Adrian L. V. Davis

Discontinuous Gas‐Exchange Cycles in Scarabaeus Dung Beetles (Coleoptera: Scarabaeidae): Mass‐Scaling and Temperature Dependence

Although discontinuous gas exchange cycles (DGC) are known from many insects, the effects of body size and temperature on DGC have not been widely examined. Here, these effects are investigated in five Scarabaeus dung beetle species from mesic and xeric habitats. The investigation tests two hypotheses: that previous estimates of the scaling exponents for the DGC and its characteristics are more broadly applicable to insects, and that, in response to temperature, both DGC frequency and the quantity of CO2 emitted during the open (O) phase (O‐phase emission volume) are modulated. Like previous workers, we find that V̇co2 scaled as mass0.968 and that O‐phase emission volume scaled as mass0.833. However, temperature‐associated increases in V̇co2 (Q10s of 2.19–2.65) were modulated mostly by increases in DGC frequency since O‐phase volumes remained constant across temperature. Flutter (F)‐phase and O‐phase durations were closely coupled to DGC duration, although the relationship between closed (C)‐phase duration and DGC duration was less pronounced. We show that ventilation phase coefficients, previously considered a measure of the proportional duration of each phase of the DGC, calculated from the slopes of these relationships are a measure of change in phase duration with change in DGC duration and not a measure of the way in which total DGC duration is apportioned among phases. We suggest that proportions be used to estimate the contribution of each of the phases to the total duration of the DGC.

Discontinuous gas exchange and the significance of respiratory water loss in Scarabaeine beetles.

SUMMARY Respiratory water loss in insects is a controversial topic. Whilst earlier studies considered respiratory transpiration a significant component of overall water loss, to the extent that it was thought to be responsible not only for the evolution of discontinuous gas exchange cycles (DGCs) but also for variation in DGC patterns, later work repeatedly questioned its importance. In particular, investigations of the proportional contribution of respiratory transpiration to total water loss in species showing DGCs suggested that respiratory transpiration was unlikely to be important in these species. In turn, these studies have been criticized on analytical grounds. In this study we investigated variation in cuticular and respiratory water loss rates in five Scarabaeus dung beetle species, all of which show discontinuous gas exchange cycles, to ascertain the significance of respiratory water loss using modern analytical techniques. In particular, we determined whether there is variation in water loss rates amongst these beetles, whether both respiratory and cuticular water loss rates contribute significantly to variation in the former, and whether metabolic rate variation and variation in the duration of the DGC periods contribute significantly to variation in respiratory water loss rate. Total water loss rate varied such that species from arid areas had the lowest rates of water loss, and both cuticular and spiracular transpiration contributed significantly to variation in overall water loss rate. Moreover, variation in metabolic rate and in the duration of the DGC periods contributed significantly to variation in respiratory water loss rate. By contrast, examination of proportional water loss revealed little other than that it varies between 6.5% and 21%, depending on the species and the temperature at which it was examined. Cuticular water loss scaled as mass0.721, but did not differ from that expected from geometric considerations alone. By contrast, respiratory water loss scaled as mass0.531, suggesting that gas exchange takes place by diffusion and convection. Our results provide direct evidence that respiratory water loss forms a significant component of water balance, and that changes in both metabolic rate and DGC characteristics contribute to modulation of respiratory water loss.

Associations of Afrotropical Coleoptera (Scarabaeidae: Aphodiidae: Staphylinidae: Hydrophilidae: Histeridae) with dung and decaying matter: implications for selection of fly-control agents for Australia

Many beetles recorded from dung are known to frequent a range of different types of decaying matter. Therefore, the range of associations shown by dung Coleoptera near Pretoria in the Transvaal, South Africa, was studied to facilitate selection of dung specialists for introduction into Australia as control agents for cattle dung and dung-breeding fly pests. Although traps were baited with three different types of dung (cattle, horse, pig) and three types of decaying matter (banana, moistened grass clippings, animal offal), beetles were predominantly attracted to dung. The distribution patterns shown by the 86 most abundant species were classified using cluster analysis in order to summarize the major trends. Four clusters of species were shown at the 55% level of similarity. All but one of the 39 species in the first cluster were attracted primarily to omnivore dung (pig). Another 31 species in a second cluster were attracted primarily to herbivore dung (cattle, horse). All but one of 14 species in a thir...

