90 EMBRYOLOGY OF INSECTS AND MYRIAPODS 



the genera Calandra and Meloe, are represented in both groups and that 

 Hydrophilus alone is unrepresented in the first. From this, one cannot 

 escape the conclusion that whatever may be the germ layer from which 

 the mid-gut is derived, it is the same for both groups and that the different 

 opinions expressed by the writers as to the derivation of the mid-gut in 

 the Coleoptera is merely a matter of interpretation. 



Hoffmann (1914) showed that in the strepsipteron Xenos hohlsi a 

 "primary mid-gut" is formed by three yolk cells which he compares to 

 the annelidan macromeres. The definitive mid-gut epithelium develops 

 from a process arising at the bhnd end of the stomodaeum. From this 

 process a pair of lateroventral ectodermal ribbons develop which broaden 

 and lengthen to form the epithelium that encloses the yolk. At the 

 blind end of the proctodaeum a small marginal proliferation occurs 

 to which the epithelium from the stomodaeum connects. Noskiewicz 

 and Poluszynski (1928) describe a similar development ior Sty lops. In 

 this insect, proliferating cells at the blind end of the stomodaeum push in 

 under and over the yolk until the epithelial wall is formed. Thus in this 

 group a primary provisional epithelium derived from yolk cells is followed 

 by a definitive mid-gut epithelium made up of ectodermal cells. 



SIPHONAPTERA AND DIPTERA 



Except for superficial differences the development of the fleas Cteno- 

 cephalides felis, Nosopsijllus fasciatus, and Hystrichopsylla dippiei studied 

 by Kessel (1939) is quite similar. The mesenteron rudiments in their 

 embryos originate from near the anterior and posterior extremities of that 

 portion of the blastoderm forming the ventral plate, making their initial 

 appearance before the formation of the inner layer is evident. Each 

 rudiment gives rise to two lateroventral entodermic ribbons from which 

 the mid-gut epithelium is derived. The very few earlier papers relating 

 to the embryology of the flea make Httle or no mention of mid-gut 

 development. 



Among the nemocerous Diptera Simulium pictipes studied by 

 Gambrell (1933) and Sciara coprophila studied by Butt (1934) exhibit a 

 similar type of development in which the bipolar mesenteron rudiments 

 give rise to paired entoderm ribbons as described above for the fleas. 

 In Chironomus, on the other hand, Hasper (1911) states that rudiments 

 from which the mid-gut epithelium is derived appear to arise from the 

 tips of the stomodaeal and proctodaeal invaginations and their cells do 

 not distinctly differ in appearance from the ectodermal cells though 

 sharply distinct from the mesoderm. From each of these rudiments the 

 usual pair of ribbons form that later develop into the mid-gut epithelium. 

 Ritter's account (1890) of the development of the mid-gut of Chironomus 

 is not convincing. Another nemocerous species in which the mid-gut 



