330 PHYLUM ARTHROPODA. 
Alimentary system.—(1) The fore-gut (stomodzum) is 
lined by a chitinous cuticle continuous with that of the outer 
surface of the body. It includes—(a) the buccal or mouth 
cavity, in which there is a tongue-like ridge, and into 
which there opens the duct of the salivary glands ; (0) the 
narrow gullet or cesophagus ; (c) the swollen crop; (@) the 
gizzard, with muscular walls, six hard cuticular teeth, and 
some bristly pads. 
There is a pair of diffuse salivary glands on each side of the crop, and 
between each pair of glands a salivary receptacle. The ducts of the 
two salivary glands on each side unite; the two ducts thus formed 
combine in a median duct, and this unites with another median duct 
formed from the union of the ducts of the receptacles. The common 
duct opens into the mouth. 
(2) The mid-gut (mesenteron) is lined by endoderm. It 
Fic. 177.—Transverse section of insect.—After Packard. 
‘., Heart; g., gut; #., nerve-cord; sz., stigma; ¢r., trachea; w., wing; 
J, femur of leg. 
is short and narrow, and with its anterior end seven or 
eight club-shaped digestive (pancreatic) outgrowths are 
connected. 
(3) The hind-gut (proctodzum) is lined by a chitinous 
cuticle. It is convoluted and divided into narrow ileum, 
wider colon, and dilated rectum with six internal ridges. 
Respiratory system.—The tracheal tubes, which have 
ten pairs of lateral apertures or stigmata, ramify throughout 
the body, and have a spirally thickened chitinous lining. 
Circulatory system.—The chambered heart lies along 
the mid-dorsal line of abdomen and thorax. It receives 
blood by lateral valvular apertures from the surrounding 
