1963 ] oumal of the Lepidopterists' Society 159

THE EVOLUTION OF MELANISM IN A POPULATION OF ILIA (NOCTUIDAE)

by D. F. OWEN and M. S. ADAMS

The evolution of melanism in many species of is one of the most striking and rapid examples of evolution ever witnessed. Since the middle of the last century, melanic forms of geometrid and noctuid moths have spread and increased in relative frequency, and, in some areas, have almost replaced the pale 'normal' forms. In North America and in western Europe, similar, presumably related, species have been similarly effected (Owen, 1961.) All the species affected rest by day upon the trunks of trees or upon similar objects and they evidently depend upon the concealing coloration of their wings for protection from predators. The genus Catocala (N oetuidae) comprises some of the best known moths that rest by day upon tree trunks. The forewings of many of the species have patterns that bear a close resemblance to the patterns on the trunks of trees, and, frequently, it is only after a has been accidentally disturbed and has taken flight that its presence can be deteeted. Many of the species of Catocala are extremely variable in the coloration of the forewings, while others hardly vary at all. Some species are known to have melanic forms (Forbes, 1954). In 1961, we commenced a study of the variation in the color and pattern of the forewings of the species of Catocala that occur in ­ hickory woodland on the University of Michigan's Edwin S. George Reserve, Livingston County, Michigan. We obtained a large unseleeted sample of 1,500 specimens of about 30 species. The moths were collected at night from a white sheet above which was suspended a 100-watt mercury vapor lamp. The lamp was operated for two or three hours on every warm night throughout the summer. Among the specimens collected were 185 Catocala ilia Cramer. It was immediately apparent from our sample that (1) there is much variation in the color and pattern of the forewings, and (2) melanics are relatively frequent. Accordingly, we classified the specimens into six categories; these are shown in the figure. The six categories may be described as follows (descriptions refer to forewings only): 160 OWEN & ADAMS: Catocala melanism Vo1.l7: no.3

a c

The six coloration categories of Catocala ilia. (Photograph by W. J. GRAHAM.)

A. Melanic: fOlfewings dark brown and all markings heavily obscured by dark scales; in some specimens reniform just discemable in outline; postmedial line black, but distinct. B. Semimelanic: markings obscured, but clearer than in A; reniform outlined in white, with an inned ring of dark gray inside of which the color is a variable grayish-brown; subrenifoml relatively distinct, variable in size, generally dark, but in some specimens dark gray. C. Semimelanic: markings as in B, but reniform solid white. Appears to be a melanic form of category D. D. Wings normal: reniform solid white. This is form "conspicua" Worthington. E. Wings normal: this is the type form; white and gray patches all over wings, with the lines distinct; area below reniform not contrasting with rest of wing, unlike F; renifonn with gray center. F. Wings normal: paler than E, and forewings divided into two by a broad ilTegular band of white or gray, which includes reniform and subreniform; reniform with gray center. The basic distinction was between melanic and normal and in general this was quite obvious. In the analysis that follows, categOlies A, B, and C are regarded as melanics, and categories D, E, and F as pale or normal forms. Categories E and F intergrade considerably and may not be distinct. Categories A and B are relatively distinct from each other and from all other forms. Categories C and D are very distinct because of the solid white renifonn. 1963 Journal of the Lepidopterists' Society 161

The relative frequency of the six color categories is as follows:

Category: A B C o E F Males: 19 35 10 7 45 56 Females: o o 2 1 2 1 Sixty-six of the 185 specimens were melanics and 112 were pale. There were seven others which could not be certainly classified because they were too damaged. Only 20 specimens with a white reniform were collected, but of these 12 were melanic. As with other species of Catocala, females rarely come to light, and hence it is not possible to say if any of the forms are sex-limited. Melanics tend to occur earlier in the season than pale specimens. This may indicate that melanics feed up more rapidly and emerge eaJ:lier. The seasonal changes in relative frequency of melanic and pale specimens are statistically significant (P

10-30 July 31 July-20 Aug. 21 Aug.-l0 Sept. Melanic forms (A-C): 6 26 34 Pale fomls ( D-F ) : 5 22 85

The high relative frequency of melanic ilia on the George Reserve in 1961 prompted us to examine earlier collections. The area has been well collected since the 1930's, and the University of Michigan Museum of Zoology has a good series from the Reserve. There are also specimens in the Michigan State University Museum, East Lansing. It can be assumed that any unusual varieties, such as melanics, would have been retained in collections. The earliest record we have of a melanic ilia on the Reserve is 1935, when one categOlY B specimen was taken. In an apparently unselected sample of seven specimens taken in 1955, three are in category A, one in category E, and three in category F. These are the only records we have been able to trace of melanic ilia on the Reserve. J. H. NEWMAN, who has collected noctuids on the Reserve for many years, does not have a single melanic ilia in hi$ oollection. Elsewhere in Michigan we have found the following records of melanics: category B: Washtenaw County (adjoining Livingston County), one in 1934; Kalamazoo County, one in 1956, one in 1958; category C: Washtenaw County, one in 1934; Wayne County, one in 1948; Otsego County, one in 1960; Osceola County, one in 1951. Undoubtedly other melanic specimens exist in other collections, but 162 OWEN & ADAMS: Catocala melanism Vo1.l7: no.3 until recently they must have been relatively rare. Thus we conclude that melanic ilia have only recently reached their present high frequency in the area. It seems that a rapid evolutionary change has taken place similar to that recorded for certain geometrids in the same area (Owen, 1961). Nothing is known of the selective forces that have produced this change, but, in view of the work of KETTLEWELL (1961) in the British Isles on the geometrid, Biston betularia, selective predation may be important. The population of ilia on the George Reserve in 1961 is so variable in the coloration of the forewings that one might suppose that any predator using its past experience as a means of searching for further moths would be persistently deceived. For instance, both melanic and pale "conspicua" look to the human eye quite different from the true melanics (A and perhaps B) and from normal pale moths (E and F). It is possible that ilia exist~ in a oondition of relatively un­ stable polymorphism, and that at the present considerable changes in the relative frequency of the various fonus are occurring that reflect adaptation to environmental changes. Such changes may be of a temporary nature; the melanics may begin to decrease in fitness if they become more common, and we would then expeot a decrease in their relative frequency - a reversal of the present trend.

Literature Cited

Forbes, W. T. M., 1954. of New York and neighboring states. Part 3. Cornell Univ. agr. expo sta. memoir 329. Kettlewell, H. B. D., 1961. The phenomenon of industrial melanism. Ann. rev. ent. 6: 245-262. Owen, D. F., 1961 Industrial melanism in North American moths. American nat. 95: 227-233.

[DFO] Department of Zoology, University of Michigan, Ann Arbor, Mich., U. S. A. [MSA] 1422 Pontiac Trail, Ann Arbor, Mich., U. S. A.