DWELLINGS. 141 



fragments of aquatic leaves, and little fragments of 

 wood which have been sufficiently long in the water 

 to have thoroughly imbibed it and so become heavy 

 enough to keep themselves at the bottom, or at least 

 to prevent them from floating to the surface. It is 

 the larva of Phryganea striata which has been best 

 studied; those of neighbouring species evidently act 

 much in the same way, with differences only in detail. 

 The little carpenter stops a fragment rather longer 

 than his own body, lies on it and brings it in contact 

 with other pieces along his own sides. He thus 

 obtains the skeleton of a cylinder. The largest holes 

 are filled up with detritus of all kinds. Then these 

 materials are agglutinated by a special secretion. 

 The larva overlays the interior of its tube with a 

 covering of soft silk which renders the cylinder water- 

 tight and consolidates the earlier labours. The insect 

 is thus in possession of a safe retreat. Resembling 

 some piece of rubbish, it completes its metamorphosis 

 in peace, undisturbed by the carnivora of the stream. 

 There is here already a tendency towards the dwellings 

 of which I shall speak later on, and which are entirely 

 formed of the external environment. 



Animals who establish their home in the natural 

 or artificial dwellings of others. — Between the beings 

 whom nature has endowed with a shelter and those 

 who construct it by their own industry, we may 

 intercept those who, deprived of a natural asylum and 

 not having the inclination or the power to make one, 

 utilise the dwellings of others, either when the latter 

 still inhabit them, or when they are empty on account 

 of the death or departure of the owner. In the 

 natural sciences there is no group of facts around 

 which may be traced a clear boundary; each of them 



