124 



ENTOMOLOGY 



FIG. 156. 



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

 proctodseum and are consequently ectodermal. A cross sec- 

 tion 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 base- 

 ment membrane is present, surrounded 

 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 contain a great 

 variety of substances, chief among 

 which are uric acid and its derivatives 

 (such as urate of sodium and of ammo- 

 nium), calcium oxalate and calcium car- 

 bonate. 



Parts of the fat-body may also be 

 concerned in excretion ; thus the fat- 

 body in Collembola and Orthop- p IG I57 

 tera serves for the permanent stor- 

 age 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 Cross section of Malpighian tube 

 eventually the dorsal vessel, which of silkworm, Bombyx mori. b, 



. , " basement membrane; c, crystals; *', 



resembles a heart merely in being intima; /, lumen; n , nucleus; />, 



a propulsatory Organ. peritoneal layer. Greatly magnified. 



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

 delicate tube extending along the median dorsal line immedi- 



Portion of Malpighian 

 tube of caterpillar, Samia 

 cecropia, surface view. 



