CHAPTER I 



WOODLICE 



Classification and distribution 



Woodlice are included in the sub-order Oniscoidea of the 

 crustacean order Isopoda. They form a reasonably homogeneous 

 group and are of particular interest because there are several com- 

 mon species showing different degrees of adaptation to life on 

 land. The majority are between one and two centimetres in length, 

 and the small size of certain genera such as Platyarthrus, Trichonis- 

 cus and Sphaerobathytropa is almost certainly a secondary feature 

 correlated with various regressive characters such as a simplifica- 

 tion in the structure of the eyes and appendices, reduction in the 

 number of body segments and loss of pigmentation. An analogous 

 phenomenon is found in the Acari. 



Brood pouch 



Fig. 3. Fifth thoracic segment of a female Oniscus asellus cut 



across to show the brood pouch with eggs. (After Webb and 



Silleni, 1906.) 



The Oniscoidea are somewhat oval in form and their bodies are 

 arched, the curve varying in different genera and species. The 

 head bears two large antennae and a smaller pair of antennules 

 anterior to them. The thorax consists of seven segments which are 

 often considerably broader than the six succeeding ones that form 



A 1 S.S.C.M. 



