406 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 1925 



insects, such as bees and wasps, whose larvae live m cells, and para- 

 sites of the same order whose larvae live in the bodies of other living 

 insects, the opening between the stomach and the intestine is not 

 made mitil near the end of the larval life. 



In adult insects, the stomodeal part of the alimentary canal becomes 

 dillercntiatcd into a pharynx, a true oesophagus, a large saclike 

 crop (fig. 21, Cr), and often a fourth part, or proventriculus. Like- 

 wise, two parts are usually distinguished in the intestine. From the 

 anterior end of the latter there grow out a varying number of excre- 

 tor}^ tubes, the Malphigian tubules (fig. 21, Mai). All the parts 

 derived from the ectodermal stomodeum and proctodeum are char- 

 acterized by being lined with an intima continuous with the cuticula 

 of the outer body wall. The stomach, being of endodermal tissue, 

 has no such lining. 



SoeGng 



Fig. 21.- — Diagrammatic lengtliwiso siection of a grasshopper, showing 

 general location of principal internal organs 

 An, anus; Ant, antenna; Br, brain; Cr, crop (a part of the stomod- 

 eum) ; Ht, heart; Int, intestine; Mai, Malpighian tubules; Mth, mouth; 

 Ql, oesophagus ; iioeOng, suba?sophageal ganglion ; Vent, stomach (ven- 

 triculus) ; VNC, ventral nerve cord ; W, wings. 



THE RESPIRATORY SYSTEM 



The chemical changes that the protoplasm of living things under- 

 goes in the process of keeping alive require a supply of oxygen and 

 produce an excess of carbon dioxide. Hence, there goes on a 

 constant " exchange " or respiration of these gases between the 

 organism and the outer world. Small, soft-bodied animals, and 

 embryos within the egg respire directly through the skin. But 

 mo.st animals in the adult stage have some special anatomical ar- 

 rangement for bringing the outside air into the body where the 

 tissues Avill have closer or easier access to it. Some have adopted 

 and evolved one method, some another. Insects have developed a 

 system of air tubes, or tracheae, that brancli profusely through- 

 out the body (fig. 22) and end in thin- walled tracheoles on all the 

 tissues that need oxygen for their maintenance. 



