Frances D. Duncan

Discontinuous gas exchange in dung beetles: patterns and ecological implications

Abstract This study correlates a distinctive pattern of external gas exchange, referred to as the discontinuous gas exchange cycle (DGC), observed in the laboratory, with habitat associations of five species of telecoprid dung beetles. The beetles were chosen from a variety of habitats that would be expected to present different amounts of water stress. All five species exhibited DGC. Sisyphus fasciculatus has been recorded only in woodland areas, and does not have strict spiracular control during its DGC. Anachalcos convexus and Scarabaeus rusticus are associated with open mesic habitats. Both species exhibit a distinct DGC, previously found in some other insect species, but intermediate within this study group. Sc. flavicornis and Circellium bacchus are typically found in arid regions, and have the most unusual form of DGC, with spiracular fluttering during the burst phase. These results support the hypothesis that spiracular fluttering reduces respiratory water loss. From this study we conclude that the DGC is an ancestral adaptation, most probably as a result of anoxic environments in underground burrows, but that spiracular control is enhanced to reduce respiratory water loss in beetle species that live in arid habitats.

The role of the subelytral cavity in respiration in a tenebrionid beetle, Onymacris multistriata (Tenebrionidae: Adesmiini).

This study measured the respiratory patterns in the tenebrionid beetle, Onymacris multistriata, using flow-through respirometry to measure carbon dioxide emission from the mesothoracic spiracles separately and simultaneously with that from around the elytral case. 96% of the total CO(2) emitted was via the mesothoracic spiracles. These spiracles used a discontinuous gas exchange cycle similar to that measured from other tenebrionid beetles. Although the circadian rhythm of the beetles resulted in changes to the period durations and cycle frequencies in the discontinuous gas exchange cycles, the mesothoracic spiracle remained the major site for gas exchange. Thus the subelytral cavity plays a different role in respiration other than the elimination of CO(2) build-up. It is expected that other arid dwelling flightless beetles will also be shown to use the mesothoracic spiracle as the major route for CO(2) emission.

Respiratory patterns and metabolism in tenebrionid and carabid beetles from the Simpson Desert, Australia

The respiratory physiology of four species of Australian desert-dwelling beetle was examined using a flow-through respirometry system over the temperature range of 20–40°C. The two species of tenebrionid beetles (Heleus waitei and Pterohelaeus sp.) did not exhibit the discontinuous gas exchange cycles (DGC), a mechanism to possibly reduce respiratory water loss rates, observed in other arid-dwelling beetles. There were small increases in metabolic rate with temperature resulting in a Q10 of 1.84 for H. waitei and 1.99 for Pterohelaeus sp. Furthermore, H. waitei has no increase in metabolic rate over the temperature range 25–35°C (Q10=1). The two species of carabid beetles (Cerotalis sp. and Carenum sp.) displayed the DGC respiratory pattern, having long flutter and burst periods. Both species also exhibited spiracular movement or muscular pumping in the burst period. Relatively low Q10 values of 1.31 and 1.64 were measured for Cerotalis sp. and Carenum sp., respectively. Cerotalis sp. had no increase in metabolic rate over the temperature range 20–35°C (Q10=1). In both species the temperature-associated increases in metabolic rate were modulated by increases in DGC frequency. All the four beetle species studied have fused elytra, a closed subelytral cavity and are nocturnally active, which should assist in reducing respiratory water loss rates, and occupy similar microhabitats. Thus we propose that the difference in respiratory patterns found between the tenebrionid and carabid beetles is related to their thorax morphology, food type and food availability.

Standard and exercise metabolism and the dynamics of gas exchange in the giant red velvet mite, Dinothrombium magnificum

Abstract Although many adult insects and adult, unfed ticks are known to exchange respiratory gases discontinuously, the respiratory physiology of other tracheate arthropods is almost unknown. In this paper we present data to test hypotheses on the gas exchange dynamics, standard metabolic rate (SMR, as a function of body mass and temperature) and activity metabolism of the giant red velvet mite Dinothrombium magnificum , which according to published accounts is a predator on termites and may forage for only a few hours every year. External gas exchange in D. magnificum is continuous, unlike the case in ticks, and its locomotion energetics are typical of other small arthropods in spite of its unsegmented, flexible exoskeleton. In its aboveground state D. magnificum has a conventional arthropod SMR, disproving our hypothesis that its SMR is low, as is the case with ticks. However, we show by modeling that D. magnificum must enter a tick-like low-SMR state if it is to survive year-long fasts and if our understanding of its ecology is correct.

The role of the mesothoracic spiracles in respiration in flighted and flightless dung beetles.

SUMMARY The relative role of the mesothoracic and abdominal spiracles in respiration was examined using flow-through respirometry in four dung beetle species from different habitats. Two species of flightless beetles, Scarabaeus (Pachysoma) gariepinus and Scarabaeus (Pachysoma) striatum, from the arid western region of southern Africa and a large flighted species, Pachylomerus femoralis, from a more mesic habitat were compared with Circellium bacchus, a flightless beetle from a low rainfall eastern area. All species showed a form of the discontinuous gas exchange pattern at rest. The mesic flighted species used a closed, flutter, open, cycle (CFO) while those species from more arid habitats used a closed, ventilation, cycle (CV) or a closed, burst cycle (CB). The relative importance of the mesothoracic spiracles in CO2 emission varied between the species, even between those from the same genus and habitat. C. bacchus and P. femoralis represent extremes of CO2 emission from the mesothoracic spiracles; from almost total to almost none, respectively. Overall, mesothoracic CO2 emission and convection were more pronounced in the dry habitat species, supporting the hypothesis that both strategies aid in the reduction of water loss.

