Diagrams illustrating the various ratios of [C II], CO and FIR emission for sources in the LMC (filled triangles, data from this paper, Israel et al. 1998; and Poglitsch et al. 1995), the SMC (filled circles, data from this paper), IC 10 (open triangles, data from Madden et al. 1997), and the Milky Way (open squares, data from Table 5 in Stacey et al. 1991). The shift of sources in the low-metallicity galaxies LMC, SMC, and IC 10 with respect to those in the Milky Way is very clear and due to relatively brighter [C II] emission caused by greater UV mean free path lengths in the low metallicities of the Magellanic Clouds and IC 10. One might expect the low-metallicity SMC sources to be shifted even more than the intermediate-metallicity LMC sources, but the opposite is true. The lesser SMC shift reflects lower [C II] photo-electric heating due to the depletion of PAHs in that galaxy. See Sect. 4 for further details.