356 THE MECHANICAL FUNCTIONS. 



laps the adjoining scales : and the whole are 

 disposed in rows with more or less regularity ; 

 one row covering the next, like tiles on the 

 roof of a house.* This imbricated arrangement, 

 together with the marks that are left on the 

 membrane of the wing where the scales have 

 been rubbed off, are shown in Fig. 165, which 

 is a faithful delineation of the appearance of the 

 wing of the Hesperia Sloanus, seen through a 

 powerful microscope. The membrane of the 

 wing itself, when stripped of its scales, is as 

 perfectly transparent as that of the bee, and 

 is, in like manner, supported by diverging 

 nervures. Many butterflies exhibit in some 

 parts of the wing smooth pearly spots, called 

 by entomologists, ocelli, or eyes, which arise 

 from those parts being naturally destitute of 

 scales. The number of these scales necessary 

 to cover the surface of the wings must, from 

 their minuteness, be exceedingly great. The 

 moth of the silk worm (Bombyx mori, Fig. 

 148), which has but a small wing, contains, 

 according to Lewenhoeck, more than two hun- 

 dred thousand of these scales in each wing. 



These scales doubtless contribute to the pro- 

 tection of the wing ; but they at the same time 



* The scales on the wing of the Lepisma are of two kinds ; 

 one set being arranged in rows, as usual, and the others, which 

 are of a different shape, being inserted between and over the 

 former, so as to fasten each firmly in its place. 



