Spiracular control in moth pupae

In conditions of low metabolic activity moth pupae of Attacus atlas show a triphasic, rhythmic pattern of CO2 release (discontinuous gas exchange, DGC). The phases, closed, fluttering and open respectively, are labeled according to the behavior of the spiracles, which control gas exchange. However, a single spiracle only possesses two states, that is open or closed. Fluttering thus might be alternatively interpreted as a resonance phenomenon, which will cease to occur if endotracheal gas composition were held constant. Experimentally, a constant composition could be achieved by perfusion of the tracheal system in live animals. Perfusion pressure served as a signal of spiracle opening state. The results of the experiment confirmed, that fluttering is not a steady state behavior. Spiracles are either continually closed or continually open during the perfusions. A plot of spiracle state vs. pO2 and pCO2 roughly confirms the conclusions of Burkett and Schneiderman (1974b). The observed thresholds for spiracle opening are lower though (pO2 approx. 2-5 kPa, pCO2 approx. 1-1.5 kPa) and, surprisingly, a distinct fluttering region is lacking completely. The higher resolution of the method suggested that the threshold curve is a bilinear function, supporting the emergent property hypothesis (Chown et al., 2006). According to this hypothesis two distinct feedback loops interact during the DGC and create the complex pattern of CO2 release. The second part of the thesis discusses possible mechanisms that may serve as the basis of the two required feedback loops. Mathematical modelling is used to show that these mechanisms are in accordance with the requirements posed by previous observation. The most simplistic CO2 based control uses pH as sensor input. In aqueous solutions CO2 hydrates to carbonic acid which dissociates quickly and acidifies the hemolymph. This drop in pH can be detected and converted into an activation of the spiracle muscle (modelled as a sigmoidal dependence of spiracle aperture on pH). If pH is too high, the spiracles close, too low and the spiracles open. Neuronal pattern generaters like dedicated respiratory centers or specialized CO2 sensing organs were obsolete in this model. Interestingly even this simple model oscillates when physiologically meaningful parameters are fed into it. All that is required is a low metabolic rate combined with relatively large tracheal conductance. Furthermore the spiracle activation has to be relatively sensitive to small changes in pH, which is in accordance to the observed two state behavior of the spiracles. If metabolic rate is increased the CO2 release pattern changes from discontinuous to continuous, as postulated by Bradley (2008). Reducing either internal conductance or spiracle conductance also resulted in a switch to continuiously open spiracles (Hetz, 2007). In contrast to the naturally observed rhythms (with period length up to 6 h) the cycles in the model were much shorter (10 20 min). The reasons for that are not fully understood at the moment. The oxygen based control, which is most prominently evident during fluttering, was modelled as a metameric extension of the mechanism proposed by Burkett and Schneiderman (1974b). Multiple independently controlled segments were diffusively coupled in sequential order. This coupling resulted in a synchronisation between neighboring segments without higher order central nervous control. Depending on the coupling strength, that is the conductance of the longitudinally running tracheae, quite different patterns of fluttering could be observed. These ranged from regular single spiracle opening with strong coupling (as observed in beetles)