OF INSECTS. 127 



[n the different stages of the nutritive process, cer- 

 tain substances are formed, which are sometimes 

 essential to the animal economy, and at other times 

 rejected as hurtful, these may he included under the 

 general name of secretions. We shall successively 

 idvert to each of the subjects just enumerated. 



Digestion. As this function is almost entirely 

 levolved on the organ named the alimentary canal, 

 shall endeavour, in the first place, to convey an 

 iccurate notion of the form and position of that rm- 

 )ortant viscus. It may be described generally as an 

 elongated tubular organ, occupying the centre of the 

 >ody, and open at both extremities. Occasionally it 

 s nearly straight and not longer than the body, but, 

 n most instances, it is twisted on itself in numerous 

 convolutions, and its length is consequently very 

 considerable, sometimes twelve times as long as the 

 hody. In this respect it is found to vary, as among 

 he higher animals, according to the nature of the 

 JDod, being long and complicated in herbivorous spe- 

 j ies, and comparatively short in such as live by prey ; 

 i ut even this law is not without numerous and striking 

 Exceptions. In most cases the form is rendered irre- 

 Hiilar by many distentions and constrictions, which are 

 BD conspicuous that they may be regarded as dividing 

 lie canal into several parts, which have received diffe- 

 pnt names according to the functions they perform. 

 he place occupied by the canal is the median line 

 If the body, immediately beneath the dorsal vessel. 

 pee PL II. fig. 1, b, c, d, e,f.) Its texture is not the 

 June throughout its whole extent, but its essential con- 



