MANUAL OF ZOOLOGY. 



is furnished with salivary glands of varying size and complexity, 

 and is provided in some of the Suctorial Insects with a dilata- 

 tion called the "sucking sto- 

 mach." Behind the pyloric 

 aperture of the stomach, with 

 very few exceptions, is a vari- 

 able number of caecal convo- 

 luted tubes (fig. 1 06, e), which 

 open into the intestine, and are 

 called the " Malpighian tubes." 

 These are often looked upon 

 as representing the liver, but 

 are by some believed to have 

 a renal function. If the Mal- 

 pighian vessels truly perform 

 the functions of a liver as their 

 position would appear to prove 

 then the kidneys will be 

 represented by a series of cse- 

 cal tubes which are only occa- 

 sionally present, and which open 

 into the rectum, close to the 

 cloaca. There are no absor- 

 bent vessels, and the products 

 of digestion simply transude 

 through the walls of the alimen- 

 tary canal into the sinuses or irre- 

 gular cavities which existbetween 

 the abdominal organs. The ap- 

 paratus of digestion does not 

 differ essentially from the above in any of 'the Insects ; but the 

 alimentary canal is, generally speaking, considerably lengthened 

 in the herbivorous species. 



There is no definite and regular course of the circulation in 

 the Insects. The propulsive organ of the circulation is a long 

 contractile cavity, situated in the back and termed the " dorsal 

 vessel." This is composed of a number of sacs (ordinarily 

 eight), opening into one another by valvular apertures, which 

 allow of a current in one direction only viz., towards the head. 

 The blood is collected from the irregular venous sinuses which 

 are formed by the lacunae and interstices between the tissues, 

 and enters the dorsal vessel from behind ; it is then driven 

 forwards, and is expelled at the anterior extremity of the body. 

 Respiration is effected by means of " trachea:;," or branched 

 tubes, which commence at the surface of the body by lateral 



ig. 1 06. Digestive system of a Beetle 

 (Carabus auratus). a CEsophagus ; b 

 Crop ; c Gizzard : d Chylific stomach ; 

 e Malpighian tubes ; f Intestine ; g 

 Cloaca ; h Supposed renal vessels. 