Novel case of a tenebrionid beetle using discontinuous gas exchange cycle when dehydrated

Abstract In this study we show a link between the respiratory method and state of hydration in an arid dwelling tenebrionid beetle (Pimelia grandis). Dehydrated beetles use discontinuous gas exchange cycles with a flutter period consisting of several discrete bursts of CO2 release, whereas beetles given access to food and water showed a form of continuous CO2 release. These data give support to the respiratory water conservation hypothesis for the discontinuous gas exchange cycle.

ENERGY COST OF LOCOMOTION: VALIDATION OF LABORATORY DATA BY IN SITU RESPIROMETRY

The energy cost of activities central to overall fitness in most animals, such as pedestrian locomotion, is traditionally measured in the laboratory and then extrapolated to field conditions for modeling or hypothesis-testing purposes. However, no accurate, direct, in situ validation of these extrapolations has been made on undisturbed animals. We present the first such validation here, on harvester ants (Messor pergandei). After measuring their energy parameters in the laboratory using a running tube, we measured the metabolic rate (MR) of a natural, undisturbed ant trail foraging through a respirometer chamber in the field. From temperature, ant numbers, ant masses, and foraging speeds (obtained through video analysis), we predicted trail MRs from separately obtained laboratory data and compared them to measured field trail MRs. No significant difference was found between actual field values and values predicted from equations produced from data acquired in the laboratory. Our results therefore serve to validate two unusual findings in ants, which would otherwise be troubling to extrapolate to field conditions: that their extrapolated MR at zero running speed does not differ from standard (resting) metabolic rate, and that their costs of transport are far lower than predicted on the basis of current allometric equations.

The role of the subelytral spiracles in respiration in the flightless dung beetle Circellium bacchus.

SUMMARY The role of the subelytral cavity in flightless beetle species as an adaptation to water saving in arid habitats is still in dispute. We found that relatively little CO2 was released from the subelytral cavity of a large apterous beetle Circellium bacchus during simultaneous measurements of CO2 emission from the anterior mesothoracic spiracles and posterior body, which included the subelytral spiracles. However, when we sampled air directly from inside the subelytral cavity, we discovered that this pattern was reversed. A discontinuous gas exchange cycle (DGC) was recorded from the posterior body half, revealing a flutter phase that had been absent from the anterior mesothoracic DGC. The anterior mesothoracic and posterior subelytral spiracles act in synchrony to maintain high CO2 and water vapour levels inside the subelytral cavity. In addition, the O2 concentration of the air within the subelytral cavity is lower than the air around the elytral case, irrespective of the time of sampling. These findings lead us to conclude that the subelytral spiracles work in a coordinated fashion with the anterior spiracles to create a DGC, which allows us to extend the hypothesis of the function of the subelytral cavity as a respiratory water-saving device.

The Burden Within: The Energy Cost of Load Carriage in the Honeypot Ant, Myrmecocystus

We measured the energy costs of locomotion and load carriage in two species of the honeypot ant, Myrmecocystus. The diurnal forager, Myrmecocystus mendax, ran voluntarily in a running tube respirometer at 40° C. After running for about an hour, foragers were removed and given access to honey water, which they drank. The energy cost of carrying this internal load was then measured. The nocturnalforager, Myrmecocystus mexicanus, ran voluntarily in the running tube at 30° C. These ants could not be persuaded to collect honey water. The minimum costs of transport (MCOT)-160 and 105 J kg⁻¹ m⁻¹ for M. mendax (6 mg) and M. mexicanus (14 mg), respectively-were substantially below values estimated from current allometric equations relating MCOT to body mass. Using our voluntary locomotion regime, we found the Y-axis intercept of the relation between metabolic rate and running speed to be equivalent to standard metabolic rate for all species and load conditions. The cost per unit mass of internal load carriage in M. mendax (the smallest animal to date for which we have data on load carriage costs) was equivalent to that of body mass carriage. Internal load carriage therefore does not appear to offer cost savings compared with external load carriage.

A comparison of the energetics of foraging of three species of Leptogenys (Hymenoptera, Formicidae)

Abstract. The three Leptogenys species L.nitida, L.schwabi and L.attenuata coexist in the coastal forests of South Africa and exhibit an array of foraging strategies ranging from individual foraging to group raiding. To determine whether there is a physiological basis for these strategies, the energetic cost of locomotion of individuals in these three species was determined. Carbon dioxide emission of voluntary running ants was measured using a flow‐through technique, in order to determine their metabolic rate when running. The minimum cost of transport was constant over a range of temperatures (20–35°C), and similar for all three species (common value 212.96 ± 17.35 Jkg_1m_1). A comparison of the energy required to sustain representative foragers at 25°C indicated that it was energetically less expensive for L. nitida workers to forage than for the other two species. This may explain why L.nitida uses group raiding, while the other two species use individual foraging with limited recruitment for retrieval of large food items.