Fat-body — Air-tubes 57 



Fat-body. — If a portion of this be examined 

 microscopically, it is seen to be made up of poly- 

 gonal fat-cells with large nuclei. In portions of the 

 tissues, however, the cell-boundaries have become 

 indistinct, the nuclei broken down, and the proto- 

 plasm replaced by granules and crystals of waste- 

 matter (urates) derived from various parts of the 

 body. These old cells burst and their contents are 

 absorbed by the blood-currents and carried to the 

 kidney-tubes by which they are passed out of the 

 system (l). The fat-body is an organ of great import- 

 ance to the insect on account of the chemical changes 

 which take place in its substance ; it is most highly 

 developed in long-lived insects. The light emitted 

 by Glow-worms, Fireflies, and other luminous insects 

 is due to chemical activity in a specialised portion of the 

 fat-body, which forms a "phosphorescent organ" with 

 an inner opaque and an outer transparent layer (34). 



Air-tubes. — Examination of the body of a cock- 

 roach shows us a pair of small oval holes, situated 

 on each of the two hindmost thoracic, and the first 

 eight abdominal segments just beneath the tergites. 

 These are the openings by which the insect breathes, 

 the spiracles through which air passes into the 

 complex system of tubes {trachea:\ which ramify 

 throughout the body, being accordingly everywhere 

 surrounded by the blood. The seven hinder spiracles 

 on each side are connected by a stout air-tube, pass- 

 ing along the side of the abdomen. From these, as 

 well as from the three foremost spiracles, branches 

 run upwards and downwards, connecting with paired 

 trunks running lengthwise near both the upper and 

 lower body-walls, so that the arrangement of tubes 

 resembles a series of loops or a ladder-like network 

 (fig. 40). Tubes branch in a complex manner over 

 the surface of the food-canal, while large trunks 



