Glands — Kidney-Tubes 71 



modified cells beneath the skin, on four of the hind- 

 body sternites (fig. /|8). Scale-insects are provided 

 with wax-glands all over their bodies ; these secrete 

 a protective covering beneath which the insect lives 

 and shelters. Plantlice too protect themselves by a 

 waxy substance, the so-called " honey-dew," which 

 is secreted by glandular cells beneath the skin, and 

 passed out through two conspicuous tubes — out- 

 growths from the back of the hind-body. 



Kidney-tubes. — The kidney -tubes {Malpighian 

 tubes) of the Cockroach open into the food-canal 

 where the stomach joins the small intestine. They 

 are sixty to seventy in number, and exceedingly 

 fine. Microscopic examination shows that each tube 

 consists of a connective tissue layer and a base- 

 ment-membrane, surrounding large cells which line 

 the interior of the tube (fig. 46). These cells and 

 the central canal contain urate crystals, which are 

 formed by the excretory action of the cells on the 

 blood, and are passed on into the intestine and so 

 cast out of the body. The number of tubes in the 

 adult cockroach is large, as it is in Locusts and their 

 allies, Earwigs, Stoneflies, Mayflies, Dragonflies, Ants, 

 Wasps, Bees, Sawflies, and Ichneumon-flies. But in 

 all immature insects the number is small (4-6), and 

 this condition persists in the adult in Springtails, 

 Booklice, Termites, Bugs and their allies, Lacewing 

 flies. Caddis-flies, Moths, and Beetles. In most insects 

 the kidney-tubes open, as in the cockroach, at the 

 junction of the mid-gut with the hind-gut, in some 

 Bugs they open into the rectum, while in a few Frog- 

 hoppers they are moved forward to the mid-gut, 

 where they open separately from each other. In 

 some cases, where numerous tubes are present, they 

 are joined in tufts, the ducts from these opening 

 together into the intestine j in the Mole-cricket, all 



