606 EVOLUTION OF ANIMALS Part V 



In herbivorous insects, the saHva contains a starch-splitting enzyme (am- 

 ylase) whose action begins in the mouth. Plant lice inject such saliva into the 

 plant tissues and digestion starts before the food is taken into the mouth. As 

 it is swallowed it is evidently pushed backward onto the base of the tongue 

 (hypopharynx). It then slips on into the crop, mainly a storage sac. The giz- 

 zard or proventriculus is equipped with chitinous teeth that thoroughly grind 

 the food by a different method but with the same result as in birds. 



Midgut. At the posterior end of the gizzard a valve keeps food from passing 

 into the stomach before it is ground. The stomach is the main organ of 

 chemical digestion and absorption. In the cockroach, its lining produces the 

 sugar enzyme — maltase, the fat enzyme — lipase, and the protein splitter — 

 trypsin. All of these enzymes are catalysts that speed digestive processes, much 

 needed in animals with low body temperatures. Insects have no mucus to pro- 

 tect the lining of the stomach as the vertebrates do. In place of it certain 

 epithelial cells produce an extremely thin sheath (peritrophic membrane) 

 which in the stomach surrounds the food like a tube. 



Hindgut {intestine). The excretory organs (Malpighian tubules) open into 

 the digestive tube at the junction of the stomach and intestines (Fig. 30.14). 

 The lining of the hindgut is permeable to water and, with the economy of water 

 usual in insects, it is there absorbed back into the body. Waste substances are 

 finally extruded from the body in dry pellets. 



Blood and Circulation. Insect blood, like vertebrate blood, is a tissue 

 fluid that distributes digested food to the tissues and carries away the waste 

 products of their metabolism. Although it holds oxygen and carbon dioxide in 

 solution it contains no such efficient oxygen carrier as the hemoglobin of 

 vertebrates and its role in respiration is secondary. It contains proteins, glu- 

 cose, salts, fats and an unusual amount of amino acids. With rare exceptions 

 such as the larvae of chironomids (midges), it does not contain hemoglobin 

 but absorbs oxygen and carbon dioxide in solution. As before mentioned, 

 while an insect is molting, muscles in the legs and abdomen contract and fill 

 the thorax with blood, swelling it till the outer cover cracks open along the 

 midline of the back. As soon as the insect sheds the old cover it contracts the 

 thorax and forces blood into the wings (Fig. 30.13). 



Blood Cells. There are several different kinds of blood cells, but no red ones. 

 They adhere to tissues and spread out often in star shapes and circulate with 

 the fluid (Fig. 30.15). Here, as in other animals, blood cells are deeply in- 

 volved in the experiences of the animal and their forms and functions change 

 with conditions in the body. 



Functions of Insect Blood. Three functions of insect blood are well estab- 

 lished. The chief function of the blood cells is phagocytosis, the ingestion of 

 minute particles and living bacteria. Blood carries digested food to the tissues 

 and metabolic waste from them to the excretory organs (Malpighian tubules). 



