l68 THE INDUSTRIES OF ANIMALS. 



stopping at a layer of summer-growth. It there 

 hollows a large grotto, leaving here and there pillars 

 of support. It lays in this space. The new genera- 

 tion working around the parents increases the space 

 and feeds on the wood removed. A second genera- 

 tion is produced, and the inhabitants become pressed 

 for space. The new-born pierce numerous passages 

 and penetrate towards the interior of the pile as far as 

 the next summer layer. There they spread them- 

 selves, always boring; they construct new rooms like 

 the first, and arrange pillars here and there. Their 

 descendants gain the subjacent zone, and so the 

 process goes on. During this time the early ancestors 

 who hollowed the surface dwellings have died, and 

 the holes which they made are no longer habitable ; 

 but they have all contributed to diminish the resist- 

 ance of the wood, and this continues as long as the 

 race which they produced makes its way towards the 

 centre of the stake. 



An insect, the Xylocopa violacea (Fig. 22), related 

 to our Humble-bee, from which it differs in several 

 anatomical characters, and by the dark violet tint of 

 its wings, brings an improvement to the formation of 

 the shelter which it makes in wood for its larvae. 

 Instead of hollowing a mere retreat to place there all 

 its eggs indiscriminately, it divides them into compart- 

 ments, separated by horizontal partitions. It is the 

 female alone who accomplishes this task, connected 

 with the function of perpetuating the race. She 

 chooses an old tree-trunk, a pole, or the post of a 

 fence, exposed to the sun and already worm-eaten, so 

 that her labour may be lightened. She first attacks 

 the wood perpendicularly to the surface, then sud- 

 denly turns and directs downwards the passage, the 



