Scratch Digging Geoscapheini

All members of the uniquely Australian Geoscapheini excavate permanent underground living quarters in the compact, semi-arid soils of Queensland and New South Wales. The unbranched burrows of M. rhinoceros can reach a meter beneath the surface (Chapter 10); the tunnel widens near the bottom into a compartment that functions as a nursery and a storage chamber for the dried vegetation that serves as food. The distal protibiae are impressively expanded to act as clawed spades, driven by the

Fig. 2.5 Macropanesthia rhinoceros, initiating descent into sand; photo courtesy of David Rentz. Inset: Detail of mole-like tibial claw used for digging; photo courtesy of Kathie Atkinson.

large muscles of the bulky body (Fig. 2.5). The hard, stout spines flick the soil out behind the cockroach as it digs. When the insect is moving through an established burrow, the spines fold neatly out of the way against the shank of the tibia. The tarsi are small and dainty (Park, 1990). The large, scoop-like pronotum probably serves as a shovel. Tepper (1894) described the behavior of Geo-scapheus robustus supplied with moist, compressed soil: "they employ not only head and forelegs, but also the other two pairs, appearing to sink into the soil without raising any considerable quantity above the surface, nor do they appear to form an unobstructed tunnel, as a part of the dislodged soil appears to be pressed against the sides, while the remainder fills up the space behind the insect. A few seconds suffice them to get out of sight." Soil texture and compaction no doubt determine the energetic costs of digging and whether burrows remain open or collapse behind the excavator.

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