234 



ZOOLOGY FOR MEDICAL STUDENTS 



CHAP. 



excretory tubes (Malpighian tubes) opening into the alimentary canaJ 

 near the front limit of the proctodaeum. 



In connexion with the disappearance of the nephridial tubes during 

 the evolution of the Arthropoda it should be remembered that in this 

 phylum the function of excretion is carried out to a great extent by the 

 skin, waste nitrogen being contained in the chitin which covers the 



* — LigM 



J 4r 



FtG.. 105. 



Record of the position of about 200 homopterous insects whichi alighted on a sheet of paper and 

 then orientated themselves towards a lamp. 



Tlie thick arrows indicate the position of the long axes of the insects' bodies 1 the two fine lines 

 indicate the direction of the source of light. 



surface and is got rid of at each ecdysis. Furthermore the coloured 

 pigments which are often present in the exoskeleton are excretory sub- 

 stances : for example, in the group to which our common Cabbage 

 Butterflies {Pieris) belong the characteristic white pigment is impure 

 uric acid, while the orange and yellow pigments are derivatives of uric 

 acid. The Arthropod then has developed the habit of depositing its 

 nitrogenous excretory material in insoluble form on the surface of its 



