353 



The cells, on tlie other hand, which aro in the region of 

 the future joint membranes, do not form a hard chitin, but 

 produce a tough, but flexible, somewhat corrugated mem- 

 brane. This is especially clearly seen in the neck region, and 

 at the points of junction of the legs- with the thorax (fig. 41). 

 The somatopleural mesoderm, so far as I can observe, 

 always undergoes a renovation during the metamorphosis, 

 and eventually persists as a delicate membrane with prominent 

 nuclei, immediately below the chitinised ectodermal cells. 



The metamorphosis of the general body integument, then, 

 closely resembles that described by Perez in Calliphora. In 

 that insect, however, the imaginal ectoderm extends over the 

 cells of the larval ectoderm, which do not disappear till much 

 later. Though cytoplasmic degeneration, somewhat similar 

 to that of Nasonia occurs, phagocytosis is much more pro- 

 minent, and the phagocytes attacking the integument cells 

 are, generally, already strongly gorged with phagocytised 

 muscle tissue (sarcolytes) . 



The Phragmas. — During the pupal period the integument 



undergoes a number of changes which result in the formation 



of the phragmas — ingrowths of the integument serving for 



the insertion of the muscles. The phragmas are of two kinds : 



there are the true phragmas, which are actual invaginations 



of the integument (fig. 43) ; a second type of structure which 



may be designated a "false phragma" is essentially an 



ingrowth of the edge of a segment into the body cavity, below 



another segment which now overlaps it. An excellent example 



of such a ''false phragma" is the anterior part of the meso- 



thoracic tergum, which, as already mentioned, is simply a 



prolongation of the mesothorax beneath the prothorax. The 



great phragmas of the ovipositor also belongs to this class. 



A false phragma, then, is a downgrowth of integument, which 



consists of only a single layer of cells. The true phragmas, 



on the other hand, are invaginations of the integument, 



gen,erally hollow at first, and consist, of course, of a double 



layer of integument. They are found early in pupal life, and 



after some thirty-six hours chitinise, the chitin. being 



secreted between the two layers. 



In the abdomen of the female a number of these phragmas 

 are developed in connection with the ovipositor. They are 

 rather short, and on them originate the great muscles of the 

 ovipositor. 



The thorax and propodeum are provided with a number 

 of transverse phragmas, for the insertion of the great thoracic 

 muscles, and those of the legs and wings. One such phragma 

 runs transversely just behind the scutum of the mesothorax; 

 below the metathorax runs a transverse horizontal phragma. 



