18 SOCIAL LIFE IN THE INSECT WORLD 



dumping-heaps, as are the burrows of the Geotrupes, 

 another notable excavator. The way in which the work 

 IS done is responsible for this difference. The dung- 

 beetle works from without inwards ; she begins to dig at 

 the mouth of the burrow, and afterwards re-ascends and 

 accumulates the excavated material on the surface. The 

 larva of the Cigale, on the contrary, works outward from 

 within, upward from below; it opens the door of exit at 

 the last moment, so that it is not free for the discharge 

 of excavated material until the work is done. The first 

 enters and raises a little rubbish-heap at the threshold 

 of her burrow ; the second emerges, and cannot, while 

 working, pile up its rubbish on a threshold which as 

 yet has no existence. 



The burrow of the Cigale descends about fifteen inches. 

 It is cylindrical, slightly twisted, according to the exi- 

 gencies of the soil, and always approaches the vertical, 

 or the direction of the shortest passage. It is perfectly 

 free along its entire length. We shall search in vain for 

 the rubbish which such an excavation must apparently 

 produce ; we shall find nothing of the sort. The burrow 

 terminates in a cul-de-sac, in a fairly roomy chamber 

 with unbroken walls, which shows not the least vestige 

 of communication with any other burrow or prolongation 

 of the shaft. 



Taking its length and diameter into account, we find 

 the excavation has a total volume of about twelve cubic 

 inches. What becomes of the earth which is removed ? 



Sunk in a very dry, crumbling soil, we should expect 

 the shaft and the chamber at the bottom to have soft, 

 powdery walls, subject to petty landslips, if no work 

 were done but that of excavation^ On the contrary, the 



