CARPENTERS AND WOOD-WORKERS 123 



only to press its head against this cap to push it 

 out. 



Before concluding this chapter a word should 

 be said about the Saw-flies (Tenthredinidae), as 

 nature has provided them with saws, though as a 

 rule they only use them upon leaves and green 

 shoots. It is only the females that are provided 

 with these useful tools, and they use them for 

 cutting a slit in which to deposit their eggs. A large 

 kind of Saw-fly, however, known as the Large 

 Horn-tail (Sirex gigas) has, instead, a powerful 

 drill with which she bores holes in the bark of 

 pine-trees for the purpose of laying an egg in each 

 of the holes she drills. The drill has a protecting 

 sheath in which it lies when not in use. It is 

 three-quarters of an inch long, and is hinged so as 

 to be used at right angles to the body. The grub 

 which issues from the egg is a notable carpenter 

 too, but it works with its jaws, and spends several 

 years in excavating a nice long tunnel in the solid 

 wood. 



It often happens that between the time of the 

 egg-laying and the completion of the insect's 

 development the woodman has come along and 

 cut down the tree ; the trunk has been cut up 

 into beams or flooring planks and used in the 

 construction of a dwelling-house. Then in due 

 time the Horn-tail, undisturbed by any of these 

 happenings, attempts to complete its destiny by 

 coming forth fully winged. But the builder who 

 used the wood for roofing beams laid upon them 



