360 ENTOMOLOGY 
found that the Sphev, after vain efforts to secure its customary 
hold, abandoned the prey. Under such unaccustomed condi- 
tions, insects often show a surprising stupidity, capable as they 
are amid ordinary circumstances. 
Flexibility of Instincts.—Notwithstanding such examples, 
the common assertion that instincts are absolutely “ blind,” or 
inflexible, is incorrect. Instinctive acts are not mechanically 
invariable, though their variations are so inconspicuous as 
frequently to escape casual observation. A precise observer 
can detect individual variations in the performance of any 
instinctive act—variations analogous to those of structure. 
Fic. 290. 
Ammophila urnaria using a stone to pound down the earth over her nest. Greatly 
enlarged.—After PeckHam, from Bull. Wisconsin Geol. and Nat. Hist. Survey. 
To take extreme examples, the Peckhams found that an 
occasional queen of Polistes fusca would occupy a comb of the 
previous year, instead of building a new one; and that an indi- 
vidual of Pompilus marginatus, instead of hiding her captured 
spider in a hole or under a lump of earth as usual, hung it up 
in the fork of a purslane plant. They observed also that one 
Ammoplila, in order to pound down the earth over her nest, 
actually used a stone, held between the mandibles (lig. 290). 
While most of the variations that one encounters are small 
