SAW-FI^Y (SIREX GIGAS) 21 



insects, some of which are wood-borers. Now 

 the host of persuasoria is the larva of Sirex 

 gigas, a very large saw-fly which, from its 

 colour resemblance, has the appearance of a 

 hornet. Like the ichneumon, it is also fur- 

 nished with an ovipositor, short and stout, an 

 appendage which adds greatly to its terrifying 

 appearance. With this instrument the Sirex 

 punctures the bark of fir-trees and deposits 

 an egg on the living wood. On hatching, the 

 grub eats its way into the tree and, by degrees, 

 excavates a tunnel in the hard wood until 

 nearly full fed, when it turns in an outward 

 direction and bores at right angles until 

 nothing but a thin partition remains between 

 it and the outside. The creature then ceases 

 work and prepares for pupation. In due 

 course the saw-fly emerges, and pushing its 

 way out is free to the world. This is what 

 happens when all goes well with the Sirex. 

 But though, apparently, so secure from out- 

 side interference, the Sirex larva has in 

 persuasoria a deadly enemy, for this wonder- 

 ful fly has the extraordinary power, not only 

 of discovering the presence of its intended 

 victim, but of locating the exact position 

 occupied by the larva which, it must be noted, 

 the ichneumon has never seen nor ever will 

 see. 



