DISTRIBUTION 1 47 



desiccation is met by this tropical species in three ways; the shell is 

 relatively impermeable so that water loss is reduced, the creature 

 can tolerate some increase in the concentration of its blood without 

 ill effect, and the Robber Crab is a good regulator, so that when it 

 finds a pool of water it can soon restore its blood to a normal level. 

 Strangely enough it is possible to drown this crab if it is forced to 

 be immersed for a day or so; this is probably due to the fact that 

 it is an air-breather and cannot obtain sufficient oxygen from that 

 dissolved in water. 



Representatives of the smaller Crustacea are also found on land, 

 notablv among the damp leaves of forest floors. Several harpacticid 

 copepods are known from such situations, and recently an ostracod, 

 Alesocvpris terrestris, has been discovered among forest humus 

 from South Africa. The only apparent modification which this 

 ostracod shows is a strengthening of the limbs for moving about 

 among the humus. 



A few amphipods also live on the floors of forests. One of these. 

 Talitroides alluaudi, has been introduced into greenhouses in 

 various parts of Europe along with tropical plants, and is now 

 established and breeding in these artificial conditions as far north 

 as Helsinki. 



The most numerous Crustacea dwelling entirely on land are the 

 Oniscoidea or woodlice. These isopods live in a variety of habitats, 

 mostly in places that are somewhat damp. The different genera and 

 species differ in their abilities to survive in dry air, but all of them 

 have to return to damp air fairlv quickly to avoid death from 

 desiccation. One wonders why they have not developed a water- 

 proof covering of the type found in the insects. The answer to this 

 may be that a permeable cuticle allows cooling by the evaporation 

 of water, as we have already seen in Ligia (p. 146), which is a 

 member of this group. The advantages of this cooling effect at 

 critical temperatures may have been sufficient to prevent water- 

 proofing from becoming established. 



The Oniscoidea are not very different in structure from other 

 isopods. The only obvious structural modification to terrestrial life 

 is that some of the more advanced members have small tubes called 

 pseudotracheae in their pleopods. These increase the respiratory 

 efficiency of the pleopods in air, by taking air nearer to the tissues, 

 but they do not penetrate beyond the pleopods and so cannot 

 approach the efficiency of the tracheae in insects, which ramify 

 throughout the body. 



