OF DIGESTION. 34/ 



traversed throughout by trachea?, which may be considered as separated 

 pulmonary passages, are deficient in a vascular system, and the frag- 

 ment of it which is present more serves to promote a motion in the fluid 

 that decomposition may be prevented by its stagnating during repose. 

 Such animals are insects, as well as a portion of the Arachnida and 

 Myriapoda. 



We have thus become acquainted with the general mode of nutri- 

 tion : we have seen that it requires two agents, viz., one to prepare the 

 nutritive fluid (the intestinal canal), and another to make it organisahle 

 (branchiae, or lungs), as well as frequently a third to conduct the fluid, 

 and which acts as a connecting member between the two others. We 

 will now investigate in detail the functions of these three agents in 

 insects in the order in which we have above noticed them. 



217. 

 I. FUNCTION OP THE INTESTINAL CANAL, DIGESTION. 



The activity of the digestive organs commences with the reception 

 of food. This in insects takes place in a double manner, namely, by 

 biting and chewing, or by the suction of fluids. 



All the mandibulate orders, it is very natural to suppose, take their 

 food by manducation ; consequently the Coleoptcra, Orthoptera, Dic- 

 iyoloptera, Neuroptera, and a portion of the Hymenoptera. In them the 

 horny mandibles, which move horizontally in opposition to each other, 

 bite the portion off which it is the function of the labrum to retain, thus 

 holding it between them ; the same is done beneath by the maxillae and 

 labium. When the part is separated it passes between the maxillae, where 

 it is readily comminuted, during which operation it is held by the labium. 

 It is then passed to the posterior parts of the cavity of the mouth, 

 whence it glides down through the pharynx and oesophagus to the 

 stomach. In many insects, namely, the Coleoptera, the mouth and 

 pharynx are upon the same plane, so that it merely requires to be 

 pushed forward to get into the stomach. Such beetles as the Cara- 

 bodea and Dytici chew but little, perhaps from their possessing a 

 proventriculus in which the food undergoes a second comminution. 

 They also feed only upon flesh, which, as in the carnivora among the 

 mammalia, requires no mastication previous to its being swallowed. 

 In the herbivora, for example, the grasshoppers, particularly of the 

 genus Gryllus, which possess no true proventriculus, but merely a 

 crop provided with teeth, the food is longer chewed, The pharynx 



