BUTTEEFLIES OF THE DISTRICT OF COLUMBIA 239 



some, indeed, giving strong images in as little as 12 hours — really 

 satisfactory results can be obtained only by exposure for at least 

 a week. 



^Yhen the pressure of the wing against the plate is uniform, all 

 the details of the color pattern are faithfully brought out, but if 

 the pressure is greater at one point than it is at another, the area 

 of greatest pressure will appear most brilliant. This is particularly 

 observable in the case of the thick veins near the bases of the wings. 



While freshly caught or raised butterflies exposed immediately after 

 death gave uniformly good results, butterflies of the same species 

 which had been dead for two years gave results which were less 

 satisfactory, the images being less bright with the same length of 

 exposure. Examples of Papilio paris and of P. polyctor that had 

 been dead for 10 years (pi. 64, fig. 4), gave images that were fainter 

 than those from similarly dark native swallowtails that had been 

 dead for two years. Specimens of Argynnis cyhcle and of Basil- 

 archia arthemis astyanax that had been dead for 30 years gave only 

 very faint images after an exposure of a week. In these two last 

 the form of the wing was barely discernible, and the heavier veins, 

 together with the lines of androconia on the males of Argynnis 

 ayhele^ were the only portions that could be clearly made out. A 

 specimen of Papilio cresphontes which had been dead for 40 years 

 after 30 days' exposure gave a very faint, but distinct and readily 

 recognizable image, while an example of Ituna Imnirus which had 

 been dead for 50 years, after 30 days' exposure gave a very faint 

 and scarcely recognizable picture. The pigmentation in this last 

 species, however, is ver}^ light. 



The best and most striking results are obtained from very dark 

 butterflies, particularly the sooty skippers {Thanaos)^ and from 

 butterflies with a large amount of black or very dark brown, such 

 as the zebra swallowtail {Papilio iiiarcellus^ pi. 62, fig. 7.), the 

 Camber well beauty {Vanessa antiopa), and the pearl crescent {Phy- 

 ciodes tharos^ pi. 59, fig. 5). Female butterflies, if colored like the 

 males, usually give a brighter image because of the heavier pig- 

 mentation. 



The image from the upper surface of the wings is almost invari- 

 ably brighter than from the lower. Usually this difference is slight, 

 but sometimes it is very marked (pi. 59, figs. 5, 6, 9, 10, 15, 16). 

 Rarely the image from the lower surface of the wings is brighter 

 than that from the upper surface. I have found this to be the case in 

 Junonia lavinia (pi. 63, figs. 4, 5.) 



The small rapidly flying butterflies, particularly the skippers (pi. 

 59, figs. 1-4, 7, 8; pi. 63, figs. 6, 7, 8-11), give the brightest images, 

 the large butterflies always giving images which are less bright with 

 the same length of exposure. 



