352 



as the hard bristks and claws, and even the delicate hairs 

 (pubescence) are to be regarded simply as chitinisations on 

 the surface of these cells, or on parts of them. The ''spiral" 

 thread of the tracheae, as will be described later, is similarly 

 merely a chitinisation of a previous protoplasmic "mould." 

 The small sculpturings on the body surface generally require 

 only a single cell to act as a mould for them; this is very 

 clearly seen, for example, on the head (fig. 33), the dorsal 

 part of which shows forwardly projecting scale-like bosses, 

 while on the antero-ventral part these project upwards; and 

 the cells, in early pupae, can be distinctly seen, one under each 

 boss, and all disposed in such a way as tO' present a "scale- 

 like" appearance similar to that of the imaginal cuticle which 

 they are secreting. The larger sculpturings, as well as such 

 structures as claws and large spines on the legs, are, as a 

 rule, moulded upon a number of cells (fig. 34). 



Bristles, on the other hand, are unicellular structures. 

 Their formation can be especially clearly seen on the ovi- 

 positor and posterior extremity of the insect. The ectodermal 

 cells begin to elongate and develop a point at their free ends ; 

 the elongation becomes more and more marked till the cell 

 assumes the' slender form of the bristle as we see it in the 

 imago. Then it begins to chitinise (fig 35) . The insertion of 

 such a bristle on the cuticle of the imago is always strengthened 

 by a small ring-like supporting structure (fig. 35), and the 

 protoplasmic mould even of this support can, if the hair and 

 cell is observed at the right moment, be clearly seen. 



The development of minute hairs (pubescence) is especi- 

 ally curious. The process can be clearly observed on the 

 second maxillae (labium). Here the ectodermal cells develop 

 a number of long delicate processes, giving the cells a curiously 

 frayed appearance at their terminations (fig. 24). Each of these 

 processes then chitinises, to form a single hair. A single cell 

 therefore acts as a mould for a number of minute hairs and the 

 co-operation of a number of such cells produces the rasp-like 

 pubescent structure which one finds on the "tongue-like" 

 labium of the adult Nasonia. The chitinisation of the 

 epidermal cells continues throughout pupal life, and the pro- 

 cess does not cease till the whole of the cells have been 

 converted into chitin. A cellular ectoderm is, therefore, 

 absent in Nasonia^ except in the region of the great eye. 



The extraordinary accumulation of epidermal cells in the 

 region of the propodeum results in the formation of an especi- 

 ally thick chitin layer there. Indeed, so active is the process 

 of chitin secretion in this neighbourhood, that sections actually 

 show minute liquid globules issuing from the chitin-secreting 

 cells. 



