OF INSECTS. 
127 
In the different stages of the nutritive process* cer- 
tain substances are formed* whicli are sometimes 
essential to the animal economy, and at other times 
rejected as hurtful, these may be included under the 
general name of secretions. We shall successively 
advert to each of the subjects just enumerated. 
Digestion . — As this function is almost entirely 
devolved on the organ named the alimentary canal , 
we shall endeavour, in the first place* to convey an 
accurate notion of the form and position of that im- 
portant viscus. It may be described generally as an 
elongated tubular organ, occupying the centre of the 
body* and open at both extremities. Occasionally it 
is nearly straight and not louger than the body, but* 
in most instances, it is twisted on itself in numerous 
convolutions* and its length is consequently very 
considerable, sometimes twelve times as long as the 
body. In this respect it is found to vary, as among 
the higher animals, according to the nature of the 
food, being long and complicated in herbivorous spe- 
cies, and comparatively short in such as live by prey ; 
but even this law is not without numerous and striking 
exceptions. In most cases the form is rendered irre- 
gular by many distentions and constrictions, which are 
so conspicuous that they may be regarded as dividing 
the canal into several parts* which have received diffe- 
rent names according to the functions they perform. 
The place occupied by the canal is the median line 
of the body* immediately beneath the dorsal vessel. 
(See PI. II. fig. 1* b } c, d , e J f.') Its texture is not the 
same throughout its whole extent, but its essential con- 
