6oo THE POPULAR SCIENCE MONTHLY. 



tend to preserve those mother-birds which adopted this device. I 

 give this explanation as the only one which either Mr. Darwin or 

 myself has been able to suggest. It will be observed, however, that 

 it is unsatisfactory, inasmuch as it fails to account for the most pe- 

 culiar feature of the instinct — I mean the trailing of the apparently 

 wounded wing. 



The instinct of migration furnishes another case of special diffi- 

 culty, but, as I have no space to dwell upon the sundry questions 

 which it presents for solution, I shall now pass on to the last of the 

 special difficulties which most urgently call for consideration. The 

 case to which I refer deserves, I think, to be regarded as the most 

 extraordinary instinct in the world. There is a species of wasp-like 

 insect, called the sphex. This insect lays its eggs in a hole excavated 

 in the ground. It then flies away and finds a spider, which it stings 

 in the main nerve-center of the animal. This has the effect of para- 

 lyzing the spider without killing it. The sphex then carries the now 

 motionless spider to its nursery, and buries it with the eggs. When 

 the eggs hatch out the grubs feed on the paralyzed prey, which is 

 then still alive and therefore quite fresh, although it has never been 

 able to move since the time when it was buried. Of course, the diffi- 

 culty here is to understand how the sphex insect can have acquired 

 so much anatomical and physiological knowledge concerning its prey 

 as the facts imply. We might indeed, suppose, as I in the first in- 

 stance was led to suppose, that the sting of the sphex and the nerve- 

 center of the spider being both organs situated on the median line 

 of their respective possessors, the striking of the nerve-center by the 

 sting might in the first instance have been thus accidentally favored, 

 and so have supplied a basis from which natural selection could work 

 to the perfecting of an instinct always to sting in one particular spot. 

 But more recently the French entomologist, M. Fabre, who first no- 

 ticed these facts with reference to the stinging of the spider, has 

 observed another species of sphex which preys upon the grasshopper, 

 and, as the nervous system of a grasshopper is more elongated than the 

 nervous system of a spider, the sphex in this case has to sting its prey 

 in three successive nerve-centers in order to induce paralysis. Again, 

 still more recently, M. Fabre has found another species of sphex 

 which preys upon a caterpillar, and in this case the animal has to sting 

 its victim in nine successive nerve-centers. On my consulting Mr. 

 Darwin in reference to these astonishing facts, he wrote me the fol- 

 lowing letter : 



" I have been thinking about Pompilius and its allies. Please take 

 the trouble to read on perforation of the corolla, by bees, page 425, of 

 my ' Cross-Fertilization,' to end of chapter. Bees show so much in- 

 telligence in their acts, that it seems not improbable to me that the 

 progenitors of Pompilius originally stung caterpillars and spiders^ 

 etc., in any part of their bodies, and then observed by their intelli- 



