24 



ECONOMIC ENTOMOLOGY 



are also valves between the chambers so that when the latter con- 

 tract the blood is forced forward. The blood on leaving the ventri- 

 cles passes into the aortic portion of the dorsal tube, toward the head, 

 and thence flows into the body cavity bathing all the organs. The 

 blood is usually colorless and consists of two portions — the watery 

 serum and the white blood corpuscles. 



Fat-bodies. — These are masses of fat-cells occupying a large part of 

 the cavity of the body, and lying between the organs. At first the fat- 

 cells are large and spherical, but they lose their structure to a large 

 extent through breaking down. They contain nourishing albuminoid 

 matter, besides uric acid and urates. Their exact function is not 



Fig. 28a. — Apparatus for closing the spiracular tracheae in a beetle (Lucanus). 

 A, Trachea opened; B, closed; b., bow; bd., band; c, external cuticula; I., lever; m., 

 muscle; s., spiracle; /., trachea. {After Judeich and Nitsche.) 



definitely known. Associated with the fat-bodies in the abdomen of 

 larvae are large yellow cells called cenocytes which, according to Glaser, 

 secrete an oxidizing enzyme. 



{d) Intestinal or Digestive System. — This system occupies the central 

 portion of the body, and is divided into distinct regions with special 

 functions (Fig. 29) . The food passes from the mouth into the pharynx 

 where it is subjected to the action of saliva secreted by the salivary 

 glands. Thence it passes in the Orthoptera and Coleoptera through 

 the oesophagus into the crop, a folded and membranous pouch, where 

 the action of the sahva is completed. The partially digested food then 

 passes into the gizzard or proventriculus , a muscular enlargement armed 

 with teeth for the purpose of straining the contents before entering the 

 true stomach or ventricuhts, In many caterpillars the food passes 



