38 



THE BIOLOGY OF INSECTS 



We are compelled to regard every cell — whatever be its 

 specific function — as a minute but marvellous laboratory 

 in which the characteristic chemical changes are continually 

 wrought through the influence of unknown ferments 

 formed within the cell, '* intracellular enzymes " as they 

 have appropriately been called. 



The chemical processes thus briefly reviewed are neces- 

 sarily accompanied by the production of waste substances 

 which must be eliminated from the body ; thus we are led 

 to consider the subject of excretion. The proteins of the 



living tissue, continually 

 built up from the materials 

 supplied in the blood, are 

 continually broken down 

 through the activities of the 

 cells, and thus nitrogenous 

 waste-products — of simple 

 chemical composition when 

 compared with the im- 

 mensely complex proteins 

 — are set free. Just as the 

 blood resigns to the tissue- 

 cells the needed food-ma- 

 terials, so it takes from 

 them the eflFete waste-sub- 

 stances, carrying these to 

 those organs of excretion whose function it is to eliminate 

 them from the body. 



In most of the great groups of animals the organs of 

 nitrogenous excretion are essentially tubular in structure, 

 consisting of tubes or groups of tubules around whose walls 

 blood may circulate — whether flowing through capillary 

 vessels or freely bathing the cells — and into whose cavities 

 the waste products may be passed, as the tubular excretory 

 systems lead directly or indirectly to some outward opening 

 of the body. In insects, as has already been mentioned, the 

 organs of nitrogenous excretion are the Malpighian or 

 kidney- tubes (Fig. lo, Mt) which grow out from the front 



Fig. 13. — Malpighian Tube of Honey 

 Bee (Apis mellifica) shown in 

 obHque cross-section, e, epithe- 

 lium ; b, basement membrane ; tr, 

 small tracheal tube with fine 

 branches on surface of Malpighian 

 tube. X 450. 



