Contributions of Morphometrics to Medical Entomology

Morphometrics quantitatively describes the morphological variation of objects.When applied to biological forms, it is a particular field of biometrics. In medical entomology, where a major interest is the biology of insects in their natural environment, morphometrics might be considered as a tool for quantifying the phenotypic variation of an organism. Morphometrics focuses on variation, its parameterization, and relation to extrinsic factors. As long as phenotypic variation has environmental and/or genetic causes, morphometrics can help detect local adaptations and genetic divergence among populations. Morphometric characters are related to growth and development, and they are usually continuous.Traditionally, they were estimates of distances between anatomical points called landmarks. More recently, they have come to be the coordinates of these landmarks in a given system of orthogonal axes. We will present here some concepts and statistical analyses related to the use of these data, insisting on their biological relevance, with some examples of applications in medical entomology. Both traditional and geometric approaches will be presented. Special attention is given to applications involving Triatominae (Hemiptera: Reduviidae), the vectors of Chagas disease in Latin America (see chapter in this book) and Phlebotominae, the vectors of leishmaniasis. Finally, some information will be given about morphometric software. 25.1.1 From Dimensions to Biology In the absence of artifactual variation, a distance between two anatomical landmarks or their relative position to other such points (see Figs. 25.1 and 25.2), depend on the morphological development of the organism under study; their variation with geography is arguably an effect of both environmental influence and adaptive changes; and their changes from one species to another reflect the process of natural evolution.When properly analyzed, metric-trait variation allows one to read some biological and evolutionary information embedded in the morphology [79,81,91]. One of the earliest morphometric studies is illustrative. After a severe storm in February 1898, among the moribund sparrows taken to the laboratory by Bumpus, some survived, others died. Examining a few measurements of their skeleton, Bumpus showed that “the birds which perished, perished not through accident, but because they were physically disqualified,’’ and “the birds which survived, survived because they possessed certain physical characters’’ [64].Thus, a simple set of measurements was able to illustrate the Darwinian concept of selection for the most fit. Morphometrics has this ability to make visible to us many aspects of the biology of an organism, such as its physiology, its pathology, and its phenotypic or genetic evolution.