206 THE LIFE OF CRUSTACEA 



that are kept by the ants for the sake of the secre- 

 tions which they yield. 



The Woodlice, like some other Isopoda, have a 

 peculiar method of moulting. Instead of the whole 

 exoskeleton being cast off at one time, as in other 

 Crustacea, that of the hinder half of the body is 

 moulted first, and it is only after two or three days, 

 when the new cuticle has hardened, that the exo- 

 skeleton of the anterior half follows. As a result of 

 this arrangement, it occasionally happens that speci- 

 mens are found with the fore part of the body differing 

 in colour from the hind part, owing to the one having 

 been moulted more recently than the other. 



Woodlice occur in most regions of the globe, 

 and one of the most remarkable features of their 

 geographical distribution is the extremely wide 

 range of certain species. This is probably due, 

 at least in many cases, to their accidental transport 

 by human agency. Thus, Porcellio scaber, so common 

 in this country, is also found in great abundance in 

 New Zealand ; but Professor Chilton notes that it 

 is usually found near buildings, and only rarely in 

 the native bush, so that there can be no doubt that 

 it has been introduced by artificial means. 



