310 FLASHLIGHTS ON NATURE 



most closely approach its own peculiar ground- 

 tint. 



It is a curious fact, however, that in spite of all 

 the apparent pains bestowed upon securing the 

 perpetuation of such destructive creatures as the 

 Hessian fly, the pest itself has its own enemies, 

 as fatal to its life as it is to the barley. Ichneumon 

 flies and other parasites prey by millions on the 

 Hessian fly in its grub condition ; and many good 

 authorities believe that the safest way of checking 

 the depredations of the barley-plague is by en- 

 couraging the multiplication of its natural enemies. 

 No. 15 shows us one of these industrious little 

 scourges actually at work. She alights on a stem 

 of barley infested by grubs of the Hessian fly, and 

 walks slowly along it, tapping gently as she goes, 

 much as a woodpecker taps with his bill on a 

 tree-trunk to discover the spot where a worm lies 

 buried. After carefully examining the surface, she 

 finds at last a place where something, either .in 

 the sound or the feeling of the stem, reveals to 

 her the presence of a Hessian fly grub within the 

 leaf-sheath. Having accurately diagnosed the spot 

 (like a doctor with a stethoscope), she brings her 

 ovipositor (in plain English, her egg-layer) just 

 above the' place where the grub is lying snug in 

 its green bed, and pierces the hard leaf-blade with 

 her sharp little lancet. Then she lays her egg in 

 the body of the larva. This egg gives rise in time 

 to a parasitic grub, inside the first one ; and the 

 parasite eats out his host's body, and emerges in 

 due time as a full-grown fly, ready to carry on the 



