360 



ENTOMOLOGY 



found that the Sphex, 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. 



FIG. 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 

 Ammophila, in order to pound down the earth over her nest, 

 actually used a stone, held between the mandibles (Fig. 290). 



While most of the variations that one encounters are small 



