INSECTS : WINGS AND THEIR APPENDAGES. 



75 



structure as before, — two clear elastic membranes stretched 

 over slender horny tubular nervures, and studded on both 

 surfaces with short spine-like hairs, which in this case, 

 however, are very numerous and minute. But along the 

 nervures, and along other lines which run (generally) 

 parallel with the front margin, and also along the whole 

 margin, there are set long leaf-like scales of very curious 

 appearance and structure. 



Confining our attention to one of these lines, suppose 

 one of the nervures, we see that its course is marked on 

 the upper membrane by five rows of minute elevated 

 warts, arranged obliquely with one another. From each 

 of these warts springs a slender stem, which gradually 

 dilates into a thin leaf-shaped plate of transparent sub- 

 stance, having from four to eight or ten longitudinal ribs. 

 They project in a radiating manner, all inclined towards 

 the tip of the wing. The same line on the under-surface 

 carries the like number of leaf-like plates, corresponding 

 in arrangement, structure, form, and direction with those 

 on the upper side. 

 The margins of 

 the wing all round 

 are furnished with 

 similar organs, 

 with this difference, 

 that whereas the 



SCALES ON A GNAT'S WING. 



plates along the 



lines are as it were cut off abruptly at their greatest 

 diameter, the marginal ones converge again with a grace- 

 fully curved outline, to a fine point : a form which is seen 

 to the greatest advantage along the hind edge of the wing, 

 while those of the front margin are thicker, and more 

 densely crowded. 



There are, however, other Insects which display these 

 or similar appendages in far greater profusion, and in 

 much variety of form and appearance. In the fissures or 



