PLIGHT or INSECTS. 249 



They are named the hdUeres, or poisers, from their sup- 

 ^Dosed use in balancing the body, or adjusting with exactness 

 the centre of gravity when the insect is flying. Whatever 

 may be their real utility, they may still be regarded as rudi- 

 ments of a second pair of wings; and they afford, therefore, 

 when thus viewed, a striking instance of the operation of 

 the tendency which prevails universally in the animal king- 

 dom, and modifies the structure of each individual part so as 

 to preserve its conformity to one general type. 



The innumerable tribes of butterflies, sphinxes, and moths, 

 are all comprehended in the order Lcjndoptcra, and are dis- 

 tinguished by having wings covered with minute plumes or 

 scales. These scales are attached so slightly to the membrane 

 of the wing as to come off when touched with the fingers, to 

 which they adhere like fine dust. When examined with 

 the microscope, their construction and arrangement a[)pear 

 to be exceedingly beautiful, being marked with parallel and 

 equidistant striae, often crossed by still finer lines, the dis- 

 tinct visibility of which in many kinds of scales, as those of 

 Pontia brassica, or cabbage butterfly, and the Morp/io 

 Menelaus of America, constitutes a good criterion of the ex- 

 cellence of the instrument. The beautiful colours which 

 these scales possess may perhaps generally be owing to the 

 presence of some colouring material: but the more delicate 

 hues are probably the result of the optical effect of the striai 

 on the surface; and in some cases they result from the thin- 

 ness of the transparent plate of which they consist; for I 

 have observed in several detached scales that the colours they 

 exhibit by transmitted light are the complementary colours 

 to those which they display when seen by reflected liglit- 



The forms of these scales arc exceedingly diversified, not 

 only in different species, but also in different parts of the 

 wings and body of the same insect; for the surface of the 

 body, generally, as well as the limbs, and even in some spe- 

 cies the antennae are more or less covered with these scales.* 



• In the posthumous work of Lyonet, which lias lately appeared, nearly the 

 whole of six quarto plates are crowded with the delineations of tlic dillcrent 

 forms of the scales found in the Bomhyx Cossus. 



Vol. I. 32 



