THE PHYLA ARTHROPODA AND ONYCHOPHORA 



molts. The buccal cavity, enclosed by the mouth parts described above, 

 receives the ducts from the salivary glands lying in the thorax. From the 

 buccal cavity a short esophagus leads dorsally, widening to produce a thin- 

 walled crop which extends into the thorax. The crop tapers posteriorly and 

 joins the stomach. The junction between crop and stomach is marked by six 

 pairs of glandular outgrowths of the digestive tract known as gastric caeca; 

 these caeca secrete a juice containing digestive enzymes. Posterior to the 

 stomach lies the intestine, which has a narrow region, the "colon," followed 

 by an expanded rectum leading to the anus. The division between stomach 

 and intestine is marked by the zone of attachment of the Malpighian tubules, 

 which are excretory organs. 



In feeding, the animal bites off pieces of grass and other vegetation with 

 its mandibles, using the labrum and labium as upper and lower lips, and 

 the palps of the maxillae and labium as tactile and gustatory organs. The 

 salivary secretion, emptied into the buccal cavity, serves as a lubricant in 

 swallowing and as a digestive fluid acting upon food stored in the crop. 

 Digestion may occur to some extent in the crop, but the cavity of the stomach 

 is the principal digestive region. Absorption of products of digestion into the 

 circulating blood occurs in the stomach and the anterior part of the intestine, 

 and assimilation follows the diffusion of these transported nutrients into the 

 cells of the body. 



As in the crayfish, the contribution of the embryonic coelom to the definitive 

 body cavities of the locust is problematical. The spaces between muscles and 

 surrounding the viscera are hemocoels and do not, on the whole, represent a 

 modified coelom. In the adult locust this space is largely occupied by an 

 irregular mass of storage tissue termed the fat body. Such fat bodies make 

 up most of the body bulk in insect larvae storing up food reserves in advance 

 of metamorphosis. 



Leading from the spiracles on the thoracic and abdominal segments are the 

 air tubes, or tracheae, which form the respiratory system. The tracheae unite 

 and branch in a complex rnanner and expand into conspicuous reservoirs, or 

 air sacs, in the abdominal region (Fig. 15.18). The fine end branches of the 

 tracheae, the tracheoles, ramify within all the tissues of the body and permit the 

 direct delivery of atmospheric oxygen to the fluids surrounding the cells. Air 

 must first enter the body through the spiracles, a fact which makes it possible 

 to kill insects by clogging these openings with dust or films of oil and soapsuds. 

 Oxygen enters the cells of the insect from the intercellular fluid and is used, 

 as in other animals, in cellular metabolism. The tracheal type of respiratory 

 mechanism is found only among the higher arthropods and their relatives, the 

 Onychophora. The fact that many insects are capable of rapid and sustained 

 metabolic activities, as in prolonged periods of flight, is an indication that the 

 tracheal system has been developed to a high point of efficiency. 



Malpighian tubules are the chief excretory organs of insects, functioning to re- 

 move nitrogenous wastes of metabolism from the blood and eliminate them 

 from the body. Each tubule, of which there may be dozens or hundreds, is 



449 



