REMARKS ON INJURIOUS AND BENEFICIAL INSECTS 21 



to them as external parasites and gradually sucking their life out. Or, 

 again, the mother fly may deposit her egg or eggs actually in the 

 grubs of the host insect through a small orifice pricked in the skin. 

 The parasitic grubs will then live inside the host-grubs as internal 

 parasites. 



Some of these parasites of wood-infesting insects are of considerable 

 size, and may be specially furnished with a long apparatus at the end of the 

 body termed an ovipositor. This the female inserts down through crevices 

 of thick-barked trees such as the spruce of the Western Himalaya, so as to 

 reach the egg or young larva of the host. An example of this class is the 

 Rhyssa Ichneumon which parasitizes the Sirex saw-fly. 



It will be found to be a common rule in the case of many of these 

 parasitic grubs that they do not immediately kill the host-grub. The latter 

 grows on until it reaches full size and has 

 carried out all its duties up to the preparation 

 of the pupal chamber. The larva gradually 

 grows feebler and feebler, and, finally, its work 

 done, dies of exhaustion. The parasitic grub 

 has by then reached full growth, and pupates 

 in the pupal chamber of its host, the fly 

 maturing here and escaping from the trees 

 by various methods, which differ for different 

 species, to seek out a mate, when the cycle is 

 commenced over again. 



The large number of parasites which prey 

 upon caterpillars live in much the same way, FlG ^_ Rhvssa p ersuasoria , 

 usually as internal parasites. When the Linn., parasitic on Sirex imperialis. 

 caterpillar has reached full growth and dies J - w - Himalaya. 



of exhaustion, they may pierce through its dried skin and pupate outside 

 close by, or they may wait till the caterpillar has formed its chrysalis case. 

 The caterpillar pupa then dies and shrivels up inside, its place being occu- 

 pied by the now fully-grown parasitic grub (or grubs), which pupates inside 

 the chrysalis case. 



A few instances of useful parasites to the forester may be quoted as 

 examples. 



The order Hymenoptera contains families of great importance in 

 this direction ; the Ichneumonidae, Chalcididae, and Braconidae, for 

 instance. 



Species of Ichneumon of use in the forest are numerous. 



Rhyssa persuasoria is parasitic upon the grubs of the Sirex imperialis, 

 which tunnel into spruce timber. Its grubs live as external parasites and 

 accompany the Sirex grubs into the heart of the tree. 



A species of Ichneumon, a small fly, lays its eggs in the tunnels of 

 the deodar pest, Scolytus major (p. 568), the grub also feeding as an 

 external parasite upon the bark-borer grub. 



