48 BEES AND WASPS [CH. 



caterpillars, Aphides, and flies. As they increase in 

 size they periodically moult their skins; while the 

 cells are made correspondingly deeper by additions of 

 "paper" put on to their lower edges by the "queen." 

 Subsequently the curved shape of the body of the 

 grub and its general plumpness cause it to impinge 

 firmly on both dorsal and ventral sides against the 

 walls of the cell, and thus a secure hold is maintained. 

 When full grown the grub spins a silken cocoon 

 within the cell, lining the entire cavity thinly but 

 closing the bottom with a thicker and tougher floor 

 which projects as a dome beyond the lower end. 

 Shortly before the pupa (chrysalis) stage is reached, 

 a black mass, composed of a sac containing the whole 

 of the excrement accumulated during larval life, is 

 ejected from the posterior end of the grub. This 

 mass is not removed from the cell, but is flattened 

 by the pressure of the larva against the cell-roof. 

 Soon after the completion of the cocoon, the larva 

 moults its skin and assumes the pupal condition. 

 In this stage the insect possesses distinct head, 

 thorax and abdomen and their respective appendages, 

 viz. a pair of antennae, three pairs of jaws, three pairs 

 of legs and two pairs of wings. These appendages 

 are folded against the body and the whole pupa is 

 of a semi-transparent white colour and very soft. 

 During this period no food is taken, but very exten- 

 sive internal changes are effected at the expense of 



