148 A BIOLOGY OF CRUSTACEA 



The isopods were in a sense pre-adapted for life on land. They 

 have hiring mouthparts for feeding on large particles; any crusta- 

 cean with a filter mechanism obviously could not leave the water 

 and still be able to feed. The possession of a brood pouch in which 

 the young develop until they are liberated as miniature adults is 

 also an important pre-adaptation. A crustacean with a nauplius, or 

 any other type of swimming larva, could not forsake the water 

 without at least the necessity of returning to liberate its young. A 

 further pre-adaptation is found in the structure of the legs, which 

 are well suited for walking on land. 



When the woodlice moved from water to land they encountered 

 a completely new type of predator — the spider. The importance of 

 spiders as predators of small land-dwelling creatures should not be 

 underestimated. It has been calculated that there is an average of 

 about 50,000 spiders per acre over the whole of England and Wales. 

 As a protection against this horde of predators the woodlice have 

 developed a series of glands along the edges of the body which 

 discharge a distasteful fluid when the woodlouse is bitten by a 

 spider. This might seem a bit late in the dav to start repelling one's 

 assailant, particularly when it has a poisonous bite, but it has been 

 found that woodlice can withstand a single spider-bite without 

 apparent ill effect. The spider, after it has bitten a woodlouse, shows 

 obvious signs of distaste and wipes its mouthparts vigorously. 



The success of woodlice as land animals is due to the combination 

 of the pre-adaptations described above, the development of distaste- 

 fulness to a particularly important group of predators, and their 

 behavioural responses (chapter 6) which keep them in damp places 

 and so protect them from the danger of desiccation. 



DISTRIBUTION OF ALLIED SPECIES 



Within the various groups of Crustacea there is a tendency for 

 allied species to live in similar places. The ostracods of Germany 

 provide a good example. Most of them can be ranged in two 

 families: the Cypridae and the Cytheridae. There are twenty 

 German genera of the Cypridae; of these, only two have representa- 

 tives in brackish water, and all the others are confined to fresh 

 water. This is in marked contrast to the Cytheridae, which have 

 twenty-three genera in the sea and only three genera in German 

 fresh waters. 



When the distribution of such a group is analysed in detail it is 



