4 THE COMMONER BUTTERFLIES. 



The thorax is divisible into three parts, called from in 

 front backward prothorax or fore-trunk, niesothorax or 

 mid-trunk, and metathorax or after-trunk. The protho- 

 rax, however, is scarcely more than a flattened plate in 

 front, and is easily overlooked; the division between the 

 other two masses is readily seen behind when the scales 

 are rubbed off, and the mesothorax is seen to be much the 

 largest part of the thorax. 



The fore wings are attached to the mesothorax, the hind 

 pair to the metathorax, and both are composed of two 

 films supported by a system of branching hollow rods and 

 the surface covered with scales. 



Of these rods there are ordinarily four or five to each 

 wing, but when all are jiresent there are six. The two 

 middle ones of the six are the only ones that branch, and 

 are called respectively the subcostal (the upper one) and 

 the median; generally they meet or nearly meet near the 

 middle of the wing and enclose what is called the discoi- 

 dal cell, and the subordinate rods or nervules appear to 

 diverge from its margin. 



The scales are hollow flattened sacs, covered with longi- 

 tudinal striae on the upper surface and generally toothed 

 or serrate at the tip, with a short bulbed stem by which 

 they are fixed in the wing membrane; upon which they lie 

 like shingles on a roof, and by their pigment and the re- 

 fraction of light by their surface stride give to the wing mII 

 its color and delicate markings. 



Certain scales, however, are peculiar to the male sex and 

 are curiously distributed in special patches or concealed 

 positions so as scarcely to be visible even under the micro- 

 scope until they have been uncovered. These are often 

 frinp-ed with tassels at the end, each thread of the tassel a 

 (»anal leading through the body of the scale to a gland at 

 the base and so serving as scent-organs — the odors being 

 sometimes appreciable to human senses and then in all 



