Comment les insectes gèrent-ils l'hiver ? Histoire d'une renaissance
Les insectes ont développé de nombreuses stratégies de survie aux conditions hivernales. Ces adaptations, toutes dictées par le patrimoine ...
les organites cellulaires
En réponse à une lente déshydratation, les concentrations en glycérol et en tréhalose sont augmentées dans les tissus de l’organisme. Ces molécules permettent de protéger <anchor>les organites cellulaires</anchor> et donc de résister à des périodes prolongées de dessiccation.
Mechanisms to reduce dehydration stress in larvae of the Antarctic midge, Belgica antarctica - ScienceDirect
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## Journal of Insect Physiology

# Mechanisms to reduce dehydration stress in larvae of the Antarctic midge, Belgica antarctica

Author links open overlay panelJoshua B.Benoita, GiancarloLopez-Martineza, M.Robert Michauda, Michael A.Elnitskyb, Richard E.LeeJrb, David L.Denlingera

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## Abstract

The Antarctic midge, Belgica antarctica, is exposed to frequent periods of dehydration during its prolonged larval development in the cold and dry Antarctic environment. In this study, we determined the water requirements of the larvae and the mechanisms it exploits to reduce the stress of drying. Larvae lost water at an exceptionally high rate (>10%/h) and tolerated losing a high portion (>70%) of their water content. Larvae were unable to absorb water from subsaturated water vapor (⩽0.98 a v) to replenish their water stores, thus this midge relies exclusively on the intake of liquid water to increase its pool of body water and maintain water balance. To reduce dehydration stress, the midge employed a variety of mechanisms. Behaviorally, the larvae suppressed water loss by clustering. In response to slow dehydration, glycerol concentration increased 2-fold and trehalose concentration increased 3-fold, responses that are known to decrease the rate of water loss and increase dehydration tolerance. No changes in the mass of cuticular lipids occurred in response to desiccation, but the observed shift to longer hydrocarbons likely contributes to reduced water loss as the larvae dehydrate. As the larvae dehydrated, their oxygen consumption rate dropped, resulting in a reduction of water loss by respiration. Lastly, one bout of slow dehydration also enhanced the larva's ability to survive subsequent dehydration, suggesting that the larvae have the capacity for drought acclimation. Thus, these hydrophilic midge larvae prevent dehydration by multiple mechanisms that collectively reduce the water loss rate and increase dehydration tolerance.

## Introduction

The habitat range of the Antarctic midge, Belgica antarctica, extends along the west coast of the Antarctic Peninsula with sporadic, but highly dense, populations in localized areas (Usher and Edwards, 1984; Sugg et al., 1983). During its 2-year life cycle, larvae feed on moss, terrestrial algae and other types of organic debris (Convey and Block, 1996; Sugg et al., 1983). All four larval instars are capable of overwintering. The adults emerge, mate, lay eggs and die with a 10–14d period during the austral summer.

During most of the year, the larvae are frozen in their hibernacula, buffered at 0 to −5°C by thick snow and ice cover (Baust and Lee, 1981; Lee et al., 2006). While frozen, the midge larvae are in vapor pressure equilibrium with the local environment, and no dehydration can occur (Holmstrup et al., 2002a, Holmstrup et al., 2002b; Lee et al., 2006). Even when not frozen, the highly permeable larvae likely are very close to vapor pressure equilibrium due to their relatively high osmolality (Lee et al., 2006; Hayward et al., 2007). Thus, with their high permeability, the midges will likely dehydrate and rehydrate in response to minor changes in the immediate habitat and continually maintain vapor pressure equilibrium, as described by Holmstrup et al. (2002a) for Collembola. Interestingly, the high permeability that allows larvae of B. antarctica to obtain vapor pressure equilibrium during most of the year is a severe problem during summer when periods of subfreezing temperatures, along with drought conditions, are common.

During our recent field seasons, we observed larvae of B. antarctica representing a range of hydration states. While most of the larvae we collected during the austral summer were fully hydrated, others were conspicuously less so. These field observations were backed by laboratory measurements showing that some of the larvae contained 30–40% less water than fully hydrated individuals. In an earlier study, we demonstrated that midge larvae tolerate a loss of up to 70% of their body water and that by slowly dehydrating they can dramatically enhance their freeze tolerance (Hayward et al., 2007). In contrast to the long-lived larvae, the short-lived adults of this species lose their body water slowly but are much less tolerant of water loss (Benoit et al., 2007).

In this study, we evaluate the water balance properties of the midge larvae and define the mechanisms it uses to prevent and reduce the stress of dehydration during the Antarctic summer. We report that as the larvae dehydrate they increase their ability to tolerate water loss and their capacity to retain their internal water pool. This is accomplished by increasing their internal concentrations of trehalose and glycerol, forming aggregations, altering the composition of their cuticular lipids, drought acclimation and reducing respiratory water loss.

## Section snippets

## Insects

The third and fourth instars of Belgica antarctica Jacobs used in these experiments were collected in substrate on Cormorant Island, Torgersen Island and Bonaparte Point, near Palmer Station on Anvers Island (64°45′ S, 64°04′ W) on the Antarctic Peninsula in January 2006. In all cases larvae were held at 4°C in their natural substrate until they were used for experimentation. Immediately before the experiments, the midges were sorted in ice-cold water. Water on the surface of the midges was

## Water pool

The total water pool for 3rd and 4th instar larvae collected on three islands near Palmer Station is presented in Table 1. Water mass, dry mass and water content did not vary significantly for larvae of the same stage collected at the three sites (ANOVA, P >0.05, F =2.01, d.f.=4, 45) but the 3rd and 4th instars were significantly different from each other (ANOVA, P <0.05, F =8.64, d.f.=1, 98). From the 3rd to 4th instars, the dry and water masses increased by 2× and 1.5×, respectively, which caused

## Discussion

To counter dehydration, terrestrial arthropods have evolved a number of adaptations to suppress water loss, to tolerate reductions in their internal water pool, and to uptake water (Wharton, 1985; Hadley, 1994). Methods to reduce water loss include the deposition of extra cuticular lipids (Yoder and Denlinger, 1991; Hadley, 1994; Benoit and Denlinger, 2007), modifications in cuticle structure (Hadley, 1994; Benoit et al., 2005), membrane changes (Hadley, 1994), reduction in metabolism (Hadley,

## Acknowledgments

Research was funded by NSF grants OPP-0337656 and OPP-0413786. The support of the Palmer Station staff is greatly appreciated.

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