How do you determine a species?

Virtual three-dimensional reconstructions of female genitalia to research the taxonomy of a cicada taxon (Cixiidae)

More than 150 years after the publication of Charles Darwin’s “On the Origin of Species” there are still many unsolved questions about the evolution of animals and plants. Taxonomy, as a branch of biological research, deals with the description and naming of species, and especially with the clarification of relationships between different species. Furthermore it helps to explain how and why many species evolved from a primitive ancestor and how these transformations happened.

Cixiidae are a family of insects known as planthoppers, with over 2100 species that are extremely varied and occur almost worldwide. Some species are crop pests which can damage plants like cereals, grapes, rice, palms and others by sucking the phloem (the living tissue of vascular plants that carries organic nutrients) and transmitting plant diseases. Despite their ecological and sometimes economic importance, the phylogenetic relationships between the species of this taxon have not yet been satisfactorily clarified.

Traditionally many external characteristics, such as structures of the head and legs or the wing venation, are used for classification and identification. Also many male genitalia are widely used for identification. Because of their complexity and role in the separation of species there are often very clear differences between different species and so they offer important characteristics. From all these differences and similarities the relation between species can finally be calculated. Genetic information is also very important for taxonomic research. Some cixiid species have been genetically analyzed, but the results of these morphological and genetic studies have, so far, been contradictory. So the origin and most of the phylogeny of this cicada family are still unknown today.

To fill the gap in our knowledge, now, for the first time, the female genitalia will be systematically used to investigate these questions. The female genitalia apparatus of Cixiidae shows numerous modifications and a high variability in both the outer and inner structures between different genera and at the species level. That is why the female genitalia apparatus is a very useful character complex for phylogenetic research and reconstruction.

For the investigation of the morphological structure of the outer and inner genitalia, classical and modern methods of comparative morphological research should be combined in a holistic approach. For example, light microscopy and classical zoological drawings will be used along with scanning electron microscopy (SEM), which allows the smallest structures of surfaces to be shown in high magnification with high resolution images. But these methods, however, require different kinds of advanced preparation which leads to the destruction of the specimen. To examine the inner and outer structures without destroying the specimen, micro-computed tomography (Micro-CT) will be used as another important technique. Computer tomography uses x-rays to create many virtual cross sections of an object, allowing three-dimensional reconstructions of the structures to be analyzed. These scans are made at the Berlin Museum of Natural History and at the Deutsches Elektronen-Synchrotron (DESY) in Hamburg. The virtual reconstruction then allows us to examine and show the whole complex more easily. For example, it is possible on the computer to take measurements and to show cross sections of every area from every side of the specimen. It also allows us to show selected tissues, or to show tissues and structures, in different colours. In this way it is hoped that a better understanding of the functional morphology and the evolutionary transformation of the female cixiid genitalia can be developed.

By getting more characters from the use of the female genitalia for systematic research, there will be new answers from which the ground pattern for the whole group can be developed. By finding and explaining these evolutionary configurations, the origin and development of another family of animals should finally be clarified.