28 



them off to suck their blood at leisure." That such is the case 

 with the above species I have had abundant evidence, as I have 

 frequently found it with its beak inserted into the stems and buds 

 of various plants, and also, as stated above, with a caterpillar 

 dangling from the end of its beak. Such being the case it is diffi- 

 cult to decide whether these insects are beneficial or injurious ; but 

 it is a fact that there is scarcely a predaceous insect which, when 

 unable to obtain its accustomed food, will not partake of a vegeta- 

 ble diet. 



The only insects which may be regarded as being wholly benefi- 

 cial, are those parasites which, in their larval state, live within 

 the bodies of caterpillars and other vegetable-feeding larvae, and 

 which ultimately cause the death of the insects which they infest. 

 They belong to the two orders Hymenoptera or wasp-like insects, 

 and Diptera or two-winged flies. I am not aware that any para- 

 site belonging to the first named order has ever been known to in- 

 fest the Yellow Canker-worm. The only parasite known to infest it 

 is the Tachina-fly spoken of above, and which belongs to the Order 

 Diptera. The Canker-worms infested by this parasite may usually 

 be distinguished from those not infested, by the white raised spots 

 which are placed on some part of the body, usually on the back 

 near the head, and which, as above stated, are the eggs of this fly. 

 These spots, however, are not always a sure indication that the 

 caterpillar on whose body they are placed is parasitized, as I have 

 on several occasions obtained moths from caterpillars on whose 

 bodies from one to five of these eggs were placed. In such cases 

 the eggs may have been infested by some minute Hymenopterous 

 parasite — probably a Chalcis-fly, as I have sometimes found a cir- 

 cular opening in the upper side of the egg, through which the par- 

 asite had evidently escaped ; or the eggs may have been unimpreg- 

 nated. Neither are they a sure indication that the caterpillar is 

 infested with the larva of a Tachina-fly, as I have on three different 

 occasions bred Hymenopterous parasites from caterpillars on whose 

 bodies several of these eggs were placed. I have also had Tachina- 

 flies from caterpillars on whose bodies not one of these eggs could 

 be found. In these instances the eggs had probably been attached 

 to the body of the caterpillar before it moulted, and after moulting, 

 the eggs, being attached only to the skin, were cast off with the 

 latter ; but the grubs, or maggots, hatching out before the eggs were 

 cast off, had entered the caterpillar's body. 



It is said of those Hymenopterous parasites which live singly 

 within the bodies of caterpillars, that the parent fly can tell 

 whether a caterpillar contains a parasite or not, 'and that she 

 never deposits an egg in a caterpillar whose body already contains 

 a parasite ; but this faculty appears to have been denied the 

 Tachina-fly. In fact, she seems to have been endowed with but 

 very little instinct, especially in regard to properly estimating the 

 number of its larvae which a certain caterpillar will nourish. I 

 have occasionally found caterpillars whose bodies were scarcely 

 large enough to maintain a single Tachina larva, and yet they had 

 five or six Tachina eggs attached to each of them. 



To many persons it may seem strange that a caterpillar can live 

 so long after being infested with these parasitic larvae. It has been 



