384 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 193 3 



might turn a knowledge of it to advantage in the war against agri- 

 cultural pests, but so far investigations have not revealed the secret. 

 Young insects are particularly constructed for the function of 

 feeding. The period in an insect's growth from the embryo to the 

 adult is above all else a digestive stage. The stomach is exalted over 

 the other members, and all activities are devoted to keeping it full of 

 food. The stomach itself at this period is a large, active, and ener- 

 getic organ (fig. lZ,Vent) ; its cells are constantly pouring out diges- 

 tive juices, and absorbing the products of digestion. The latter are 

 used partly by the growing insect, and partly are stored in the body 

 as reserves for the use of the adult. 



Imcl 



cmcl 

 Epth 



BMb 



FiQDEK 14. — Diagrammatic cross-section of the mature stomach, or ventriculus, of an 



insect. 



BMb, basement membrane ; cmcl, circular muscles ; dg, digestive cells ; Epth, endo- 

 dermal epithelium ; F, food material ; Imcl, lengthwise muscles ; PMb, peritrophic mem- 

 brane ; rg, regenerative cells ; sb, striated border of epithelial cells. 



The structure of the stomach is best seen in a cross-section (fig. 14) . 

 The principal part of its wall is a cellular layer, the epithelium 

 (Epth), which is bounded externally by a thin hasement membrane 

 (BMb). On the outside is a muscular sheath composed of inner 

 circular fibers {cmcl) and outer lengthwise fibers {hncl). The inner 

 margin of the epithelium has the appearance of being penetrated by 

 numerous fine pores perpendicular to the surface, and for this reason 

 it is called the striated border {sb). Within the cavity of the stom- 

 ach is the mass of food material (F) , but usually it is to be seen that 

 the food is enclosed in a sack formed of a delicate film, the peritrophic 

 membrane (PMb), which is given off from time to time from the cell 

 surface, and probably serves to protect the latter from direct contact 

 with the food. The peritrophic membrane, however, must be pene- 



