72 INSECT ARTIZANS AND THEIR WORK 



branches below into three or four cells ; but 

 instead of sweeping away the pellets it quarries, it 

 uses them for building up around the mouth of 

 its excavation a round tower, at first straight, then 

 curved to correspond with the curvature of its 

 own body. The use of this tower appears to be 

 to make it difficult for any parasitical insect, during 

 its own absence in obtaining provisions, to en er 

 and lay eggs in the cavity. It is only a temporary 

 erection, and therefore it is not made solid ; the 

 stones of which it is constructed, though firmly 

 connected, leave little interstices, as though it 

 were anxious to make its material go as far as 

 possible. 



The nest is furnished with a number of small 

 green caterpillars which naturally curl into a circle 

 when alarmed. These, of course, are first stung, 

 so that they have little or no power of movement. 

 The egg is laid in the far end of the cell, so that 

 on hatching the grub first attacks the caterpillar 

 that was first stung. 



After filling up the cell with from twenty to 

 forty small caterpillars, the Mason Wasp takes down 

 her tower, stone by stone, and uses the material's 

 for building up the mouth of the nest solidly. The 

 building of this tower is therefore a fine example 

 of economy of labour. Instead of dropping the 

 material excavated from the hard sand-bank and 

 having to collect it or similar material again for 

 building up the entrance, she stacks it ready to 

 hand, and in the doing of it contrives a shelter 



