642 



ZOOLOGY 



SECT. 



elongated, and its posterior portion {ed.) is dilated to form a wide 

 rectum {r.), which opens on the exterior by an anal aperture 

 situated on the ventral side of the last segment of the abdomen. 

 Anal glands {ad.), producing an odoriferous secretion, often open 

 into the rectum. 



The tracheal system (Fig. 520) communicates with the ex- 

 terior through a number of apertures— the stigmata (st) — which 



vary in the details of 

 their arrangement in 

 the different orders. 

 They are always pro- 

 tected against the 

 entry of foreign par- 

 ticles by some means 

 — either by being 

 surrounded by spe- 

 cial bundles of hairs, 

 or by being provided 

 with a special sieve- 

 like membrane. In 

 most cases they are 

 capable of being 

 closed by muscular 

 action. In some In- 

 sects, mainly those 

 adapted for active 

 flight, such as the 

 Hymenoptera, the 

 tracheal system is 

 dilated in certain 

 parts of the body to 

 form comparatively 

 large air-sacs or air- 

 rcservoirs (tb). In the 

 aquatic larvse of some 

 Insects there is a 

 series of soft external, 

 simple or divided, 

 processes — the tracheal gills (Fig. 521) — attached to the abdominal 

 segments and richly supplied with trachese, which have no com- 

 munication with the exterior. 



The blood-vascular system is, in comparison with the other 

 systems of organs, not very highly developed, the need of an 

 elaborate system of vessels being greatly diminished by the way in 

 which all the tissues and organs are supplied with oxygen through 

 the system of trachea?. The blood is colourless or faintly yellowish 

 or greenish, and contains colourless corpuscles. A contractile 



Fig. 520. — Nervouw, tracheal, and digestive sj'stems of the 

 Honey-bee. a. antenna ; aic, compound eye ; foj, b2, 63, 

 the three pairs of legs ; nn, stomach ; f', hind-gut ; luii, 

 honey stomach (crop) ; rd, rectal glands ; st, stigmata ; 

 tbf vesicle of tracheal system ; vm, Malpighian vessels. 

 (From Lang's Comparative Anatotny.) 



