8 WOODLICE 



the Middle East. The animals live together in vertical holes 5-6 

 cm in diameter and many centimetres in depth. A digging re- 

 action is released by a temperature of 35° C if the soil is dry, and 

 45° C if it is moist. The head is placed against a small stone or 

 some other rigid body: the anterior legs then lift the sand back- 

 wards while the posterior ones throw it away. Sometimes several 

 animals combine to dig a single hole and frequently two woodlice 

 can be seen digging head to head. 



The pill-woodlice or Armadillidiidae are so called because of 

 their habit of rolling into a ball like a little armadillo. The ability 

 to do this is by no means restricted to this family however, but has 

 evolved independently in several diverse groups. In forms that 

 can curl up completely the head has become flattened in an antero- 

 posterior direction so that its height is much greater than its 

 length and the front part is covered by the last abdominal appen- 

 dages or uropods when the animal rolls up. (In other woodlice the 

 uropods project like a couple of small tails from the hinder end of 

 the body). The most common British species is Armadillidium 

 vulgar e, sometimes called the Till bug', which reaches a length of 

 18 mm and is a little more than twice as long as broad. The colour 

 varies from completely black to pale yellow, but the more usual 

 shades are light grey. 



As already mentioned, woodlice have little ability to prevent 

 loss of water by evaporation and excretion, and although they can 

 regain lost water both by actively drinking and absorbing mois- 

 ture through their pleopods, they can only survive on land as 

 a result of behaviour mechanisms that keep them in cool, moist 

 places. 



During the day they normally collect at the moist end of a 

 humidity gradient and avoid the light: it is at night that dispersal 

 to new environments mostly takes place. Changes in behaviour 

 between day and night have recently been demonstrated in Oniscus 

 asellus that can perhaps be correlated with the ecology of the 

 species as follows: a fall in the intensity of the humidity response 

 after dark enables the animals for a time to walk in drier places 

 than their day-time retreats, but increased photo-negative be- 

 haviour after exposure to dark ensures that they return to cover at 

 daybreak, and thus no doubt avoid the early bird (Cloudsley- 



