A template-dependent semilandmarks treatment and its use in medical entomology

In medical entomology, as well as in many other groups of arthropods, geometric morphometrics has become a powerful tool for species identification and population characterization. The approach lies on the relative position of some anatomical points (landmarks) or, more recently, of curved features (semilandmarks). Landmarks are described by coordinates of points easy to recognize from one individual to another. According to this criterion decreasing levels of homology have been recognized, going from strong (type I) to weak (type III landmarks). Semilandmarks (or sliding landmarks) are points having poor homology like landmarks III, but making it possible to capture curves or surfaces where landmarks are sparse. Their use is becoming increasingly routine. Superimposition of semilandmarks differ from what is currently applied to landmarks, ways and tools for collecting them may also differ from collecting landmarks. They can be collected by simply digitizing points along a curve or a surface but can also be collected in a more systematic way by the use of a template. In the CLIC package (https://xyom-clic.eu), as well as in the XYOM software (https://xyom.io), we created an algorithm-based template to both collect and align semilandmarks or landmarks III. The use of such template for the final alignment of these special points represents an original approach, so that a comprehensive explanation is required. Using a published example, we compare in details the results of our method with the ones produced by the currently applied approaches. A close parallelism of information is found. The specificities and limitations of our method are discussed.