Seasonal dung beetle activity and dung dispersal in selected South African habitats: implications for pasture improvement in Australia

Abstract Spatio-temporal variation in dung beetle distribution and cattle dung dispersal was quantified across a spectrum of summer rainfall habitat types in South Africa, i.e. grassland, open woodland and thickets on both sand and clay. This study facilitates the selection of further Afrotropical dung beetle species to optimize dung and fly control in similar Australian habitats. Substantial dung beetle activity and dung removal were recorded during the warm rainy season. However, there was considerable spatial variance for which soil type was primarily responsible. Differences between harder (clay) and softer soil types (sand) include, lower biomass of dung-burying and lower abundance of predatory dung beetles, lower rates and amounts of dung removal, and greater shredding of pad remnants on clay. Positive correlations between dung beetle biomass and dung removal were most frequent for large, fast-burying tunnellers on sand and for ball rollers on clay. The implications of spatio-temporal differences in dung beetle functional group structure and dung dispersal are discussed with regards to the necessity of further introductions of both dung-burying and predatory dung beetles from African into Australia.

A comparative analysis of metabolic rate in six Scarabaeus species (Coleoptera: Scarabaeidae) from southern Africa: further caveats when inferring adaptation

Alterations in VO(2) or VCO(2) are amongst the more polemical physiological adaptations ascribed to insects. Generally, metabolic rate is thought to be lowered in response to arid conditions, and elevated in species from cold environments compared to their more temperate relatives. However, most studies have rarely addressed the influence of both environmental factors in unison. To this end, standard metabolic rate and its temperature dependence were measured (at 4 degrees C intervals from 16 to 32 degrees C) in six Scarabaeus dung beetle species (three flightless, three volant) from a variety of habitats (warm, arid to cool, mesic) in southern Africa using flow-through respirometry. Mass specific VCO(2) varied from 0.0158 ml g(-1) h(-1) at 16 degrees C to 0.1839 ml g (-1) h(-1) at 32 degrees C. The slopes of the rate temperature curves were similar for all species (Q(10)s of 2.14-2.84), although the intercepts differed significantly in the direction (warm arid to cool mesic): S. gariepinus<S. hippocrates<S. striatum<S. galenus<S. rusticus<S. westwoodi. General linear models were fitted to the VCO(2) data with treatment temperature, wing status, mean annual rainfall and mean annual temperature as the independent variables. For all six species, the best fit model included treatment temperature and wing status only. Flightless species had significantly lower VCO(2)s than did winged ones. Amongst the flightless species, S. gariepinus, from the driest area, had the lowest VCO(2). Among the winged species it was impossible to determine whether environmental temperature or water availability were most important in influencing VCO(2); both were significant. We conclude that investigations of metabolic rate modulation, and especially metabolic cold adaptation, in insects must take both temperature and water availability into account.

Nesting of Afrotropical Oniticellus (Coleoptera, Scarabaeidae) and its evolutionary trend from soil to dung

Abstract. 1. Oniticellus egregius Klug constructs brood ovoids of dung in the soil immediately under the edge of animal droppings. Each successive brood ovoid is enveloped within a soil shell. After completion of brood construction, loose earth is cleared from around the broods to produce a brood chamber. The immatures are then abandoned as eggs or first instar larvae.

Trophic associations of a dung beetle assemblage (Scarabaeidae: Scarabaeinae) in a woodland savanna of Botswana.

