3 i4 PHYLVM ARTHROPODA. 



cavity, in which there is a tongue-like ridge, and into 

 which there opens the duct of the salivary glands ; (b) the 

 narrow gullet or (esophagus ; (c) the swollen crop ; (d} 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 is 



short and narrow, 

 and with its anterior 

 end seven or eight 

 club-shaped diges- 

 tive (pancreatic) 

 outgrowths are con- 

 nected. 



(3) The hind-gut 

 (proctodaeum) is 

 _ _ lined by a chitin- 



FIG. 15^- Transverse section of insect. ous CUtlCle. 



After Packard. Convoluted and 



//., Heart ; g., gut ; ., nerve-cord ; st., stigma ; tr., divided into 



trachea ; u>., wing ;/., femur of leg. ., ., , 



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 chitinous lining throughout. 



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 

 pericardial space, and drives it forwards by a slender aorta. 

 The blood circulates, however, within ill-defined spaces in 

 the body. 



Excretory system. There are sixty or so fine (Mal- 

 pighian) tubules, w r hich rise in six bundles from the begin- 

 ning of the ileum, and twine through the " fatty body '' 

 and in the abdominal cavity. The absence of nephridia 

 in insects has been already mentioned. 



