THE WOOD-WASP loi 



common plan ; moreover, they are found only in female 

 insects. We conclude that they are homologous, or equivalent 

 structures ; at first, in all probability, framed as egg-forceps 

 for depositing the eggs in narrow crevices, but afterwards 

 taking on highly special functions, and becoming fit for boring, 

 cutting, or stinging. In all Hymenoptera the implement, 

 whatever its functions, is developed from three pairs of promin- 

 ences, which in the embryo stand out from the under surface 

 of the hinder segments. 



When the female Sirex is ready to lay, she chooses a fir- 

 tree, by preference one that is felled, uprooted, or sickly, bores 

 a hole through the bark into the young sapwood, and deposits 

 in it a single egg. The operation has rarely been witnessed, 

 and we know few of the details. Since the larvae are usually 

 found several or many together in one trunk, it is probable 

 that the female lays egg after egg in the same tree. In 

 boring, the valves are parted, and only the central part of 

 the ovipositor enters the hole. A fresh-hatched female con- 

 tains about a hundred eggs. After the majority are safely 

 lodged in trees, she grows faint and weary, and often dies 

 with her ovipositor inserted in the wood. Insects found dead 

 in this attitude are nearly always spent females, which have 

 died from exhaustion, sometimes, perhaps, because the borer 

 got jammed in the wood. 



The larva of the wood-wasp is long, cylindrical, pale and 

 fleshy, the head small and hard. Living always in the dark, 

 it has no eyes. It crushes its food with a pair of strong, 

 toothed mandibles, which do not form a symmetrical pair, 

 but differ in shape. Sedentary life has reduced the legs to 

 vestiges ; the thoracic legs are represented by small, unjointed 

 prominences, while there is hardly a trace of the pseudopods, 

 or false feet, which are so conspicuous in the caterpillar of the 

 saw-fly. The body ends in a hard and pointed tip, as in 

 the fly. Eating is the same thing as working to this larva, for 

 it mines by eating the wood, which is its only food. It works 

 lengthwise along the trunk, keeping at first to the sapwood. 

 Old larvae penetrate to a greater depth, but as the time of 

 pupation approaches, they take care to lie near the bark.. 

 In May or June the sound of their jaws can be heard if the 

 observer places his ear close to the trunk of an infected tree.. 

 Larvae which begin to feed late in summer resume their 



