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Cribellate Spiders and the Production of Their Capture Threads

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Cribellate Spiders and the Production of Their Capture Threads Cribellate spiders and the production of their capture threads Von der Fakultät für Mathematik, Informatik und Naturwissenschaften der RWTH Aachen University zur Erlangung des akademischen Grades eines Doktors der Naturwissenschaften genehmigte Dissertation vorgelegt von Anna-Christin Joel, M.Sc. aus Neuss in Nordrhein-Westfalen, Deutschland Berichter: Universitätsprofessor Dr. tech. Werner Baumgartner Universitätsprofessor Dr. rer. nat. Peter Bräunig Tag der mündlichen Prüfung: 15.07.2016 Diese Dissertation ist auf den Internetseiten der Universitätsbibliothek online verfügbar. Abstract Spider silk production has been studied intensively during the last years due to their mechanical properties. However, spiders do not only produce silk with outstanding mechanical properties: Some of them, the cribellate spiders, are specialized in producing nanofibres as their capture threads. In general, these threads are highly interesting, because their production involves not only a controlled arrangement of three types of silks with one being nanofibres (cribellate fibres), but also a special comb-like structure on the metatarsus of their fourth legs (calamistrum). There are several hypotheses about how this thread assembly takes place, but none of them is able to explain all shapes in which capture threads of different species occur. Hence, the cribellate thread production has to be examined more closely to establish a model describing the process in general. Evaluating the structure of the capture thread, I found the cribellate fibres organized as a mat, forming the typical puffy structure of cribellate threads. Due to this shape, they enclosed two larger parallel fibres (axial fibres). Mat and axial fibres were linked to each other between two puffs presumably by the action of the median spinnerets. This linkage alone cannot lead to the typical puffy shape of a cribellate thread. After removing the calamistrum, a functional capture thread was still produced, but the puffs of the thread were not shaped anymore. Therefore, the calamistrum is not necessary for the extraction or combination of fibres, but for further processing of the cribellate fibres. Using data from Uloborus plumipes I was able to develop a model of the cribellate thread production, connecting morphological data with the movement of the single limbs and determining the influence of this action on the structure of the thread. Although I was not able to determine, how cribellate fibres are processed by the calamistrum to form the puffy structure of the capture thread, I was able to refute some hypotheses, for example that Coulomb-forces are involved in keeping the nanofibres separated. The main features of the cribellate thread production were found conserved between species of very distant related families, suggesting the model to be generally valid. Differences in the morphology of the calamistrum were found to be an adaption to the web producing behaviour, not influencing the processing of the cribellate fibres. A teeth-like structure for example might help picking up fibres for transportation of threads after production in non-orb-weaving spiders. Only the asynchronous movement of the posterior spinnerets observed for Kukulcania hibernalis has to have an impact on the thread structuring according to the previously established model. Indeed their threads showed a pronounced looping structure, probably established by fewer linkages between axial fibres and the cribellate mat. Zusammenfassung Die bemerkenswerte mechanische Eigenschaft von Spinnenseide hat dazu geführt, dass die Produktion dieser Seide in den letzten Jahren intensiv erforscht wurde. Betrachtet man allerdings nicht nur die Eigenschaften der Spinnenseide, sondern auch deren Produktion, entdeckt man, dass einige Spinnen, nämlich die cribellaten Spinnen, sogar Nanofasern als Fangfäden herstellen. Für die Herstellung dieser speziellen Fäden werden die Nanofasern (cribellate Fasern) mit zwei weiteren Seidenarten verwoben. Weiterhin ist eine Art Kamm am Metatarsus des vierten Beines, das Calamistrum, an der Produktion beteiligt. Es gibt verschiedene Hypothesen, wie diese Fäden genau hergestellt werden und wie sich die besondere Struktur dieser Fäden ausprägt. Es gibt jedoch bisher keine These, welche alle Strukturunterschiede erklären könnte, die bei den Fäden verschiedener Spinnen vorkommen. Um diese Lücke zu schließen muss die cribellate Fangfadenproduktion näher untersucht und ein Modell erstellt werden, welches den Prozess im Allgemein beschreiben kann. Im Rahmen dieser Arbeit konnte gezeigt werden, dass die cribellaten Fasern sich als umhüllende Matte um zwei größere, parallel zueinander laufende Fäden, den Axialfasern, anordnen. Die Matte und die Axialfasern sind miteinander verbunden, vermutlich durch die Bewegung der medianen Spinnwarzen. Diese Verbindung alleine sorgt allerdings nicht für die typische puffige Struktur der cribellaten Fangfäden. Wenn man das Calamistrum entfernt, wird wider Erwarten ein funktionierender Fangfaden erzeugt, jedoch ohne die charakteristische Struktur. Das Calamistrum ist dementsprechend nicht notwendig, um die cribellaten Fasern zu extrahieren, sondern sorgt durch eine Kräuselung dieser für die puffige Struktur. Mit Hilfe der Spinne Uloborus plumipes konnte ein Model des cribellaten Spinnprozesses erstellt werden, welches morphologische Daten und Bewegungsanalysen vereint und damit erklärt, wie die Struktur des Fadens erzeugt wird. Da die charakteristischen Merkmale des Spinnprozesses von U. plumipes auch in weit entfernt verwandten Spinnen entdeckt wurden, kann davon ausgegangen werden, dass das Model allgemeingültig ist. Einige Unterschiede, wie z.B. die zeitversetze Bewegung der posterioren Spinnwarzen bei Kukulcania hibernalis können mit dem Model zusammen die schleifenförmige Struktur von deren Fangfäden erklären, da hier weniger Verbindungen zwischen cribellater Matte und Axialfasern entstehen. Unterschiede in der Morphologie des Calamistrums hingegen scheinen keinen Einfluss auf die Struktur des Fadens zu haben. Eine Zahn-artige Struktur am Calamistrum zum Beispiel scheint erst nach Fertigstellung des Fadens zu helfen, den Faden zu packen und unabhängig von den Spinnwarzen zum gewünschten Zielort zu transportieren. Table of contents List of abbreviations and nomenclature ..................................................................................... v List of figures ........................................................................................................................... vii List of tables .............................................................................................................................. ix 1. Introduction ............................................................................................................................ 1 1.1. Spiders and their silk ....................................................................................................... 1 1.2. A spider’s web ................................................................................................................. 2 1.3. Capture threads ................................................................................................................ 5 1.4. Cribellate capture threads ................................................................................................ 5 1.5. Differences in the production of the cribellate thread ..................................................... 9 1.6. Aim of this thesis ........................................................................................................... 11 2. Material and Methods ........................................................................................................... 13 2.1. Study animals ................................................................................................................ 13 2.1.1. Keeping and breeding spiders for in vivo experiments .......................................... 13 2.1.2. Preserved specimens .............................................................................................. 14 2.1.3. Thread samples ....................................................................................................... 14 2.2. Behavioural experiments ............................................................................................... 14 2.2.1. Observations in the natural habitat and the lab colony .......................................... 14 2.2.2. Standardized interactions ....................................................................................... 15 2.2.3. Characterization of vibrational patterns ................................................................. 17 2.2.4. Statistical analysis .................................................................................................. 17 2.3. Recording cribellate spinning process ........................................................................... 18 2.3.1. Animal housing for recordings ............................................................................... 18 2.3.2. Nocturnal observations ........................................................................................... 18 2.3.3. Recordings with low speed camera ........................................................................ 19 2.3.4. Recordings with high speed camera ....................................................................... 20 2.3.5. Using freely available clips ...................................................................................
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