246 CHANGE OF INSTINCTS — BEMBEX. 



own burrow and stores it with paralyzed prey for its 

 own larv98 to feed on, yet that, when this insect finds a 

 burrow already made and stored by another Sphex, it 

 takes advantage of the prize, and becomes for the 

 occasion parasitic. On which Mr. Darwin has justly 

 observed that he could see no difficulty in natural 

 selection making an occasional habit permanent, if of 

 advantage to the species, and if the insect whose nest 

 and stored food are thus feloniously appropriated be 

 not thus exterminated. 



The problem is certainly one of great difficulty, and 

 it is with diffidence that I would suggest to M. Fabre 

 certain considerations which may perhaps throw some 

 light on it. Let us examine some of the other solitary 

 wasps, and see whether their habits afford us any clue. 

 That an animal of prey knows where its victim is 

 most vulnerable, has not in itself anything unusual or 

 unaccountable. 



The genus Bembex kills the insects on which its 

 young are fed, and supplies the cell with a fresh 

 victim from time to time. Eumenes, like Ammophila 

 and Sphex, stores up the victims once for all. They 

 are grievously wounded, but not altogether paralyzed. 

 Here, then, we have the very condition which M. Fabre 

 considers would be fatal to the tender egg of the wasp. 

 But not necessarily so. The wretched caterpillars lie 

 in a wriggling mass at the bottom of the cell; a clear 

 space is left above them, and from the summit of the 

 cell the delicate egg is suspended by a fine thread, so 

 that, even if touched by a caterpillar in one of its con- 

 vulsive struggles, it would simply swing away in safety. 

 When the young grub is hatched, it suspends itself to 

 this thread by a silken sheath, in which it hangs head 



