SRµCT-data helps to develop a phylogenetic character matrix for head structures of lower Pterygota. Alexander Blanke, Manuela Thelen Zoologisches Forschungsmuseum Alexander Koenig, Adenauerallee 160, 53113 Bonn, Germany The basal branching patterns of pterygote insects - Odonata, Ephemeroptera and neopterans - are one of the major unsolved problems of insect systematics (Klass 2007). All three possibilities are subject of the discussion and are favoured by studies (Haas and Kukalová-Peck 2001; Hennig 1969; Kristensen 1981; Kristensen 1991; Kukalová-Peck 1991; Staniczek 2000; Staniczek 2001). One major setback of morphologic analyses in the past were incomplete studies and an insufficient taxon sampling. Especially the odonatan head was rarely studied (Mathur and Mathur 1961; Short 1955). In contrast, the head has proven to be a highly informative character system (Beutel et al. 2010), especially addressing the pterygote base (Wipfler et al. 2011). Since the odonatan thorax with its direct flight musculature highly diverges from all other pterygote insects, it is of limited use in our context. The present study will address the basal pterygote relationships by providing a comprehensive morphological data set: 200 cephalic characters for a total of 26 species of all major pterygote insect lineages and two outgroups (Archaeognatha and Zygentoma) are studied. Evolutionary scenarios for all three possible clades (Metapterygota, Palaeoptera, Chiastomyaria) will be presented. Prior to scanning samples were critical point dried (CPD) (Model E4850, BioRad) and mounted on sample holders. Except for Siphlonurus and Lepisma all specimens were scanned using the beamline BW2 of the storage ring DORIS III at DESY (operated by HZG) with a stable energy of 8 keV and a high density resolution (Beckmann et al. 2008). For scanning electron microscopy (SEM) species were transferred to 100% ethanol, CP-dried (Model E4850, BioRad) and subsequently sputter coated (Model Anatech Hummer VII). Microscopy was performed on a Hitachi S-2460N using a special sample holder (Pohl 2010). Some of the results obtained by SR-microCT scanning at BW2 are shown below (Fig 1, B). We received detailed images of the inner anatomy of our taxon set. The muscle equipment and our results from the external analysis (Fig 1, A) will be incorporated into a data matrix for phylogenetic analysis and subsequently analysed with TNT and Winclada. With the help of SR-microCT at DESY we plan to conduct studies on the functional anatomy of pterygote heads. The 3D models generated from the CT-data will be the basis for developing physically realistic simulations of mouthpart movement. This will aid us in reconstructing a scenario for the mouthpart evolution which is robust and testable against conflicting hypotheses. Figure 1 The head of L. virens in frontal view. A SEM picture show relevant head structures and general organisation. B 3D-reconstruction from SR-microCT data showing head muscles, brain and ocellar ganglia. acl anteclypeus, anm antennal muscles, e eye, fl flagellum, la labium, lb labrum, lbm labral muscles, md mandible, mdm mandibular muscles, mxm maxillar muscles, oc ocellus, ocg ocellar ganglion, ol optical lobe, pcl postclypeus, pe pedicellus, sc sca pus, v vertex References Beckmann, F., J. Herzen, A. Haibel, B. Müller, and A. Schreyer. 2008. High density resolution in synchrotron-radiation-based attenuation-contrast microtomography. Proceedings of SPIE 7078:70781D-3. Beutel, R. G., D. Zimmermann, M. Krauß, S. Randolf, and B. Wipfler. 2010. Head morphology of Osmylus fulvicephalus (Osmylidae, Neuroptera) and its phylogenetic implications. Organisms Diversity & Evolution 10:311-329. Haas, F., and J. Kukalová-Peck. 2001. 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Microscopy Research and Technique 73:1073-1076. Short, J. R. T. 1955. The morphology of the head of Aeshna cyanea (Müller) (Odonata, Anisoptera). Transactions of the Royal Entomological Society London 106:197-211. Staniczek, A. H. 2000. The mandible of silverfish (Insecta : Zygentoma) and mayflies (Ephemeroptera): Its morphology and phylogenetic significance. Zoologischer Anzeiger 239:147-178. Staniczek, A. H. 2001. Der Larvenkopf von Oniscigaster wakefieldi McLachlan, 1873 (Insecta: Ephemeroptera: Oniscigastridae). ein Beitrag zur vergleichenden Anatomie und Phylogenie der Eintagsfliegen. Pp. 160. Biological Department. Eberhard-Karls-Universität Tübingen, Tübingen. Wipfler, B., R. Machida, B. Müller, and R. G. Beutel. 2011. On the head morphology of Grylloblattodea (Insecta) and the systematic position of the order, with a new nomenclature for the head muscles of Dicondylia. Systematic Entomology 36 .