INSECTS 



387 



she bores holes in the trunks of sickly fir-trees, and lays her 

 eggs therein. The structure of this organ is complex, but may be 

 understood by reference to figs. 908 and 909. It consists of two 

 protective pieces, which can be brought together so as to en- 

 sheathe the part by which the boring is effected. This is made 

 up of three rods, one of which is relatively broad, and provided 

 below with a pair of ridges, on which two boring spines can 

 slide up and down. Each of these is studded with saw-like teeth 

 near its tip. The larva is a pale grub-like creature practically 

 devoid of limbs, though three pairs of little projections near 

 the front end of the body correspond to the legs of the adult. 

 It feeds on the wood of the tree, biting out long galleries, and 

 is said to remain in the larval state 

 for as much as two years. When full 

 grown it travels towards the surface 

 of the trunk or branch, and becomes 

 a pupa, as which it may remain for a 

 long time. The imago, after emerg- 

 ing from the pupal skin, gnaws its 

 way to the exterior. Fabre states that 

 in an allied species {^Sirex augur) the 

 galleries of the larva are longitudinal, 

 and that it becomes a pupa in one of 

 these, making no attempt to come near the surface before doing 

 so. When the imago makes its appearance it does not try to 

 get to the open air by travelling along the gallery in which it 

 finds itself, but bores a transverse passage that takes it direct 

 to the exterior. This is only one of many instances in which 

 animals possess a sense of direction far more highly developed 

 than our own somewhat imperfectly developed locality sense. 

 There are possibly special sense organs related to this important 

 faculty, which is here of vital importance, as without it many of 

 the adult insects would never get out of the tree at all. 



The Corn Saw- Fly {Cephus pygmceus, fig. 910) passes through 

 a life-history presenting similar stages to those described for the 

 Wood-Wasp. Among cultivated plants wheat and rye are the 

 special objects of its attentions. In such plants the stem or haulm 

 is marked by a series of swollen solid nodes or "knots", from 

 which the leaves grow out, and between which are long hollow 

 internodes. The female Saw- Fly lays about a dozen eggs, de- 



Fig. 909. — Cross-section through Ovipositor 

 and Sheath of Wood - Wasp [Sirex gigas), 

 greatly enlarged, ab and a' b' , sheath; e and 

 e , boring spines, which slide on the ridges of 

 a directing piece [c d). 



