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Performance and adaptive value of tarsal morphology in rove beetles of the genus Stenus (Coleoptera, Staphylinidae)

Oliver Betz^*

Zoologisches Institut der Universität, Ökologie, Olshausenstraße 40, D-24098 Kiel, Germany

View larger version (16K): [in a new window]   Fig. 1. Representative example of the continuously recorded pulling force exerted by an individual beetle (Stenus pubescens) on a dry reed (Phragmites) surface. The maximal attained pulling force is indicated.

View larger version (130K): [in a new window]   Fig. 2. Scanning electron micrographs of the surfaces used for the measurements of the pulling forces of the beetles: (A) a glass microscope slide, (B) photographic paper, (C) a fresh leaf of Glyceria maxima, (D) a fresh and (E) a dry leaf of Phragmites communis and (F) filter paper. (G) Lateral view of the contact zone between the tarsal tenent setae (ts) of tarsomere IV of Stenus pubescens and a dry Phragmites leaf. Scale bars: A—F, 100 µm; G, 3 µm. The plant surfaces represent the adaxial surfaces of young (uppermost) leaves of plants collected in the field. Fresh leaves of Glyceria maxima and Phragmites communis were observed directly using low-voltage scanning electron microscopy, whereas all the other surfaces were air-dried and gold-coated before examination at high voltage. wb, wax blooms.

View larger version (93K): [in a new window]   Fig. 3. Two-dimensional colour plots of the surface profiles of the test surfaces used in experiment 1. The plots were obtained using an optical surface profiler. The surface topographies can be inferred from the colour profiles and the Ra values (i.e. the means of the profile ordinates). Values of Ra (arithmetic means ± S.D.; N=3) are as follows: (A) glass microscope slide (Ra=0.034±0.002 µm), (B) photographic paper (Ra=1.09±0.2 µm), (C) dry leaf of Phragmites communis (Ra=7.95±0.4 µm) and (D) filter paper (Ra=8.96±0.6 µm). The dimensions (length and width) of the measured surface areas can be read off the x- and y-axes.

View larger version (24K): [in a new window]   Fig. 4. Left: plots of maximum vertical pulling forces per body weight as achieved by representatives of 18 Stenus and one Dianous species on various surfaces (species-specific arithmetic means ± S.D.). Note the considerably higher pulling forces on filter paper (D). For sample sizes, see Table 2. Right: mean maximum vertical pulling forces as a function of the mean number of tarsal tenent setae, i.e. the number of tenent setae on one hind tarsus as reported in table 1 in Betz (2002). Before the analyses, both variables were corrected for body mass, as described in the text. (A) photographic paper, (B) glass slide, (C) dry Phragmites leaf, (D) filter paper. Red represents species with slender tarsi (subgenera Stenus s. str., Nestus), whereas green represents species with wide tarsi (subgenera Hypostenus, Metastenus and Hemistenus). Asterisks beside the coefficient of determination r2 indicate different significant levels of the regression analysis: *P0.05; ***P0.001. b, slope. 1, Stenus comma; 2, S. biguttatus; 3, S. fossulatus; 4, S. bimaculatus; 5, S. juno; 6, S. providus; 7, S. boops; 8, S. canaliculatus; 9, S. cicindeloides; 10, S. solutus; 11, S. similis; 12, S. tarsalis; 13, S. latifrons; 14, S. bifoveolatus; 15, S. binotatus; 16, S. pubescens; 17, S. nitidiusculus; 18, S. impressus; 19, Dianous coerulescens.

View larger version (27K): [in a new window]   Fig. 5. Consequences of the `neutralization' of tenent setae (plots on the left) and removal of the claws (plots on the right) on the attainable pulling forces (species-specific arithmetic means ± S.D.) on various surfaces in three different Stenus species with different tarsal morphologies: (A) S. comma, (B) S. pubescens and (C) S. cicindeloides. The test surfaces are arranged in order of increasing surface topography. The asterisks above the error bars refer to the significance levels of Wilcoxon tests for paired comparisons: *P0.05; **P0.005; NS, not significant. Values of N are given below the x-axes.

View larger version (23K): [in a new window]   Fig. 6. Comparisons of pulling forces per single tenent seta between claw-amputated beetles of three Stenus species on various test surfaces (arithmetic means ± S.D.). Values of N are given below the x-axes. During maximum pulling performance, all six tarsi usually have contact with the test surface, so the maximum pulling forces were divided by the total number of tenent setae on all six tarsi. The latter was approximated by multiplying the number of tenent setae counted on the hind tarsi (see table 1 in Betz, 2002) by six. S. pubescens and S. cicindeloides are species with wide tarsi, whereas S. comma has slender tarsi. S. cicindeloides was tested on only three of the five surfaces. The letters above the error bars indicate statistically significant interspecific differences (Mann-Whitney U-test; P<0.05).

View larger version (103K): [in a new window]   Fig. 7. Video frame of a Dianous coerulescens walking on the surface of water in a test chamber illuminated obliquely from above. Note the ovoid shadows on the ground produced by the tarsi and the superimposed luminous points presumably produced by the claws. The length of the beetle is approximately 6 mm.

View larger version (26K): [in a new window]   Fig. 8. Functional model illustrating the suggested synergistic behaviour of the claws and tarsal tenent setae during vertical upward climbing on plants that combine attributes of both rough and smooth surfaces. (A) First, the claws cling to a surface irregularity; (B) because of the downward-directed pull of the leg, the ventral sides of the tarsomeres are subsequently pressed against the substratum, compressing the tarsal tenent setae. Note that this model assumes sub-apically recurved setae, but is also valid for smoothly curved setae.