124 



ALIMENTARY SYSTEM 



and the muscular layer of the whole canal from the meso- 



derm. 



The alimentary canal is more complex anatomically than it is 



morphologically, and various parts are distinguished, viz. the 



canal and its appendicula ; the former consisting of oesophagus, 



crop, gizzard, true stomach, and an 

 intestine divided into two or more 

 parts. It should be remarked that 

 though it is probable that the mor- 

 phological distinctions correspond to 

 a great extent with the anatomical 

 lines of demarcation, yet this has 

 not been sufficiently ascertained : the 

 origin of the proctodaeum in Musca 

 is indeed a point of special difficulty, 

 and one on which there is consider- 

 able diversity of opinion. In some 

 Hemiptera the division of the canal 

 into three parts is very obscure, 

 so that it would be more correct, as 

 Dufour says, to define it as consist- 

 ing in these Insects of two main 

 divisions one anterior to, the other 

 posterior to, the insertion of the Mal- 

 pighian tubes. 



It should be borne in mind that 

 the alimentary canal is very different 

 in different Insects, so that the brief 

 general description we must confine 

 ourselves to will not be found to 



FIG. 69. Digestive system of , ,. ~ ., 



Xyphidria camelus (after Du- aPPty Satisfactorily to any one Ill- 

 four) : a, head capsule ; b, sal- se ct. The oesophagus is the part be- 



ivary glands ; c, oesophagus ; d, . . , . A 



crop ; e, proventricuius; /, chyle, hind the mouth, and is usually narrow, 



or true stomach ; g, small intes- as ft } las to paSS through the most 

 tine; h, large intestine ; i, Mai- . 



pighian tubes ; k, termination of important nervous centres ; extremely 

 body ' variable in length, it dilates behind 



to form the crop. It may, too, have a dilatation immediately 

 behind the mouth, and in such case a pharynx is considered to 

 exist. The crop is broader than the oesophagus, and must be 

 looked on as a mere dilatation of the latter, as no line of 



