264 THE BIOLOGY OF INSECTS 



adaptability for permanent residence in northern latitudes 

 through its intolerance of the cold or damp of winter. 



Other butterflies of the same family (Nymphalidae), 

 however, such as the " Peacock " (Vanessa to) and the Small 

 Tortoiseshell (Aglais urticae), whose caterpillars feed on 

 nettles, are not only commonly conspicuous members of 

 the British insect fauna, but permanently resident species, 

 because the butterflies of the second or third brood, which 

 emerge from the pupa in autumn, are able to survive the 

 winter in various shelters whither they betake themselves 

 at the onset of cold weather. So strong is the adaptation 

 of Aglais urticae to severe climatic conditions that it is a 

 member of the Arctic fauna, a resident in Greenland. In 

 such a case the presence of the insect in the far northern 

 portion of its range is a demonstration of its power to 

 endure extreme severity of climate. 



Dragon-flies, as a group, are also remarkable for their 

 strong flight, and many of them occupy a wide range of 

 territory. One of our common British species, Lihellula 

 quadrimaculata, has a range extending all round the northern 

 hemisphere, and the immense migratory swarms in which it 

 sometimes appears must be an important factor in its 

 distribution. R. J. Tillyard (19 17) points out that the large 

 Pantala flavescens and some species of the allied Tramea 

 '' travel far and wide and have overspread the whole of the 

 tropics." He records how an AustraHan dragon-fly, Hemi- 

 cordulia tau, " has recently colonised Tasmania across a 

 strait two hundred miles wide." 



Very different from such strong flying insects as butter- 

 flies and dragon-flies are the tiny, lowly wingless springtails 

 (Collembola), many kinds of which may be found in our 

 country beneath stones, under bark, among fallen leaves, 

 or feeding on soft plant-tissues or on decaying vegetable 

 or animal substances. Some of these, despite their small 

 size and feeble cuticle, have an enormously wide range. 

 There is a dark, almost black, species Achorutes viaticus 

 (Fig. 66), not more than j^ in. in length, found commonly 

 all over our islands and able to adapt itself to what seem the 



