GIRDERS, TIES, AND BUTTRESSES. 



193 



When it is full-fed, and about to enter the pupal state, it 

 proceeds to prepare its habitation. As a larva, when it desired 

 to feed, it protruded its head and the front of its body from the 

 mouth of the tube, and then crawled about in search of nourish- 

 ment, dragging the tube with it, and holding it firmly by 

 means of the claspers with which the end of the body is fur- 

 nished. But when it becomes a pupa it is no longer able to 

 defend itself, and is instinctively compelled to secure its safety 

 in some peculiar manner. 



It cannot fasten up the entrance entirely, because it would 

 not be able to breathe unless water could pass over its body. 

 Accordingly, it constructs a grated window precisely like those 

 of the old castles, so that water can pass freely, while no enemy 

 can gain admittance. Unlike, however, the grated windows of 

 the castle, which had no pretence to beauty, the Caddis always 

 constructs its barriers in some definite pattern. Each species 

 appears to have its own peculiar pattern, but all agree in making 

 their window, if we may so call it, exactly like a wheel-window 

 before the glass is inserted. 



When the pupa is about to make its final change into the 

 perfect form, it cuts away the tracery with a pair of sharp 

 jaws, with which it is furnished for this sole purpose, emerges 

 from the water, throws off the pupa- skin, and issues forth as 

 a Stone-fly. 



Girders, Ties, and Buttresses. 



Next in order come the means by which walls are supported 

 internally by Girders and Ties, and externally by Buttresses. 



Of late years the Girder, in its many varieties, has come into 

 general use, especially in the construction of railway bridges 

 and similar edifices. 



RADIUS AND ULNA OF HUMAN ARM. GIRDER (FROM A HOUSE IN BERMONDSEY). 



On the right of the accompanying illustration is shown the 

 Girder in its simplest form. The figure was taken from a 

 Girder which is used in supporting the walls of a large 



