Fig. 4. An abbreviated cladogram (redrawn from Regier and Schultz, 1998; Strausfeld, 1998) showing the phylogenetic relationships between the major tracheated arthropod classes, the occurrence of discontinuous gas exchange cycles (DGCs) and the earliest known fossil evidence of terrestriality. ^*The tracheated Onychophora is shown as an outgroup. ^**See Lighton (1998). mya, million years ago.