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TEXT-BOOK OF ENTOMOLOGY 



takes a little liquid food, it is slender, delicate, and highly differenti- 

 ated. In the larva the mid-gut forms the largest part of the canal ; in 

 the imago, the intestine becomes very long and coiled into numerous 

 turns ; at the same time the food-reservoir (the " sucking stomach ") 

 develops, and the excretory tubes are longer. 



a. The digestive canal 



It will greatly simplify our conception of the anatomy of the 

 digestive canal if we take into account its mode of origin in the 

 embryo, bearing in mind the fact that during the gastrula condition 

 the ectoderm is invaginated at each pole to form the primitive mouth 

 and fore-gut (stomodaeum) and hind-gut (proctodaeum). The cells of 

 the ectoderm secrete a chitinous lining (intima), which forms the 

 continuation of the outer chitinous crust, and thus the lining of each 

 end of the digestive canal is cast whenever the insect molts ; while 

 the mid-intestine (mesenteroii), arising independently of the rest of 



the canal much later 

 in embryonic life 

 from the uiesoderm, 

 is not the result of 

 any invagination, 

 being directly derived 

 from the mesoderm, 



an( l * s n k li ne< l with 

 chitin. 



The mouth, or oral 

 cavity, and pharynx. 

 This is the begin- 

 ning of the aliment- 



FIG. 803. Interior view of the bottom of the head of Danais _ f n V. p r> a <* q i n 



archippus, the top having been cut away, showing, in the middle, a>1 J J e, JJ ct 



the pharvngeal sac with its five muscles : the frontal ( f. m), dorsal ^.-po ,1 Q 1 1 -.7- infn fVio 



pair (d.m), and the lateral pair (l.m)\ cl, clypeus; cor, cornea; & ld/ 411 J 



*, oesophagus; p.m, one of the large muscles which move the rocrmlnrmo Tf ia 



labial palp. After Burgess. 



bounded above by the 



clypeus, and labrum, with the epipharynx, and below by the hypo- 

 pharynx, or tongue, as well as the labium. Into it pour the secretion 

 of the salivary glands, which passes out through an opening at the 

 base of the tongue or hypopharynx. On each side of the mouth are 

 the mandibles and first maxillae. 



The sucking or pharyngeal pump. This organ has been observed l>y 

 Graber in flies and Hemiptera, but the fullest account is that by 

 Burgess, who was the^ first to discover it in Lepidoptera. In the 

 milkweed butterfly (Danais archippus) the canal traversing the pro- 



