144: Mr. George J, Bomanes [Feb. 8, 



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 pecu- 

 liar 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-centre of the animal. This has the effect of para- 

 lysing 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 paralysed prey, which is 

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

 able to move since ^he 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- 

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

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

 the sting might in the first instance have been thus accidentally 

 favoured, 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 noticed 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-centres in order to in- 

 duce 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- 

 centres. On my consulting Mr. Darwin in reference to these 

 astonishing facts, he wrote me the following letter : — 



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

 the trouble to read on perforation of the corolla, by Bees, p. 425, of 

 my ' Cross-fertilisation,' to end of chapter. Bees shows so much 

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

 progenitors of Pompilius originally stung caterpillars and spiders, 

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

 gence that if they stung them in one particular place, as between 

 certain segments on the lower side, their prey was at once paralysed. 

 It does not seem to me at all incredible that this action should then 



