Water Balance

Maintenance of homeostatic water balance also is a challenge for organisms with high ratios of surface area to volume (Edney 1977, N. Hadley 1994). The arthropod exoskeleton is an important mechanism for control of water loss. Larger, more heavily sclerotized arthropods are less susceptible to desiccation than are smaller, more delicate species (Alstad et al. 1982, Kharboutli and Mack 1993).

Arthropods in xeric environments usually are larger, have a thicker cuticle, and secrete more waxes to inhibit water loss, compared to insects in mesic environments (Crawford 1986, Edney 1977, N. Hadley 1994, Kharboutli and Mack 1993). Cuticular lipids with higher melting points might be expected to be less permeable to water loss than are lipids with lower melting points. Gibbs (2002a)

Tent caterpillars, Malacosoma spp., and other tent-constructing Lepidoptera reduce airflow and variation in temperatures within their tents.

Tent caterpillars, Malacosoma spp., and other tent-constructing Lepidoptera reduce airflow and variation in temperatures within their tents.

evaluated cuticular permeability relative to water loss for several arthropod species and found that all species produced lipids with low melting points as well as high melting points, tending to increase water loss. Furthermore, lipids with high melting points did not reduce rates of water loss (Gibbs 2002a, Gibbs et al. 2003).

Some species in xeric environments conserve metabolic water (from oxidation of food) or acquire water from condensation on hairs or spines (R. Chapman 1982, N. Hadley 1994). Carbohydrate metabolism, to release bound water, increases several-fold in some insects subjected to desiccation stress (Marron et al. 2003). Others tolerate water loss of 17-89% of total body water content (Gibbs 2002b, N. Hadley 1994). Dehydration tolerance in Drosophila apparently reflects phylogeny rather than adaptation to desert environments (Gibbs and Matzkin 2001). Some insects regulate respiratory water loss by controlling spirac-ular activity under dry conditions (Fielden et al. 1994, N. Hadley 1994, Kharboutli and Mack 1993). Water conservation is under hormonal control in some species. An antidiuretic hormone is released in desert locusts, Schistocerca gregaria, and other species under conditions of water loss (Delphin 1965).

Gibbs et al. (2003) compared the three main water loss pathways among Drosophila species from xeric and mesic habitats. Excretory loss was <6% of the total and did not differ among species from different habitats. No consistent relationship was found between cuticular properties and water loss. Cuticular water loss rates did not appear to differ among flies from different habitats. Respiratory water loss differed significantly between xeric and mesic species. Xeric species of the same size had lower metabolic rates, were less active, and showed a cyclic pattern of CO2 release, compared to mesic species, indicating adaptation to reduce respiratory loss.

Extreme dehydration may trigger the onset of anhydrobiosis, a physiological state characterized by an absence of free water and of measurable metabolism (N. Hadley 1994, Whitford 1992). Survival during anhydrobiosis requires stabilization of membranes and enzymes by compounds other than water (e.g., glyc-erol and trehalose), whose synthesis is stimulated by dehydration (N. Hadley 1994). Anhydrobiosis is common among plant seeds, fungi, and lower invertebrates, but among insects only some larval Diptera and adult Collembola have been shown to undergo anhydrobiosis (N. Hadley 1994). Hinton (1960a, b) reported that a chironomid fly, Polypedilum vanderplancki, found in temporary pools in central Africa, withstands repeated dehydration to 8% of body water content. At 3% body water content, this midge is capable of surviving temperatures from -270°C to 100°C, a range that contrasts dramatically with its tolerance range when hydrated.

Insects and other arthropods are most vulnerable to desiccation at times when a new exoskeleton is forming (i.e., during eclosion from eggs, during molts, and during diapause) (Crawford 1978,Willmer et al. 1996).Tisdale and Wagner (1990) found that percentage of sawfly, Neodiprion fulviceps, eggs hatched was significantly higher at relative humidities >50%. Yoder et al. (1996) found that slow water loss through the integument and respiration by diapausing fly pupae were balanced by passive water vapor absorption from the air at sufficiently high humidities. The ability of adult insects to regulate water loss may decline with age (Gibbs and Markow 2001).

