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Psyche 
[Vol. 92 
a wide range of soils show highly variable tool-using behavior, 
pounding when nesting in compactable soil and not when nesting in 
sand (Peckham and Peckham, 1898; Thorpe, 1956). Unfortunately, 
the data were too sparse to include substrate in Table 1 and hence 
no test of this hypothesis is possible at present. However, it is possi- 
ble to say that while this association may hold within the Ammo- 
phila, it does not explain the diversity of packing behavior found 
among the ground-nesting Sphecinae. I have observed two species 
of Sphex ( S . ichneumoneus, S. pensylvanicus) and one species of 
Ammophila (. A . urnaria) nesting side-by-side and only the Ammo- 
phila showed the pounding behavior. Table 1A reveals that this 
behavior is unique to Ammophila — but this is not the only unique 
behavior found in the genus. They also dig their burrows in quite a 
different manner from the other Sphecinae, depositing soil at some 
distance from the nest entrance. Therefore, Ammophila have no 
mound of soil from which to scrape nest fill and they must use 
whatever they can find around the nest. Some of these objects are 
not particularly appropriate for the purpose and may require more 
persistent and strenuous packing to achieve the same level of com- 
paction; certainly hunting around for objects takes more time than 
simply filling from the mound (Adriaanse, 1943). Sphex, on the 
other hand, (and other species with a mound) have a ready supply of 
suitable material with which they can rapidly fill their nest. It might 
also be noted that Ammophila are particularly slender, light-bodied 
wasps (as compared with other Sphecinae) and it may be difficult 
for these animals to apply any significant force to the substrate 
without the added weight of a stone (which can be as much as ten 
times the weight of the wasp, Iwata, 1976). 
No one has ever demonstrated the survival value of packing for 
digger wasps, although there is no doubt that they expend consider- 
able time and effort in carefully compacting the fill and disguising 
the location of the nest entrance. There are no observations of par- 
asites or predators digging into closed nests, although the very 
common parasitic flies (Sarcophagidae) and velvet ants (Mutillidae) 
must be able to dig their way out. I have observed several species 
working their way in through temporary closures of Sphex ichneu- 
moneus, including various species of ants (which rob the nest of its 
provisions), a cleptoparasitic sphecid, Nysson plagiatus, and possi- 
bly sarcophagid flies ( Senotainia trilineata) and cuckoo wasps 
