316 THE MECHANICAL FUNCTIONS. 



fly, and the gnat, which have only two. These 

 compose the order Diptera (Fig. 162). In these 

 insects we meet with two organs, consisting of 

 cylindrical filaments, terminated by a clubbed ex- 

 tremity ; one arising from each side of the thorax 

 (as seen in the above figure), in the situation in 

 which the second pair of wings originate in those 

 insects which have four wings. They are named 

 the halteres, or poisers, from their supposed 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 rudiments 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 kingdom, 

 and modifies the structure of each individual part 

 so as to preserve its conformity to one general type. 

 The innumerable tribes of butterflies, sphinges, 

 and moths, are all comprehended in the order Le- 

 pidoptera, and are distinguished 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 ex- 

 amined with the microscope, their construction and 

 arrangement appear to be exceedingly beautiful, 

 being marked with parallel and equidistant striae, 

 often crossed by still finer lines, the distinct visi- 

 bility of which, in many kinds of scales, as those of 

 Pontia Brassier, or cabbage butterfly, and the 

 3Iorp/io 3Ienel(ms of America, constitutes a good 

 criterion of the excellence of the instrument. The 

 beautiful colours which these scales possess may 



