INSTINCTS OF THE SPI1EX. 299 



which Mr. Darwin has observed, and which is of much 

 importance as indicating that the different castes of neuters 

 have arisen by degrees, and therefore presumably under the 

 influence of natural selection. This fact is that, when care- 

 fully searched for, neuters presenting more or less well- 

 marked gradations of structure between one caste and another 

 may be occasionally found in the same nest.* On the whole, 

 therefore, I conclude, with regard to this particular case of 

 difficulty, that it is not so formidable as to exclude the 

 explanation furnished by the hypothesis of natural selection, 

 supposing that we have already accepted this hypothesis as 

 explanatory of other and less difficult cases. 



Instincts of the Sphex. 



Several species of this division of the Hymenoptera dis- 

 play what I think may be justly deemed the most remarkable 

 instincts in the world. These consist in stinging spiders, 

 insects, and caterpillars in their chief nerve-centres, in con- 

 sequence of which the victims are not killed outright, but 

 rendered motionless ; they are then conveyed to a burrow 

 previously formed by the Sphex, and, continuing to live in 

 their paralyzed condition for several weeks, are at last avail- 

 able as food for the larva.' when these are hatched. Of course 

 the extraordinary fact which stands to be explained is that 

 of the precise anatomical, not to say also physiological know- 

 ledge which appears to be displayed by the insect in stinging 

 only the nerve-centres of its prey. The following, so far as 

 is at present known, are the main features of this very sur- 

 prising case. 



The same species of Sphex always preys upon the same 

 species of victim. When the victim is a spider, the instinct 

 of its assailant dictates that a single sting shall be given in 

 the large ganglion where, in the case of the spider, most of 

 the central nervous matter is aggregated. When the victim 

 i a beetle, the Sphex which preys upon it — there are eight 

 species which prey upon two genera — first throws the insect 

 Upon its hack, then embraces it and plunges the sting through 



the membranes between the first and the second pairs of Legs ; 

 the sting thus 1 1 ikes the main nerve-centre, w Inch is unusually 

 agglomerated in hectics of this genus. When the prey is a 



• Sec Origin qfBptcUt, 231 -ii. 



