441] COLORS OF TIGER BEETLES—SHELFORD 47 



v.ithout trace of color and acts like a piece of black paper. They are 

 merely without the filin over the surface. 



Professor Michelson states further that the colors are chiefly 

 if not entirely true surface or metallic colors. They are produced by a 

 film of ultra microscopic thickness probably less than a ten-thousandth 

 of a millimeter. He is inclined to attribute differences in the colors 

 to differences in the chemical constitution of the film and color changes 

 during ontogeny to changes in chemical constitution, but states that this 

 would be very ditficult to demonstrate on account of the minuteness of 

 the film. The work of Heylaerts (1870) (see page 48) would seem to 

 indicate that physical conditions or differences cause a change or differ- 

 ence in color in dried specimens. 



Tower's figure copied by Folsom must be incorrect as he shows such 

 a film as seen under the microscope. This line which he draws appears 

 as a dark line under the oil immersion lens; it is probably a total re- 

 flection line which he misinterpreted under the influence of Professor 

 Michelson "s verbal statement, that surface films must be responsible for 

 brilliant colors, which preceded the latter 's investigation by several 

 years. 



The colors of the group which are on the whole exceptionally brill- 

 iant are to be attributed to a brown or black pigment either without or 

 with any film or with films of varying effectiveness and with varying 

 effects on the light reflected from the surface. A change in color with 

 a change in the angle of incidence indicates the presence of metallic 

 film. 



ONTOGENY OP COLOR 



One of the striking phenomena in connection with the study of the 

 ontogeny of patterns is the ontogeny of color as opposed to pigment. 

 Plates XXIX, XXX, and XXXI are devoted to this subject and show 

 a series of radical changes in the character of the coloration associated 

 with stages of development. Plate XXIX is devoted to the ontogeny 

 of color in C. scutellaris lecontci. Figure 543 shows the beginning of 

 color on the ventral side which consists of bluish reflections, at first 

 about the center of segments which later become green. Later figure 

 541 shows purple reflections at points which remain so throughout with- 

 out change, showing that changes do not always take place. The tip 

 of the abdomen and trochanters appears not to have a surface film. 



Considering the colors of the dorsal side and elytra we note that 

 at the beginning the color is a yellow, the usual color of the cuticula 

 when backed up by the tissues, with greenish reflections. After a little 

 time green color begins to appear more prominent and the elytron of 



