202 NATURAL HISTORY OF WASPS. 
inner walls, or of the adjoming cells. Still, as the 
comb grows by constant additions to the outer edge, 
its weakest point, its line of fracture, runs in concen- 
tric circles. And any attempt to wash an old comb 
is likely, without the greatest care, to induce a tear 
in this direction. The sides are built up, streak by 
streak, as the cells expand from narrow pouches into 
large regular hexagons formed with geometrical pre- 
cision. To allow for this gradual expansion, the 
comb takes a curved form, presenting the convexity 
downwards. The central cells are vertical; a little 
farther off from the centre they only maintain this 
direction by having the first narrow portion turned 
round, like a little horn. Still farther out this expe- 
dient is insufficient to meet the increasing divergence, 
and at last, in some species, the cells are placed 
almost horizontally. 
With this exception of the direction of the outer 
cells, all the species alike make their comb on the 
same pattern, or as nearly so as the capabilities of the 
material employed will allow. The brittle wood chips 
which V. Crabro and V. vulgaris generally employ do 
not allow of the same lightness of construction as 
the tough fibres which other species prefer. So their 
combs are thick, and the cells stand parallel, quite up 
to the edge; and the structure has none of the con- 
vexity which gives such a light, graceful appearance 
to the large combs of the tree-wasps. 
The upper surface of the comb has little to attract 
our attention. It is of a dull dirty aspect, studded 
over with little round points, each of these eminences 
corresponding to the bottom of a cell. The open 
space between this and the under surface of the stage 
