40 THE BUTTERFLIES OF NEW ENGLAND. 



reduced to a mere appressed pellicle in those groups which do not have the 

 fore legs aborted in both sexes, as if in compensation for their atrophy. 

 Cholodkovsky has recently tried to show that these lobes were homologues 

 of the wings on the other thoracic segments, but entirely without success. 

 The meso- and metathorax, which, although perfectly distinct, seem to 

 compose one homogeneous whole, form a com[)act oval mass, the upper 

 surface of which is arched and sides sometimes compressed. The meso- 

 thorax is far the most important and is composed of two unequal parts, a 

 mesoscutum in front and a much smaller, somewhat wedge-shaped piece, 

 the mesoscutellum, the apex of which enters the middle of the mesoscutum. 

 The metathorax, on the other hand, as far as it is readily seen, is composed 

 above of three pieces placed transversely, the central piece being the mc- 

 tascutellum, also more or less wedged between the others, the metascuta. 

 Viewed from the side, the metathorax assumes a much greater importance. 

 All these parts vary in comparative dimensions and relative positions in the 

 different groups. It may be added, in general terms, that as regards the 

 trunk of the imago, the families seem to be distinguished by the contour of 

 the ])rincipal masses, while the subfamilies are dependent upon the sha[)e 

 of the grander parts which compose the masses, and generic characters are 

 to be sought for in the structural details within the diiferent parts. 



The appendages of the thorax are two pairs of wings and one pair of 

 pat agia above, and three pairs of legs below. 



The wings, never wanting in butterflies, are attached res[)ectively to 

 the meso- and metathorax ; they are very large in comparison with the 

 body and seldom more than tAvice as long as broad, although always longer 

 than broad. The o-eneral form of the fore wings is a triangle whose trun- 

 cated apex is at the point of attachment to the body ; that of the hind- 

 wings subcircular, with a tendency also to the triangular outline. They 

 are fiat, excepting that sometimes the inner border and its outer angle in 

 the hind wings may be guttered or plaited or bent at an angle. They are 

 each composed of a thin dou1)le film of membrane, covered on both siu"- 

 faces Math imbricated scales and with scattered hairs, the latter particularly 

 U})on the upper surface and upon the hind wings near the base. This 

 double membrane is stretched upon a regular system of tubular rods, 

 termed nervures or veins, which hinge at the base upon the body; they 

 conduct the nerves and air tubes to all parts of the wing. The normal 

 number of veins in the wings of insects (38-42) is six, disposed to a certain 

 extent in pairs ; the middle pair usually branches to a greater extent than 

 the others, and supports most of the membrane of the wing. In butter- 

 flies the foremost vein is always absent (except in the front wings of some 

 male Ilesperidi) ; the hindmost is also very commonly wanting, so that 

 there are usually only five (often only four) principal veins, rather inap- 

 propriately designated costal, subcostal, median, submedian, and, when 



