Polymorphism in Featherwing Beetles, with a Revision of the Genus Ptinellodes (Coleoptera: Ptiliidae)

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Abstract
Ptinellodes Matthews 1872, and its polymorphic type species lecontei (Gemminger and Harold) 1868 from the southeastern United States are redescribed and the following 4 polymorphic species are newly described: suteri n. sp. (southeastern United States), similis n. sp. (Jamaica), malkini n. sp. (Nicaragua), and heterosternus n. sp. (Bolivia and Brazil). Ptinellodes and several other genera in the family exhibit a characteristic kind of polymorphism. Two strongly differentiated morphs occur in each sex, a normal morph with normal eyes, wings, and body pigmentation, and a vestigial morph, in which the eyes, wings and other structures are reduced or absent. The vestigial morph is by far the more abundant (ca. 90% or more of all individuals). Typically, each species of polymorphic Ptiliidae is represented by all 4 phenotypes (males and females of both morphs), although the degree of regression in particular vestigial characters varies according to species. The morphs occur together at the same time and place, but in very unequal numbers. Sex and morph data are presented for all the specimens (ca. 1100) of the 5 species of Ptinellodes in the study. The characters involved in the polymorphism, the relative proportions of the different phenotypes, and the taxonomic, geographic, and ecological distribution of the polymorphism in the family are reviewed. Other topics discussed are the possible mechanisms controlling the expression of the morphs, the proportion of phenotypes in relation to habitat, sex ratios, and the possible adaptive significance or selective advantage of the polymorphism. Ptiliidae are the smallest beetles in the vast order Coleoptera, and it can be assumed that availability of space and materials for essential structures is a limiting condition in such minute forms. Among the various explanations that have been proposed to account for loss or reduction of structure (regressive evolution), the principle of material compensation, as a means of conserving developmental energy and space in favor of reproductive effort, is considered to account best for the selective advantage and high incidence of polymorphism in the Ptiliidae, as well as for the great numerical preponderance of the vestigial morph in all the polymorphic species. Parthenogenesis, like polymorphism, also has an exceptionally high incidence in the Ptiliidae. Only one egg is accomodated and matured in the abdomen at a time, and a low fecundity is postulated. Thelytokous parthenogenesis (production of females from unfertilized eggs) in this family has been previously interpreted as an adaptation that enhances fecundity (because no eggs need to be “wasted” on males). I hypothesize that the 2 phenomena, polymorphism and parthenogenesis, represent adaptations or solutions to the limiting factors resulting from evolution to small size in the Ptiliidae. If the hypothesis has validity, there should be a higher incidence of both phenomena in the smallest members of various lineages of arthropods in which evolution to small size has occurred.