6 WOODLICE 



However, a reversal of the response to light occurs when wood- 

 lice become somewhat desiccated, so that if their daytime habitat 

 should dry up they are not restrained there until they die, but 

 become attracted to light and are then able to wander in the open 

 until they find some other damp dark hiding place where they 

 again become photo-negative (Cloudsley-Thompson, 1952). 



The clearest adaptation to terrestrial life is to be found in the 

 pleopods. These are variously modified and in the more advanced 

 forms bear tufts of invaginated tubules forming 'lung-trees' or 

 'pseudotracheae'. Each tree opens to the exterior by a slit-like 

 aperture near the edge of the pleopod, and the minute ramifying 

 tracheae are thin-walled tubes surrounded by blood which carries 

 oxygen to the tissues of the body. When the air is dry, the pleopods 

 are probably kept moist by water that diffuses from the body 

 fluids of the animal. 



Like most other woodlice, Ligia oceanica is nocturnal in habit 

 and emerges during the night at low tide to feed on seaweeds such 

 as Fucus and other algae. The species is strongly photo-negative 

 and tends to remain under cover on moonlight nights. Edney 

 (1954) has recently pointed out that Ligia can live on land as a 

 result of wide osmotic tolerance rather than by developing os- 

 motic independence. Since other species of woodlouse can lose 

 much water by evaporation without dying, it seems likely that 

 such osmotic independence is characteristic of the group as a 

 whole. The colour oi Ligia ranges from a dark greyish-green to a 

 light, dirty brown, while young specimens have two light- 

 coloured patches on the middle of the dorsal side. The British 

 Ligia oceanica^ as well as the American L. baudiniana and L. 

 exotica, have been found to show well marked colour responses 

 due to the expansion and contraction of colour pigment cells or 

 chromatophores, so that they become light when placed on a 

 white background and turn dark on a black background. In ad- 

 dition there is a diurnal rhythm of colour change and they tend to 

 be dark by day and pale by night. 



The family Trichoniscidae includes a number of small, elonga- 

 ted woodlice that are fairly widely distributed in damp places 

 under moss, bark, fallen leaves, logs and so on. The Oniscidae are 

 less dependent on moisture, but the common Philoscia muscorum 