Abstract Species richness and abundance of dung beetles were assessed across a range of bait types that acted as surrogates for the food resources available in Chobe National Park, Botswana. These bait types were comprised of the dung of pig (omnivore), cattle (ruminant herbivore dropping fine-fiberd pads), sheep (pellet-dropping ruminant herbivore), and elephant (monogastric, nonruminant herbivore producing coarse-fibered droppings), and chicken livers (carrion). Species richness was similar between traps baited with pig, cattle, and elephant dung but was relatively lower in those baited with sheep dung and carrion. In traps baited with pig dung, abundance was relatively greater than in all other bait types. A cluster analysis of species abundance distributions for the 30 most abundant species identified four different patterns of bait type association at a 60% level of similarity. All but 1 of the 15 species in cluster A were attracted primarily to the dung of omnivores and pad-dropping ruminant herbivores (pig and cattle). All seven species of cluster B were attracted primarily to coarse-fibered, nonruminant herbivore dung (elephant). All four species of cluster C were primarily carrion and pig dung associated, whereas all four species of cluster D were carrion specialists. In conclusion, the most abundant species were attracted to all bait types, but most species were largely specialized to different dung types or carrion, with dung attracting the majority of the fauna in terms of both species richness and abundance.

Habitat associations and seasonal abundance of coprophilous coleoptera (Staphylinidae, Hydrophilidae and Histeridae) in the Hluhluwe region of South Africa

A total of 134 species of Coleoptera (100 of staphylinids, 13 of hydrophilids and 21 of histerids) was extracted from pads of cattle dung placed in four contrasting habitats in both Hluhluwe Game Reserve, Natal, South Africa, and the surrounding pastoral regions. Pads were exposed to colonists for 24 h on eight occasions over one year. Forty-six species and species complexes were captured in numbers sufficient to permit analysis of their distribution patterns. Of 27 species significantly influenced by vegetation type, 22 were more abundant in unshaded situations and five more abundant in situations shaded by high profile vegetation. The total numbers of individual predatory staphylinids were evenly distributed between vegetation types, but members of coprophagous staphylinid and other predatory taxa were more numerous in unshaded situations. Of 13 species significantly influenced by soil type, six were more abundant on clay and seven more abundant on deep sand. The total numbers of individual coprophagous oxyteline staphylinids were greater on clay than on sand, but the members of predatory taxa, including staphylinids, were evenly distributed across soil types. Of 14 species showing significant seasonal changes in abundance, 13 were more abundant in hot wet months and one in cool dry months. The total numbers of hydrophilid individuals were greater in the cool dry season, but the other taxa were active predominantly in the rainy season. Most species were present in similar numbers in both the game reserve and pastoral regions. Enclosure of dung pads within wiremesh cones to exclude large dung-burying beetles resulted in reduced colonization by most beetle species regardless of their size.

How might intensification of farming influence dung beetle diversity (Coleoptera: Scarabaeidae) in Maputo Special Reserve (Mozambique)?

There are concerns over the increasing encroachment of humans, domestic livestock, and farming onto Maputo Special Reserve because of the potential for habitat modification. Therefore, differences between an undisturbed area of the reserve and a neighbouring farming area are assessed using dung beetle as indicators. In each of the two areas, pig-dung-baited pitfall traps were used to sample dung beetle assemblages in two contrasting habitats, grassland and forest. Distributional analysis of the 57 species and 36 942 individuals that were captured, showed that species richness, species turnover, relative abundance patterns, and biogeographical composition differed strongly between both habitats and areas under different land usage. However, in analyses that apportion variation, the greatest amounts were related to habitat rather than land usage. Even so, in both habitats, the total and mean number of species per trap was higher in the farmed area than in the reserve although this was a significant trend only in grassland. Furthermore, in grassland, widespread species were better represented in the farmed area than in the reserve whereas in forest, widespread species were poorly represented compared to grassland. Also in forest, Maputaland endemics were better represented in the reserve than in the farmed area. Further work is necessary to separate the different geographical, ecological, and land usage factors responsible for the patterns detected in this preliminary study. Even so, there are clearly differences between the Maputo Special Reserve and the farmed area.

Functional implications of temperature‐correlated colour polymorphism in an iridescent, scarabaeine dung beetle

Abstract 1. In many Coleoptera, iridescent colouration is generated by exoskeleton ultra‐structure, within which multilayer interference reflects only certain wavelengths. Published work indicates that the colour polymorphism shown by some iridescent beetles is under genetic control. However, the present study suggests environmental involvement in the polymorphic southern African dung beetle, Gymnopleurus humanus Macleay.