INSECTS 277 



guts, lined with cuticle as in the crayfish. The fore-gut 

 comprises (i) the mouth, with a tongue-like ridge 

 System tery an d the opening of the duct of the salivary 

 glands, (ii) the narrow gullet, lying in the neck, 

 (iii) the swollen crop, (iv) the proventriculus or gizzard, which 

 has muscular walls and contains six hard, cuticular teeth 

 and some pads covered with bristles which form a strainer. 

 The mid-gut or mesenteron, lined by soft endoderm, is 

 short and narrow and bears at its beginning seven or eight 

 club-shaped hepatic azca, which secrete a digestive fluid. 

 The gizzard projects funnel-wise into the mid-gut. The 

 hind-gut is coiled and divided into a narrow ileum, a 

 wider colon, and a still wider rectum, which has six in- 

 ternal ridges. A pair of diffuse salivary glands lie on 

 each side of the gut, and between each pair lies a salivary 

 bladder or receptacle. The ducts of the two glands of each 

 side join ; the ducts of the two sides then unite to form a 

 median duct, and this is joined by another median duct 

 formed by the union of the ducts of the receptacles. The 

 common duct opens into the floor of the mouth between 

 the tongue and the lower lip. At the beginning of the 

 hind-gut a number of long, fine Malpighian tubules are 

 attached. They have an excretory function, and their 

 lumina often contain uric acid which has been shed out 

 by the glandular epithelium which lines them. 



The respiratory system consists of branching tubes or 

 trachea, with a spirally thickened, chitinous lining, which 

 arise from ten pairs of openings or stigmata at the sides 

 of the body. There are two large stigmata on 

 organs* '" 1 ' eacn s ^ e °f the thorax, one between prothorax 

 and mesothorax, one between mesothorax 

 and metathorax, and in each of the first eight abdominal 

 segments a stigma is placed on each side between the 

 tergum and the sternum. Air is pumped in and out by 

 contraction and expansion of the abdomen, and is carried 

 to the tissues by the fine branches of the tracheal system. 



The direct supply of air to the tissues is no doubt the 



reason for the simple condition of the blood- 



Biood vessels. vascu j ar S y Ste m, which consists of a long heart 



(Fig. 407), lying along the mid-dorsal line of the abdomen 



and thorax, an anterior aorta, and a system of ill-defined 



