THE INSECT WORLD. 



379 



a, two-jointed trochanter 

 of parasitic Hymenoptera ; 

 b, normal structure. 



pointed out, since the insects are usually small in size ; indeed, it 

 is a matter of little importance to the farmer, because in any case 

 he can look upon these gall-wasps with indifference, and without 

 much fear of possible injury to himself 



There is a very large series of parasitic Hymenoptera, and it 

 contains a number of families, nearly all the species of which are 

 beneficial. To the practised eye a doubt rarely exists as to 

 whether an insect is parasitic or not ; but 

 that the student may be able to decide 

 this matter for himself if he chooses, it 

 may be pointed out that the trochanter, 

 the little segment between the coxa and 

 femur of the leg, is rather peculiarly de- 

 veloped here. It is normally composed 

 of one joint only in all the bees, wasps, and 

 ants, whatever their size or appearance, 

 but it is two-jointed in all the parasites. 

 This character can be easily seen on large 

 specimens with or without a lens, but on 



minute species it becomes more difficult ; and, after all, in most 

 cases the farmer is justified in assuming that most of the minute 

 wasp-like creatures that he notices are parasites and beneficial. 



One feature characteristic of a great number of species is an 

 external ovipositor, or egg-laying tube, and this may vary from 

 a scarcely visible projection to an enormously developed hair- 

 like appendage five to six or even more inches in length. 



There may be frequently seen on the trunks of various trees a 

 quite large ichneumon, a Thalessa, yellow or black in color ac- 

 cording to the species, which has its long, bristle-like ovipositor 

 forced deep into the wood, and so firmly fixed, occasionally, that 

 it is unable to withdraw it, and perishes miserably. Wherever 

 this occurs the trees will be found infested by borers, and all but 

 universally this ichneumon is accused of being the parent of the 

 larva that produces the injury. The circumstantial evidence is 

 all against it ; but we really have here a beneficial species which 

 is after the wood-boring larva of the ' ' pigeon Tremex. ' ' The 

 insects seem able to recognize an infested tree, and pierce the 

 trunk until the ovipositor reaches one of the burrows of the wood- 

 feeding larva. In this an egg is laid, and the larval parasite, 



