Theme and Variation in the Development of Insect Mouthparts

Abstract

Insect mouthparts are serially homologous appendages. As such, their development and evolution are nonindependent. Arthropod appendages share similarities in their developmental origins and underlying genetics. Here, we review the development, specification, and patterning of insect mouthparts, with comparisons to the legs of Drosophila melanogaster. The expression and function of genes in the arthropod head give clues as to the homology of the labrum. The activity of Hox genes establishes appendage-specific gene expression and interactions allowing for the development of unique appendage types. Many similarities exist in the patterning of gnathal appendages and legs; however, unique variations in gene function in each appendage type provide clues to the developmental origins of mouthpart morphologies. We examine what is known about mouthpart patterning in mandibulates, as exemplified from several beetle species, as well as in the proboscis of Drosophila melanogaster and in the hemipteran rostrum of Oncopeltus fasciatus. With these findings in mind, we reflect on the evolution of serially homologous structures.