2 WOODLICE 



the abdomen. Each of the thoracic segments carries a pair of 

 walking legs and in the female, at the time when the eggs are laid, 

 a pair of plates arises on segments two to five. These plates to- 

 gether form a brood pouch in which the eggs are carried until they 

 hatch and in which the young remain for some time afterwards. 

 The appendages of the abdomen are also plate-like, with the ex- 

 ception of the last pair or uropods. The endopodites or pleopods 

 are homologous with the gills of aquatic Isopods and have a 

 respiratory function, while the outer exopodites act as a protective 

 cover. In the male the first two pairs of abdominal appendages are 

 specially modified, their endopodites being long and pointed, 

 while the uropods are often considerably larger than in the female 

 and their shape is sometimes of value in classification. 



Fig. 4. Pleopods of first thoracic segment of Porcellio scaber 

 showing pseudotracheae. Male on the left, female on the right. 

 (Diagrammatic.) 



In a comprehensive survey of the Oniscoidea, Vandel (1943) 

 recognises some 18 families, of which five are included in the 

 British fauna. These are the Ligiidae, Trichoniscidae, Oniscidae, 

 Porcellionidae and Armadillidiidae. The geological histor}' of the 

 Crustacea is a long one and fossils occur in the Old Red Sandstone 

 of the Devonian period and in the Carboniferous Coal Measures. 

 Fossil woodlice, however, have not been found below the Upper 

 Eocene which might suggest that colonisation of the land has been 

 achieved somewhat late in the history of the group. When they do 



