"Every trace begins with a wall. No wall, no discontinuity with the substrate, no trace. The wall is the minimal expression of a trace. Even with no filling a trace may exist, but never without a wall. The paramount importance of the wall is not only restricted to the existence of traces in itself and how they are produced, but also with the possibility to attribute them to particular producers and to extract different types of paleoenvironmental inferences. In addition, another equally important aspect is that the wall is directly related to the potential of preservation of the traces (Genise and Bown 1994a ). In other words, how and why traces become preserved. Why they fall under the gaze of Medusa, the goddess of taphonomy, to be converted into stone.
The micromorphological perspective is fundamental for describing walls. Thin sections are necessary for a complete description and identification of the different modifications of the wall involved. Micromorphology is also a very complete source for identifying insect behaviors preserved in trace fossils. Taphonomy is a barrier that in some cases results impassable for micromorphological characters, either because originally they were hidden by the insect’s work, or because later they were hidden by diagenetic processes. Accordingly, in some cases micromorphological characters may be unpreserved, or it may be necessary to prepare as many thin sections as possible to find some character preserved in only one section. Micromorphological analysis was motivated primarily by the necessity of identifying positive diagnostic characters to recognize insect trace fossils in paleosols. Such diagnostic characters will be described for each lining. Wall morphology was also used as a main morphological ichnotaxobase to group insect trace fossils in paleosols in ichnofamilies (Genise 2000 , 2004a)".
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"The shape of insect trace fossils in paleosols is completed in some cases by the presence of burrows or systems, which in the case of social insects may be very complex involving many chambers and burrows. A three dimensional burrow system, as in many ant and termite nests, is called boxwork in ichnology (Ekdale et al. 1984 ). In some cases the boxworks include chambers. For instance, Tacuruichnus is a termite nest composed of a single large chamber, surrounded by a wall whose interior shows a burrow boxwork with chambers (Genise 1997 ). Tabular chambers of Krausichnus trompitus are grouped in spindles, which are connected with other spindles (Genise and Bown 1994b ; Chap. 12).
In contrast to burrows produced by solitary insects which show a constant diameter reflecting their body size, in nests of social insects the burrow system shows different diameters (and even nests show different morphology) reflecting collective behavior and cooperative work (Theraulaz et al. 1998 ; Mizumoto and Matsuura 2013 ). The shape of other parts of the nests may follow different templates: physical or chemical heterogeneities of the environment (e.g. Theraulaz et al. 1998 ). In some ants the construction of nests and distribution of their parts are controlled by temperature and moisture gradients, whereas the pheromonal gradient around a termite queen may act as a template of the royal cell shape (Theraulaz et al. 1998 ). Buhl et al. ( 2006 ) provided a predictive model for the growth of boxworks constructed by ants. Stigmergy is a concept developed by Grassé ( 1984 ) for explaining some aspects of the collective building behavior of termites. The idea is that the shape of a recently constructed structure becomes a stimulus for triggering a particular building behavior of a worker, which changes the shape again, which in turn becomes a stimulus for a further transformation and successively on (Grassé 1984 ). Even the size of workers may result in different shapes and arrangement of structures. In Velocitermes heteropterus nests, differences in the size of workers are reflected in architecture: longer tunnels are constructed by larger workers, whereas shorter and more branching ones are constructed by smaller workers (Haifig et al. 2011)".
Jorge F. Genise
