Microhabitats and activity

The significance of microclimate (= climate of the microhabitat, see Section 4.4.1) in the thermal and water balance of insects was reviewed by Willmer (1982). The critical parameters are temperature and the moisture content of the air, continuously modified by solar radiation and air movement. Steep gradients are common near the ground (both above and below the surface), and climate can be greatly modified in boundary layers, especially when vegetation is present to transpire, provide shade and slow air movement. It is flight which enables insects to use the three-dimensional structure of vegetation and its associated microhabitats. The thermoregulation literature is replete with studies recording activity times, body temperatures, and microclimate parameters for insect ectotherms.

Desert beetles

Flightless tenebrionid beetles are abundant in arid and semi-arid habitats around the world. Many are diurnally active and black in colour, and their thermal biology has attracted much attention from ecophysiologists. Their preferred body temperatures and thermal tolerances are broadly correlated with habitat, those of some North American arid-land tenebrionids (mainly Eleodes species) (e.g. Kenagy and Stevenson 1982; Parmenter et al. 1989) being considerably lower than those of Namib desert tenebrionids of the genus Onymacris, which exhibit high preferred Tb (among the highest of any ectotherms) in both the laboratory and field, and high upper lethal temperatures (Roberts et al. 1991). Mechanisms of thermoregulation in Onymacris species occupying different sandy habitats were investigated using continuous measurement of field Tb with implanted thermocouples (Ward and Seely 1996b). High preferred Tb was maintained primarily by shuttling behaviour, climbing on bushes, and burying in the sand (a depth of 20 cm is enough to maintain constant Tb day and night). There was little evidence of basking or of stilting behaviour; although Onymacris species have long legs, stilting may be effective only in a narrow range of wind velocities. Microhabitat shifts were thus more important than postural adjustments in controlling Tb (Ward and Seely 1996a). Although the beetles are exposed to high surface temperatures, sand is a readily available thermal refuge providing access to a broad range of Ta. Comparative phylogenetic analysis shows perfect coadaptation of preferred and field Tb in Onymacris (Ward and Seely 1996b). Many desert tenebrionids show biphasic activity to avoid high midday temperatures. However, this is not the case in all biphasic desert beetles. For example, in keratin-feeding Omorgus species (Trogidae) of the Kalahari desert, the relationship between temperature and surface activity is more complex; a dawn peak of activity, at moderate temperatures and high humidity, when feeding predominates, and a sunset peak for social interactions when temperatures are still high (Scholtz and Caveney 1992).

Thermal respites for heat-tolerant ants Small body size and the inability to fly restrict many ant species (Formicidae) to severe surface temperatures (Willmer 1982), but a mosaic of microclimates can be utilized even in sandy desert habitats, and ants are important desert fauna throughout the world. Extreme heat tolerance is seen in thermophilic desert ants which forage at nearly lethal temperatures for insect prey that has already succumbed to heat stress. The Namib Desert ant Ocymyrmex robustior begins foraging when sand surface temperatures reach 30oC, but when they exceed 51oC it forages intermittently, pausing in thermal refuges such as the shade of a grass stalk or running up stems to cool off (Marsh 1985). The frequency and duration of thermal respite behaviour increases with sand temperatures over 51oC, and refuge temperatures are 7-15oC lower than those an ant would experience on the sand surface. Live mass of O. robustior is about 4 mg and its low thermal inertia ensures rapid heat exchange. Even more extreme heat tolerance is seen in the synchronized midday foraging of the

Saharan ant Cataglyphis bombycina, which occurs during a brief thermal window with its upper limit set by the critical thermal maximum of 53.6°C and the lower limit by the retreat of a lizard predator into burrows (Wehner et al. 1992). Again, thermal respite behaviour is vital during foraging. Heat shock proteins are accumulated by Cataglyphis species prior to heat exposure (Gehring and Wehner 1995) (Section 5.2.2). Most ants are too small for direct measurement of Tb using thermocouples, and Tb is estimated from Te thermometers placed at ant height and in other appropriate locations. However, Tb has been measured directly in a larger thermophilic species, Melophorus bagoti, which is the Australian equivalent of Ocymyrmex and Cataglyphis (Christian and Morton 1992). Both large and small workers of M. bagoti maintain Tb of 45-46°C through much of the day. Size effects may be insignificant in the steep temperature and convection gradients next to the surface.

