286 HYMENOPTERA 



There are often to be seen in empty cells of the nest the . 

 pupa cases of flies, one to a cell, probably Tachinidae, 

 but I have not actually seen a fly depositing its egg on 

 the wasp larva. These wasps belong to the Diploptera, 

 or true wasps, whose wings are doubled up longi- 

 tudinally when not in use, giving them a very narrow 

 appearance. 



In the group of wasp-like insects now to be discussed, 

 known as the Fossores, the wings are not folded. While 

 some of them, such as Bembex, have a certain resemblance 

 to a true wasp, most of them are readily distinguished. 



The group is remarkable from the habits whence the 

 name Fossores, which means " diggers," is derived ; 

 they are also called " Sand wasps." The wasp Belonogaster 

 has been described above as feeding its young on insects 

 which it has first killed and pulped. It is obvious that 

 meat of such kind must be troublesome to prepare, and 

 must be supplied fresh at frequent intervals. 



Some other wasps, which are of solitary habits (Odynerus, 

 etc.), do not pulp the prey, but store them up whole in 

 their burrow. But if one examines the store that has 

 been laid up for the larva to feed upon, one finds that 

 in the case of small geometrid caterpillars, some of them 

 shrivel and dry up, and are not in very good condition to 

 form food. 



One family of fossors, the Bembecidae, has slightly 

 improved upon this method. They feed their larvae 

 almost entirely upon whole flies, which they do not 

 kill, but sting so that they remain alive and juicy but 

 cannot move, since the central nervous system has been 

 paralysed ; probably, therefore, they are insensitive as 

 well. Regular supplies of these paralysed flies are taken 

 down into the burrow by the mother Bembex. 



A further step in the saving of trouble is exemplified 

 by other fossors of the families Sphegidae and Pompilidae. 

 These lay up in a burrow a store of paralysed insects 



