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- 



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- I, lumen; n, nucleus; p, peritoneal layer. Greatly magnified. 

 cropia, surface view. 



tain a great variety of substances, chief among which are uric acid aiid 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- 



