1 8 INJURIOUS AND USEFUL INSECTS 



The eight blind tubes are believed to secrete a fluid which aids 

 in digestion. The Malpighian tubules contribute no useful 

 product, but remove waste nitrogenous matter from the blood, 

 and deliver it into the intestine. The intestine receives and 

 discharges the waste products. The food undergoes a twofold 

 digestion, (i) In the crop it is acted upon by the salivary 



Fig. t6. — Salivary glands and receptacle of cockroach, right side. The arrow marks 

 the opening of the common duct on the back of the lingua. A, side view of lingua ; 

 B, front view of lingua (a tongue-like projection from the floor of the mouth). 



fluid, which changes starch into glucose, and thus renders 

 starchy food soluble. The dissolved glucose is absorbed by 

 the wall of the crop, and passes into the blood. (2) In the 

 stomach fats are emulsified, so that they become capable of 

 absorption, and flesh or foods of similar composition become 

 dissolved by the secretion of the eight tubes. The fluid 

 secretion of these tubes can be forced into the crop, while the 



