Water loss in insects: An environmental change perspective

Abstract

In the context of global environmental change much of the focus has been on changing temperatures. However, patterns of rainfall and water availability have also been changing and are expected to continue doing so. In consequence, understanding the responses of insects to water availability is important, especially because it has a pronounced influence on insect activity, distribution patterns, and species richness. Here we therefore provide a critical review of key questions that either are being or need to be addressed in this field. First, an overview of insect behavioural responses to changing humidity conditions and the mechanisms underlying sensing of humidity variation is provided. The primary sensors in insects belong to the temperature receptor protein superfamily of cation channels. Temperature-activated transient receptor potential ion channels, or thermoTRPs, respond to a diverse range of stimuli and may be a primary integrator of sensory information, such as environmental temperature and moisture. Next we touch briefly on the components of water loss, drawing attention to a new, universal model of the water costs of gas exchange and its implications for responses to a warming, and in places drying, world. We also provide an overview of new understanding of the role of the sub-elytral chamber for water conservation, and developments in understanding of the role of cuticular hydrocarbons in preventing water loss. Because of an increasing focus on the molecular basis of responses to dehydration stress we touch briefly on this area, drawing attention to the role of sugars, heat shock proteins, aquaporins, and LEA proteins. Next we consider phenotypic plasticity or acclimation responses in insect water balance after initial exposures to altered humidity, temperature or nutrition. Although beneficial acclimation has been demonstrated in several instances, this is not always the case. Laboratory studies show that responses to selection for enhanced ability to survive water stress do evolve and that genetic variation for traits underlying such responses does exist in many species. However, in others, especially tropical, typically narrowly distributed species, this appears not to be the case. Using the above information we then demonstrate that habitat alteration, climate change, biological invasions, pollution and overexploitation are likely to be having considerable effects on insect populations mediated through physiological responses (or the lack thereof) to water stress, and that these effects may often be non-intuitive.

Floral humidity as a reliable sensory cue for profitability assessment by nectar-foraging hawkmoths

“…37). When insects such as H. lineata use local humidity differences in a nectar-foraging context, the humidity gradients are more likely to convey information about floral profitability than microhabitat suitability.…”

Lepidopteran species differ in susceptibility to winter warming

“…*Significant energy depletion (p < 0.05) from the beginning to the end of winter; sample sizes above bars larvae (~40% of their initial water content) may be a cause of the high mortality observed in this species and may result from increased permeability of the larval compared to pupal integument, or interspecific differences in cuticular hydrocarbons (Chown et al 2011). It is possible that the mortality in winterwarmed P. canadensis pupae (which also lose a lot of mass, Mercader & Scriber 2008) may be mediated by desiccation stress rather than energetic drain.…”

Insect capa neuropeptides impact desiccation and cold tolerance

“…Insects are ectotherms with high surface area to volume ratios; maintaining water balance and tolerating temperature fluctuations thus are essential adaptations. In desiccating environments, a key mechanism used by insects to maintain water balance is to reduce the rate of water loss (1,2). In lowtemperature environments insects face both chilling and low availability of water, thus requiring that they be both cold and desiccation tolerant (3).…”