1985] 
Brockmann — Digger wasps 
319 
Origin and Adaptive Significance of Tool Use 
The preceding review of nest-closing behavior reveals that “tool 
use” is widespread among the Sphecinae, and is not limited to the 
Ammophila (Table 1 A). Many species pick up stones and press and 
vibrate them down into the substrate during nest closure, an action 
that almost certainly serves to compact the underlying soil at the 
nest entrance (Newcomer, 1930; Frisch, 1940; Spangler, 1973; per- 
sonal observation). Whether the object is removed or not, whether it 
is used repeatedly or only once, whether the wasp pounds or pushes 
with it, the result is almost certainly the same. If the stone is not 
removed, then in addition to being used as a compacting tool, it 
serves to block the nest entrance at the surface. The origin of this 
tool-using behavior among digging wasps coincides with the evolu- 
tion of packing by means of the head and mandibles rather than the 
abdomen. Most species of Sphecinae which have been observed to 
pack soil with their mandibles have been observed to manipulate 
and press stones or other objects into the substrate (Table 1A). 
Evans (1959) describes a gradual progression from packing move- 
ments involving only the mandibles, to packing by pressing down 
with a stone, to the more complex pattern of pounding with a 
pebble which is later discarded. Table 1 A demonstrates that pound- 
ing with a stone is confined to some species of Ammophila, whereas 
pressing and pressing while holding an object are found among most 
of the ground-nesting Sphecinae. The intriguing question now 
becomes, what is the adaptive significance of these different sorts of 
packing techniques? 
Several hypotheses have been proposed to explain the origin of 
the pounding-with-a-stone behavior found in Ammophila. Rau and 
Rau (1918) and Hicks (1932b) suggest that the availability of stones 
affects the frequency of pounding behavior in Ammophila. How- 
ever, Table 1A and IB reveal no association between the material 
used in nest closure and the kind of packing behavior. Eleven spe- 
cies of Ammophila use stones during nest closure, nine pound, two 
do not; six species do not use stones (or use only small pebbles) and 
three pound while three do not ( \2 = 0.67, p = 0.4, df = 1). Evans 
(1959) suggests that species such as A. aberti and A. zanthoptera 
which nest in compactable soil, show frequent pounding behavior 
whereas those living in sandy soils that resist compaction (such as A. 
harti) do not. He argues that A. urnaria and A.juncea which nest in 
