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Discontinuous gas-exchange in centipedes and its convergent evolution in tracheated arthropods

C. Jaco Klok^1,*, Richard D. Mercer² and Steven L. Chown¹

¹ Department of Zoology, University of Stellenbosch, Private Bag X1, Matieland 7602, South Africa
² Department of Zoology and Entomology, University of Pretoria, Pretoria 0002, South Africa

View larger version (34K): [in a new window]   Fig. 1. (A-E) Recordings of the gas-exchange patterns in the five centipede species at 20 °C. Cormocephalus morsitans was recorded in dry air, while the other centipedes were recorded in rehumidified air. DGC, discontinuous gas-exchange cycle.

View larger version (14K): [in a new window]   Fig. 2. A typical normoxic—anoxic—normoxic respirometry recording for Cormocephalus morsitans at 20 °C in dry air. The markers indicate when the airflow was changed from normoxic (21 % O2, balance N2) to anoxic (pure N2) and back to normoxic again. The large bursts of CO2 emission indicate the end of the closed phase, when the centipede initiates a flutter phase (functionally equivalent to those observed in insects) to maintain O2 partial pressure sufficient for cellular respiration.

View larger version (140K): [in a new window]   Fig. 3. (A) External view of a spiracle of Cormocephalus elegans. Note the uniform appearance of the ostial trichomes. (B) A tranverse section of the spiracle of C. elegans. Across the ostium (O), the ostial trichomes (OT) are separated from the atrial trichomes (AT) by a denuded cuticular fold (CF). The long atrial trichomes obscure the openings of the tracheae (T) in the atrium (Atr). (C) This external view of the spiracle of C. morsitans shows the elongated tufts of the ostial trichome. The three septa of the Y-shaped atrial valve are visible through the ostium. (D) A tranverse section of the spiracle of C. morsitans. Across the ostium (O), the ostial trichomes (OT) are separated from the atrial trichomes (AT) by the smooth denuded strip of cuticle of the atrial valve (AV). The shorter atrial trichomes leave the openings of tracheae (T) in the atrium (Atr) unobscured and only form a tracheal fimbrium (TF) around each opening. Scale bars, 100 µm.

View larger version (17K): [in a new window]   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.

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