CHAPTER VII 

 THE ALIMENTARY CANAL 



The alimentary canal of the arthropods has three primary divisions, 

 commonly spoken of as the fore, mid, and hind intestine, or fore-, mid-, 

 and hind-gut. The rudiments of these parts appear very early, the fore 

 and hind parts developing from the ectoderm as anterior and posterior 

 invaginations (Figs. 435, 44) the former constituting the stomodaeum; 

 the latter, the proctodaeum. In general, the stomodaeum forms before 

 the proctodaeum, but in a number of cases the proctodaeum forms first, 

 as in the head louse, the flour moth (Ephestia) , some chrysomelids, the 

 sheep tick, etc. The esophagus, the proventiculus, the salivary glands, 

 and other annexes have their origin in the stomodaeal invagination. 

 The ileum, colon, rectum, rectal glands, and Malpighian tubules originate 

 in the proctodaeum. The last mentioned arise as evaginations from the 

 blind end of the proctodaeum, either as a single pair of buds which later 

 branch or as two or three pairs depending on the species, three pairs 

 apparently being the primitive number. Henson (1932) only, of recent 

 writers, considers the Malpighian tubules of mesodermic origin. 



The stomodaeum is a simple invagination easy to identify. The 

 proctodaeum, on the other hand, by reason of its position may sometimes 

 be confused with the posterior amniotic cavity in cases where the two 

 appear simultaneously and the transition from one into the other is not 

 sharp. The resulting invagination has consequently been termed the 

 "amnioproctodaeal cavity." 



Within the lumen of the mid-gut of insects that subsist on solid food, 

 the thin peritrophic membrane is found. Its presence in the embryo has 

 been recorded in but few cases. Strindberg (19136) found it in the 

 embryo of Formica fusca in the form of two membranes: an outer and 

 younger membrane closely applied to the mid-gut epithelium and an 

 inner and older membrane in contact with the yolk, the space between 

 them filled with a coagulate. Strindberg argues that since the mem- 

 brane, at least in the ant embryo, forms a closed sac around the yolk 

 including its posterior end, it cannot develop from a ring of cells in the 

 proventriculus but must be secreted by the mid-gut epithelium. 



Gambrell (1933) in her study of the development of Simulium pidipes 

 believes that the closing membrane of the stomodaeum after rupture 

 forms the rudiment of the peritrophic membrane. Butt (1934) independ- 



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