Interspecific interactions lead to temporal resource partitioning in ants occupying different thermal niches. Subordinate species in a Mediterranean grassland are forced to be active in more stressful conditions during the day and to forage at temperatures closer to their critical thermal limits; however, this risk-prone strategy has foraging benefits (Cerda and Retana 2000). Similarly, in an Argentinian ant community (Bestelmeyer 2000), subordinate species are active at temperature extremes (both high and low) when they have almost exclusive access to resources. The balance between stress and competition in ant assemblages is thought to be responsible for the unimodal relationship between species richness and the abundance of dominant ants (Andersen 1992). During stressful conditions, richness is low and most species are thermal specialists. As stress declines, more species become active, and under the most favourable conditions numerically abundant and aggressive dominant ants exclude other species. This unimodal relationship between species richness and the abundance of dominant ants is convergent across three continents (Africa, Australia and North America). However, modelling work by Parr (2003) has demonstrated that interactions between stress and competition are not necessarily required to explain this unimodal relationship.

Rather, it may also be the simple consequence of the constraints imposed by abundance frequency distributions, themselves the outcome of a variety of processes (Tokeshi 1999).

Sun patches in forests

Short-term selection of sunlit or shaded substrates is probably the most common mechanism for control of body temperature in insects (May 1979). Tiger beetles in the genus Cicindela (Cicindelidae) are active diurnal predators of open sandy habitats which maintain high Tb around 35°C through a combination of basking, stilting, and shuttling between sun and shade (Dreisig 1980). However, a few Cicindela species inhabit forest floor environments: C. sexguttata adults aggregate in light gaps to maintain their preferred Tb of 33°C, behaviour which increases prey capture rates and intra-specific encounters, but also the risk of predation (Schultz 1998). Robber flies (Asilidae) are ambush predators which spend 98 per cent of their time perched (O'Neill et al. 1990), and those in a tropical rainforest can be grouped into light- and shade-seeking species: The former bask but the latter, which belong to a more recent lineage, do not thermoregulate at all and their foraging and activity patterns differ accordingly (Morgan et al. 1985). This occupation of either sunlit or shaded habitats occurs on a long-term basis compared to shuttling behaviour, and is also evident in damselflies in a tropical forest (Shelly 1982) and in butterflies (Srygley and Chai 1990). Herrera (1997) examined the varied insect pollinators of summer-flowering Lavandula latifolia in a mosaic of shade and sun on a Spanish forest floor. Compared to hymenopterans, the dipteran pollinators (mainly hoverflies; Syrphidae) tended to be restricted to shade and had significantly lower Tth, lower temperature excesses (the difference between Tb and Ta) and higher Tth/Ta regression slopes. Insolation of plants thus determines the pollinator assemblages to which they are exposed. Individual plants of L. latifolia show characteristic temporal patterns of exposure to direct sunlight, and patterns of insect visitation vary accordingly (Herrera 1995a). Microclimate effects on pollinator activity at flowers were reviewed by Corbet (1990). Physical factors affect plant reproduction through both the hygrothermal balance of pollinators and the floral rewards on offer, and these effects depend on the local microclimate within the flower. Temperature excesses as high as 8°C inside flowers are of critical importance for small ectothermic pollinators crawling deep into the flowers (Herrera 1995b). The thermal benefits of flower basking have also been demonstrated for insect pollinators visiting helio-tropic and other Arctic flowers (Kevan 1973,1975), and for scarab pollinators resting inside endo-thermic flowers in neotropical forests (Seymour et al. 2003).

Males of the speckled wood butterfly Pararge aegeria (Nymphalidae) perch in sun spots in woodland habitats and defend these territories against patrolling intruders. The contest takes the form of a short spiral flight and is usually won by the owner of the sun spot. Patrollers have a lower Tth than perchers, due to flying through shade, but their darker wings have basking benefits (Van Dyck and Matthysen 1998). This butterfly has been widely used in behavioural research, and Stutt and Willmer (1998) have elegantly separated temperature and ownership asymmetries by releasing pairs of butterflies in sun spots after manipulating their body temperatures: contests were won by the warmer butterflies. Sun spots can be considered as limited solar energy resources.

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