Class 3. Insecta. 



239 



connection ; it is also the absorptive region ; it is saccular, 

 and sometimes separated into several sections. The epithelium 

 of the alimentary canal secretes the digestive fluid ; sometimes 

 small evaginations, which project externally from the alimentary 

 canal as warts or papillae, may have this function ; there is never 

 a specialised liver. The proctodseum is usually divided into an 

 anterior narrower and a posterior wider portion ; the anus is situated 



Fig. 201. 



Fig. 202. 



Fig. 201. Diagram of the 

 chief trunks of the tracheal 

 system of an Insect ; the 

 central nervous system is also 

 shown, a antenna, o eye, st' 

 anterior stigma, I longitudinal 

 trunk. — After Kolbe. 



Fig. 202. Portion of a 

 trachea from a, Gall - fly 

 larva (somewhat diagrammatic). 

 z a cell of the wall. — Orig. 



on the terminal segment. The Malpighian tubules, delicate 

 unbranched, brightly-coloured (white, yellow^ brown, or green) tubes, 

 open into the proctodiBum at its junction with the mesenteron. There 

 is usually only a small number, four to six, when they reach a con- 

 siderable length ; in the Hymenoptera and some of the Orthoptera, 

 however, there is a much larger number of shorter tubes. These 

 constitute the excretory apparatus. 