Insects in diapause at subfreezing temperatures are subject to freeze-drying. Lundheim and Zachariassen (1993) reported that beetles that tolerate ice formation in extracellular fluids have lower rates of water loss than do insects that have supercooled body fluids, perhaps because the hemolymph in frozen beetles is in vapor pressure equilibrium with surrounding ice, whereas the hemolymph in supercooled insects has vapor pressure higher than the environment.

However, some insects must contend with excess water. Termites, ants, and other insects that live underground must survive periods of flooding. Subterranean termite species apparently survive extended periods of inundation by entering a quiescent state; relative abilities of species to withstand periods of flooding correspond to their utilization of above-ground or below-ground wood resources (Forschler and Henderson 1995).

Insects that ingest liquid food immediately excrete large amounts of water to concentrate dissolved nutrients. Elimination of excess water (and carbohydrates) in sap-feeding Homoptera is accomplished in the midgut by rapid diffusion

Adelges Cooleyi
| Sap-feeding Homoptera, such as Adelges cooleyi on Douglas fir, egest excess water and carbohydrates as honeydew.

across a steep moisture gradient created by a filter loop (R. Chapman 1982). The resulting concentration of sugars in honeydew excreted by phloem-feeding Homoptera (Fig. 2.13) is an important resource for ants, hummingbirds, preda-ceous Hymenoptera, and sooty molds (Dixon 1985, E. Edwards 1982, N. Elliott et al. 1987, Huxley and Cutler 1991). The abundant water excreted by xylem-feeding spittlebugs is used to create the frothy mass that hides the insect. Excretion in some species, such as the blood-feeding Rhodnius (Heteroptera) is controlled by a diuretic hormone (Maddrell 1962).

Water balance also can be maintained behaviorally, to some extent, by retreating to cooler or moister areas to prevent desiccation. Burrowing provides access to more mesic subterranean environments (Polis et al. 1986). The small size of most insects makes them vulnerable to desiccation but also permits habitation within the relatively humid boundary layer around plant surfaces or at the soil surface.

Termites construct their colonies to optimize temperature and moisture conditions. Formosan subterranean termites, Coptitermes formosanus, prefer nest sites with high moisture availability (Fei and Henderson 1998). Metabolic heat generated in the core of the nest rises by convection into large upper cavities and diffuses to the sides of the nest where air is cooled and gaseous exchange occurs through the thin walls. Cooled air sinks into lower passages (Luscher 1961). The interior chambers of termite colonies usually have high relative humidities.

C. Air and Water Chemistry

Air and water chemistry affect insect physiology. Oxygen supply is critical to survival but may be limited under certain conditions. Airborne or dissolved chemicals can affect respiration and development. Soil or water pH can affect exoskeleton function and other physiological processes. Changes in concentrations of various chemicals, especially those affected by industrial activities, affect many organisms, including insects.

Oxygen supply can limit activity and survival of aquatic species and some terrestrial species living in enclosed habitats. Less oxygen can remain dissolved in warm water than in cold water. Stagnant water can undergo oxygen depletion as a result of algal and bacterial respiration (Ward 1992). Some insect species living in oxygen-poor environments have more efficient oxygen delivery systems, such as increased tracheal supply, gills, or breathing tubes that extend to air supply (R. Chapman 1982, L. Chapman et al. 2004). For example, the hemolymph of some aquatic chironomid larvae and endoparasitic fly larvae is unique among insects in containing a hemoglobin that has a higher affinity for oxygen than does mammalian hemoglobin (R. Chapman 1982, Pinder and Morley 1995). Oxygen supply can be enhanced by ventilatory movement (i.e., movement of gills or other body parts to create currents that maintain oxygen supply and reduce the diffusion barrier) (Ward 1992). Other species must use siphon tubes (e.g., mosquito and syrphid fly larvae) or return to the surface (diving beetles) to obtain atmospheric oxygen (L. Chapman et al. 2004). Some wood-boring species must be able to tolerate low oxygen concentrations deep in decomposing wood, although O2 limitation may occur only in relatively sound wood or water-soaked wood (Hicks and Harmon 2002).

