The larvae exhibit
a color polymorphism first described by Edwards in 1893. Nigra larvae are
solid black, bicolor is black with a mid dorsal orange band, and rufa are
solid orange to rust colored (although there can be a fair amount of melanic
coloration between body segments, especially in the lower instars).
The larvae are leaf skeletonizers, and large stands of wild sunflower can
be entirely skeletonized, i.e., denuded, due to population explosions occurring
later in the year, usually September. There are five larval instars.
The larvae feed gregariously, primarily as sibships, during the first four
instar. This gregarious behavior confers greater protection from parasitism.
At the last instar, the fifth instar, the larvae wonder and feed individually.
Third and fourth instar larvae enter diapause at the onset of winter.
The season can end with a huge larval population, but from personal observation
few survive to start the next season. As a result, populations annually
go through bottlenecks which may have some evolutionary significance.
The pupae are variable
in the extent of melanization ranging from one nearly devoid of black markings
to one almost blackened by them. There also appears to be a non sex
linked hemolymphic polymorphism. Two discrete forms exist, a creamy
white vs. a pale yellow background coloration. However, there appears
to be some linkage between the hemolymphic morphs and the degree of melanization
because heavily marked pupae are never yellow, and, alternatively, those
nearly devoid of markings are never white.
I initiated studies in 1966 to determine the genetic mechanism underlying the larval polymorphism as part of my M.S. degree thesis at Arizona State University. Surprisingly, up to this time very little scientific work of this nature had been done. I solved the genetic mechanism in 1967, and two years later published the results in Genetical Research ('The Genetics of Three Polymorphic Larval Colour Forms of Chlosyne lacinia'. Genetical Research, 1969. Vol 14: pp. 333-336. Page 333, Page 334, Page 335, Page336).
My research showed
the mechanism to be an autosomal non-linked, two locus epistatic system.
The bicolor and nigra determining alleles are at one locus, and the rufa
determining alleles at another locus of a non-homologous chromosome.
Bicolor and rufa are determined by dominant alleles, and the dominant rufa
allele is epistatic over the bicolor locus, i.e., the bicolor and nigra
phenotypes are surpressed when the dominant rufa allele is present on the
other chromosome. When Raymond Neck, Guy Bush, and Boyce Drummond
learned of the mechanism from my work, they extended this research further,
and in 1971 published results indicating the existence of lethals closely
linked to the dominant rufa allele.