562 



PSYCHE. 



[December 1893. 



Mingazzini * too in his study of the 

 alimentary canal of the phytophagous 

 Lamellicornia concludes that the 

 hexagonal structure of the rectum is 

 "di grande importanza filogenetico, 

 perche si riscontra in quasi tutti gli 

 ordini d'insetti finora studiati." Min- 

 gazzina adds as further evidence to Miall 

 and Denny's suggestion that the proc- 

 todaeum presents 6 bands of longitudinal 

 muscles corresponding to the 6 longitud- 

 inal muscles of a segment. 



Miall and Denny's hypothesis is cer- 

 tainly ingenious, but I fear that it is 

 untenable. To be admissible it would 

 require that each of the 6 sides of the 

 hind-gut should be parallel to one of the 

 6 exoskeletal elements of the segment 

 (Fig. i). The median dorsoventral 



would contain three entire planes. But 

 in reality this is not the case, as sections 

 through the embryo show (Fig. 2.) ; 

 there being in the proctodaeuni one 

 dorsal, one ventral, and on either side 

 two pleural planes. The median dor- 

 soventral plane A A cuts the procto- 

 daeuni into two symmetrical halves, 



plane, represented by the line A A, 

 should cut the prismatic hind-gut into 

 two symmetrical halves, either of which 



* Ricerche sul canale digerente delle larve dei 

 lamellicorni fitofagi. Mitlh. zool. stat. Neapel. bd. ix 

 heft i, 1SS9, p. 95 at seq. 



Fig. 2. 



either of which consists of two entire 

 and two half sides. It is therefore nec- 

 essary to suppose either that the rectum 

 has undergone a i - otation of 30 on its 

 median longitudinal axis — and this 

 would produce a torsion of which there 

 is not the slightest evidence in the 

 embryo or adult — or that a single side 

 of the proctodaeum does not correspond 

 to a segmental element and this is, of 

 course, equivalent to abandoning the 

 hypothesis altogether.* 



* Miall and Denny's remarks are hardly explicit 

 enough. I have interpreted their hypothesis as refer- 

 ing to the sides and not to the angles of the hind-gut. On 

 the supposition that each corner, or horn with the adja- 

 cent half sides is regarded as equivalent to an exoskele. 

 tal element, no very serious objection can be raised to 

 the view. 



