WOODLICE 3 



appear, however, they are generically indistinct from hving forms, 

 and Vandel beHeves that the ancestors of woodUce became ter- 

 restrial during the second half of the Palaeozoic era. He bases this 

 conclusion on the fact that all the main types of organisation 

 within the Oniscoidea have a world-wide distribution and con- 

 sequently must have a very ancient origin. 



The most primitive and at the same time the least well adapted 

 of the woodlice to terrestrial conditions are littoral species belong- 

 ing to the family Ligiidae. There are two British species in this 

 family, Ligia oceanica, the largest of the British woodlice — up to 

 30 mm in length and rather more than twice as long as broad — and 

 the smaller Ligidium hypnorum which is sometimes found far in- 

 land, nearly always in the neighbourhood of water. 



Ligia oceanica has a wide distribution around the shores of 

 Great Britain and indeed of practically the whole north coast of 

 Europe. It also occurs in France, Spain, Morocco and America. 

 Although never found far from the sea, it is truly terrestrial and can 

 withstand prolonged submersion in sea water only if this is well 

 aerated. The normal habitat is in deep narrow crevices in the 

 rocks just above high-tide level, under stones on sandy beaches or 

 on the sides of quays: hence the name 'quay-louse' or 'quay- 

 lowder'. In St. Kilda Ligia has been found in the crevices of 

 boulders over 450 ft. above sea level, but on that exposed islet 

 spray is often blown to this height (Nicholls, 1931). I have found 

 specimens over a mile inland on the Isle of Man. 



The family Trichoniscidae also occurs in very moist places, but 

 the Porcellionidae and Armadillidiidae are found in progressively 

 drier localities. Now this sequence is also one of increasing morpho- 

 logical specialisation within the group, the significance of which 

 will be considered below. 



Hatchett (1947) has found that in Michigan Armadillidium 

 nasatum does not occur outside buildings, while A. vulgar e is 

 chiefly found around human habitations; Cylisticus convexus prefers 

 rocky regions and Ligidium longicaudatum occurs only in very wet 

 situations. Metoponorthus pruinosus is somewhat urban in its dis- 

 tribution. Porcellio scaber lives on beaches, river banks and other 

 moist situations. P. spinicornis requires a habitat where plenty of 

 lime is available and P. rathkei is generally distributed but more 