Increased atmospheric CO2 appears to have little direct effect on insects or other arthropods. However, relatively few insect species have been studied with respect to CO2 enrichment. Increased atmospheric CO2 can significantly affect the quality of plant material for some herbivore (Arnone et al. 1995, Bezemer and Jones 1998, Bezemer et al. 1998, Fajer et al. 1989, Kinney et al. 1997, Lincoln et al. 1993, Roth and Lindroth 1994) and decomposer (Grime et al. 1996, Hirschel et al. 1997) species, although plant response to CO2 enrichment depends on a variety of environmental factors (e.g., Lawton 1995, Watt et al. 1995, see Chapter 3). In general, leaf chewers compensate for effects of elevated CO2 by increasing consumption rates, whereas sap-suckers show reduced development times and increased population size (Bezemer and Jones 1998). At least some herbivorous species are likely to become more abundant and cause greater crop losses as a result of increased atmospheric CO2 (Bezemer et al. 1998).

Airborne and dissolved materials can include volatile emissions or secretions from plant, animal, and industrial origin. Fluorides, sulfur compounds, nitrogen oxides, and ozone affect many insect species directly, although the physiological mechanisms of toxicity are not well-known (Alstad et al. 1982, Heliovaara 1986, Heliovaara and Vaisanen 1986,1993, Pinder and Morley 1995). Disruption of epi-cuticular or spiracular tissues by these reactive chemicals may be involved. Dust and ash kill many insects, apparently because they absorb and abrade the thin epicuticular wax-lipid film that is the principal barrier to water loss. Insects then die of desiccation (Alstad et al. 1982). V. C. Brown (1995) concluded that there is little evidence for direct effects of realistic concentrations of these major air pol lutants on terrestrial herbivores, but there is considerable evidence that many herbivorous species respond to changes in the quality of plant resources or abundance of predators resulting from exposure to these pollutants. Kainulainen et al. (1994) found that exposure of Scots pine, Pinus sylvestris, seedlings to ozone significantly reduced amounts of starch and total amino acids at the highest ozone concentration (0.3 ppm), but it did not affect other sugars or other secondary compounds. Reproduction of grey pine aphids, Schizolachnus pineti, was not significantly affected by ozone exposure. However, pollutants may interfere with olfactory detection of hosts. Gate et al. (1995) exposed braconid parasitoids, Asobara tabida, to ozone, sulfur dioxide, and nitrogen dioxide in chambers with aggregations of its host, Drosophila subobscura. Ozone, but not sulfur dioxide or nitrogen dioxide, significantly reduced searching efficiency and the proportion of hosts that were parasitized. Parasitoids were able to avoid patches with no hosts but appeared to be less able to distinguish different host densities, indicating that air pollutants could reduce the effect of predation or parasitism.

Soil and water pH affect a variety of chemical reactions, including enzymatic activity. Changes in pH resulting from acidification (such as from volcanic or anthropogenic activity) affect osmotic exchange, gill and spiracular surfaces, and digestive processes. Changes in pH often are correlated with other chemical changes, such as increased N or S, and effects of pH change may be difficult to separate from other factors. Pinder and Morley (1995) reported that many chi-ronomid species are relatively tolerant of alkaline water, but few are tolerant of pH < 6.3. Other aquatic species also may be unable to survive in low pH water (Batzer and Wissinger 1996). Acid deposition and loss of pH buffering capacity likely will affect survival and reproduction of aquatic and soil/litter arthropods (Curry 1994, Pinder and Morley 1995).

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