64 



INSECT PHYSIOLOGY 



fig. 10. — Types of excretory system in insects: schematic 



Hind-gut indicated by a heavy line internally. Explanation in text 



In many beetles, phytophagous, carnivorous, and omnivorous 

 species, as well as those that feed on dry substances, such as the meal- 

 worm (Tenebrio), the upper parts of the Malpighian tubes closely 

 invest the rectum, being bound to it by a delicate membrane (Fig. 

 10, B). The precise physiology of this arrangement is not fully 

 known; but it is clear that it serves to add the absorptive powers of 

 the Malpighian tubes to those of the rectal epithelium. The arrange- 

 ment seems always to be associated with remarkable powers of drying 

 the excrement. The Malpighian tubes of the mealworm produce a 



