PHYLUM ARTHROPODA. INSECTS 



255 



hemocoel, not a coelom. All of the systems 

 characteristic of higher animals are repre- 

 sented. 



Muscular system 



The muscles are of the striated type, very 

 soft and delicate, but strong. They are seg- 

 mentally arranged in the abdomen but not 

 in the head and thorax. The most con- 

 spicuous muscles are those that move the 

 mandibles, the wings, the metathoracie legs, 

 and the ovipositor. 



Digestive system 



The principal parts of the digestive tract 

 (Fig. 136) are the foregut, midgut, and 

 hindgut. The foregut consists of (1) the 

 mouth, on each side of which opens a 

 salivary gland that produces an enzyme- 

 containing secretion; (2) a tubular eso- 

 phagus, which enlarges into (3) a crop in 

 the mesothoracic and metathoracie seg- 

 ments. This leads into (4) the proven- 

 triculus, which is a grinding organ (gizzard). 

 Next is the midgut, which is the ventriculus 

 (stomach), reaching posteriorly into the ab- 

 domen; and into it, 6 double cone-shaped 

 pouches, the gastric ceca, pour the digestive 

 enzymes they secrete. The products of diges- 

 tion are absorbed through the wall of the 

 stomach and pass into the blood around it. 

 From this point, the food is carried through 

 the circulatory system to the cells, where it 

 is utilized. Then the hindgut is made up of 

 (1) the ileum, into the anterior end of 

 which the delicate Malpighian tubules open 

 and (2) the colon, which expands into the 

 (3) rectum and opens through the (4) 

 anus. Since both the foregut and the hind- 

 gut are lined with cuticle, little absorption 

 takes place in them. 



Circulatory system 



This (Fig. 136) is not a closed system of 

 blood vessels as in vertebrates and some in- 

 vertebrates, but consists of a single tube lo- 

 cated in the abdomen just under the body 

 wall in the middorsal line, and of spaces 



(sinuses). The heart is divided into a row 

 of chambers; into the base of each opens a 

 pair of ostia. These ostia are closed by valves 

 when the heart contracts. The pericardial 

 cavity, in which the heart lies, is formed by 

 a transverse diaphragm beneath it. Blood 

 enters the heart and is forced anteriorly 

 through the aorta into the hemocoel, where 

 it bathes all the organs. The blood system is 

 an open one, as is that of other arthropods, 

 for there are no capillaries or veins. The 

 blood serves chiefly to carry food and wastes; 

 there is a separate respiratory system. The 

 blood consists of a clear plasma in which are 

 suspended white blood cells that act as 

 phagocytes to remove foreign organisms and 

 other substances. 



Respiratory system 



The respiratory system (Fig. 137) con- 

 sists of a network of tubes, the tracheae, 

 that communicate with every part of the 

 body. The tracheae consist of a single layer 

 of cells lined with a layer of cuticle, which 

 is thickened to form spiral rings that pre- 

 vent the tracheae from collapsing. A tracheal 

 branch extends from each spiracle to a 

 longitudinal trunk on each side of the body. 

 The finest tracheae, the tracheoles, are con- 

 nected directly with the tissue to which they 

 supply oxygen and from which they carry 

 away carbon dioxide. The smallest trache- 

 oles contain fluid in which oxygen dis- 

 solves before actually reaching the cells; 

 this fluid serves in internal respiration like 

 the blood in other animals. In the grass- 

 hopper and certain other insects, some of 

 the tracheae become expanded into thin- 

 walled air sacs which are easily compressed 

 and thus aid in the movement of air. Con- 

 traction and expansion of the body expels 

 air from and draws it into the tracheal 

 system. 



The utilization of oxygen in the metabolic 

 processes of the cell, with the production 

 of carbon dioxide, is accomplished through 

 the action of respiratory enzymes called the 

 cytochrome system. The cytochrome sys- 



