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Polymorphism in Lepidoptera
Many types of polymorphism can be seen in the insect order Lepidoptera. Polymorphism is appearance of forms or "morphs" differing in colour and number of attributes within a single species. In Lepidoptera, polymorphism can be seen not only between individuals in a population, but also between the sexes as ''sexual dimorphism'', between geographically separated populations in ''geographical polymorphism'' and also between generations flying at different seasons of the year (''seasonal polymorphism''). It also includes the phenomenon of mimicry when mimetic morphs fly alongside non-mimetic morphs in a population of a particular species. Polymorphism occurs both at specific level with heritable variation in the overall morphological design of individuals as well as in certain specific morphological or physiological traits within a species.〔
== Genetic polymorphism ==
Genetic polymorphism occurs when the morphs are a result of genetic determination only. The extreme case of genetic polymorphism is that of the papilionid Great Mormon (''Papilio memnon''), where four male forms and many as twenty-six female forms are reported. This species, and others in its genus, have been extensively studied for understanding the genetic basis for polymorphism and Batesian mimicry.
In the case of the Scarlet Tiger Moth ''Callimorpha'' (''Panaxia'') ''dominula'' (family Arctiidae), which is a diurnal moth occurs in continental Europe, western Asia and southern England, three forms occur in England : the typical homozygote; the rare homozygote (''bimacula'') and the heterozygote (''medionigra''). It was studied there by E. B. Ford, and later by P. M. Sheppard and their co-workers over many years. Data is available from 1939 to the present day, got by the usual field method of capture-mark-release-recapture and by genetic analysis from breeding in captivity. The records cover gene frequency and population size for much of the twentieth century.〔Ford, E. B. 1971. ''Ecological genetics''. 3rd ed. London 1971, chapter 7.〕 In this instance the genetics appears to be simple: two alleles at a single locus, producing the three phenotypes. Total captures over 26 years 1939–1964 came to 15,784 homozygous ''dominula'' (i.e. ''typica''), 1,221 heterozygous ''medionigra'' and 28 homozygous ''bimacula''. Now, assuming equal viability of the genotypes 1,209 heterozygotes would be expected, so the field results do not suggest any heterozygous advantage. It was Sheppard who found that the polymorphism is maintained by selective mating: each genotype preferentially mates with other morphs. This is sufficient to maintain the system despite the fact that in this case the heterozygote has slightly lower viability. Another example is the genetic polyrmorphism of larval developmental rates seen in the ''Phengaris rebeli'', in which there exist slow-developing larvae (SDL) (75% of the total ''P. rebeli'' larval population) and fast-developing larvae (FDL) (25% of the total ''P. rebeli'' larvae population). The slow-developing larvae do not grow much during the first year, but grow rapidly during the early part of the second summer and remain a second winter within the ant colonies. On the other hand, the FDL complete their growth the following spring after they are taken into the ant nest.
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