SOLITARY WASPS 269 



difference in the position of the two galls or a variation 

 in the quality of the sap at different seasons will lead 

 to an explanation of the fact that the gall-fly has 

 two differing generations. 



The act of piercing the skin of plants, and of then 

 laying eggs in the puncture so produced, is performed 

 independently in many groups of insects by the same 

 complex surgical instrument. It is, however, only in 

 the higher Hymenoptera that the organ becomes, by the 

 addition of a poison-bag, a sting. The value of this 

 office is best seen in the solitary wasps. 



Solitary Wasps. — The social wasps are so familiar 

 that the term wasp has become almost synonymous 

 with the species that form communities ; and to many 

 the less impressive solitary wasps are quite unknown. 

 Yet they are common enough in this country, and their 

 habits excited the admiration of observers before the 

 beginning of our era. Few of those who have lived 

 in the country will have failed to notice the absorption 

 and persistence with which a ' fly ' will have estab- 

 lished its quarters in a keyhole, dressing-table, or some 

 other part of the furniture which may be reached 

 from an open window or door. The movement and 

 the life of the house makes no difference to the constant 

 visits paid by this insect when its mind is once made 

 up to fix its abode there. With unerring certainty 

 it flies in by a straight course, executes its business, 

 and flies away by an undeviating line. Should the 

 spot be examined the insect will return and hover 

 impatiently until its task can be resumed. The work 



