ANATOMY AND PHYSIOLOGY 



99 



Not more than six and frequently only four occur in the embryo (Wheeler ) , 

 though these few embryonic tubes may subsequently branch into many. 

 The Malpighian tubes (Fig. 156) are evaginations of the proctodaeum 

 and are consequently ectodermal. A cross-section of a tube shows a 

 ring of from one to six or more large polygonal cells (Fig. 157), which 

 often project into the lumen of the tube; the nuclei are usually large and 



may be branched, as in Lepidoptera. A chitinous 

 intima, traversed by pore-canals, lines the tube, 

 and a delicate basement membrane is present, sur- 

 rounded by a peritoneal layer of connective tissue. 

 Furthermore, the urinary tubes are richly supplied 

 with tracheae. In function, the Malpighian tubes 

 are analogous to the vertebrate kidneys and con- 



Ojt 



n 



FIG. 156. Portion FIG. 157. Cross-section of Malpighian tube of silkworm, 



of Malpighian tube of Bombyx mori. b, basement membrane; c, crystals; I, intima; 



caterpillar, Samia ce- 1, lumen; n, nucleus; p, peritoneal layer. Greatly magnified. 

 cropia, surface view. 



tain a great variety of substances, chief among which are uric acid and its 

 derivatives (such as urate of sodium and of ammonium), calcium oxalate 

 and calcium carbonate. 



Parts of the fat-body may also be concerned in excretion; thus the 

 fat-body in Collembola and Orthoptera serves for the permanent storage 

 of urates. 



7. CIRCULATORY SYSTEM 



Insects, unlike vertebrates, have no system of closed blood-vessels, 

 but the blood wanders freely through the body cavity to enter eventually 

 the dorsal vessel, which resembles a heart merely in being a propulsatory 

 organ. 



Dorsal Vessel. The dorsal vessel (Figs. 158, 162) is a delicate tube 

 extending along the median dorsal line immediately under the integu- 



