competent observers. The results so far have not been 

 marked, and but one positive report as to the acclimatisation 

 of the parasite has been received — viz., from Professor S. A. 

 Forbes, of Champaign, Illinois. I am, however, of opinion 

 that the lack of evidence is due almost entirely to lack of 

 proper examination, and I have every hope that the species 

 before long will be found to have obtained a secure foothold 

 at all the several points of distribution. It is very difficult 

 to ascertain the existence of a parasite of this minute size,, 

 except where it occurs in great numbers. The reason for 

 attempting the introduction of this particular species was 

 simply that in England it was found to be far more abundant 

 and far more beneficial than any of our native species have 

 so far proved." 



It is due to this ichneumon fly, as well as to the unfavour- 

 able climatic conditions, that the Hessian fly has not caused 

 any important harm in Great Britain. The fly is only about 

 the eighth of an inch long, with a wing expanse of about the 

 fourth of an inch. It is black, with yellow colourings on parts 

 of its body, and a metallic lustre upon the thorax. 



There are many other species of the Hymenoptera which 

 parasitise and destroy noxious insects ; as, for example, the 



