Fig. 1 Standardized data output of thermolimit respirometry outlined as (A,B) changes in metabolic rate (_CO2; ml CO₂ h^-1) and (C) activity (V) for a tracheal breathing beetle (A,C) and a pleopodal breathing terrestrial isopod (B,C) at increasing temperatures starting at 30°C. Analogous to the law of tolerance (Schwerdfeger, 1977; Shelford, 1931) as interpreted by Pörtner (2001, 2002a), the CO₂ profiles characterise the rise in metabolic rate in both species across their range of aerobic capacity (above 30°C), culminating in a maximum metabolic rate _CO2max corresponding to a temperature here termed the T_MetMax (equivalent to Pörtner's T_pII - upper pejus temperature). Beyond the T_MetMax (=T_pII) increasing temperatures cause the onset of a progressive decrease in metabolic rate. This short temperature range is called the deleterious range. This metabolic rate decline culminates in the respirometry CT_max, signalled by a brief spike in CO₂ emission. With temperature increases beyond the CT_max, metabolic breakdown continues and eventually leads to death and the subsequent release of residual CO₂ from the body. Motor activity also increases with increasing temperature and these responses are equivalent in both test species. The cessation of coordinated motor function characterises onset of the activity CT_max (sensu Lutterschmidt and Hutchison, 1997). The respirometry CT_max often corresponds closely with the activity CT_max (see vertical line). Furthermore, respiratory recordings in the tracheal breather show spiracular activity and this ceases in concert with cessation in coordinated motor function shown in the activity recordings (J. R. B. Lighton and R. J. Turner, personal communication; see also Lighton and Turner, 2004). The primarily diffusive pleopodal breathing isopod does not show this.