124 
EN TOMOLOGY 
The Malpighian tubes (Fig. 156) are evaginations of the 
proctodzum and are consequently ectodermal. 
A cross sec- 
tion of a tube shows a ring of from one 
variety of 
bonate. 
Portion of Malpighian 
tube of caterpillar, Samia 
cecropia, surface view. 
concerned i 
body in Collembola and Orthop- 
tera serves for the permanent stor- 
age of urates. 
7. CIRCULATORY SYSTEM 
Insects, unlike vertebrates, have 
no system of closed blood-vessels, 
but blood 
through the body cavity to enter 
the wanders freely 
eventually the dorsal vessel, which 
resembles a heart merely in being 
a propulsatory organ. 
supplied with trachez. 
substances, 
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 
In function, the 
Malpighian tubes are analogous to the 
vertebrate kidneys and contain a great 
chief among 
which are uric acid and its derivatives 
(such as urate of sodium and of ammo- 
nium), calcium oxalate and calcium car- 
Parts of the fat-body may also be 
excretion: thus the fat- 
Cross section of Malpighian tube 
of silkworm, Bombyx mori. b, 
basement membrane; c, crystals; 1, 
lumen; mn, nucleus; ff, 
Greatly magnified. 
intima; 7, 
peritoneal layer. 
Dorsal Vessel.—The dorsal vessel (Figs. 158, 162) is a 
delicate tube extending along the median dorsal line immedi- 
