Desiccation tolerance
Encyclopedia
Desiccation tolerance refers to the ability of an organism to withstand or endure extreme dryness, or drought
-like conditions. Plants and animals living in arid or periodically arid environments such as temporary streams or ponds may face the challenge of desiccation, therefore physiological or behavioral adaptation
s to withstand these periods are necessary to ensure survival. In particular, insects occupy a wide range of ecologically diverse niches and so exhibit a variety of strategies to avoid desiccation.
Desiccation resistance in insects is generally measured by the change in mass during dry conditions. The overall mass difference between measurements before and after aridity exposure is attributed to body water loss, as respiratory water loss is generally considered negligible.
.
The accumulation of glycogen
during the insect larval stage has been linked to increased body water content and is likely a source of metabolic water during dry conditions. Glycogen, a glucose
polysaccharide
, acts as an oxidative energy source during times of physiological stress. Because it binds up to five times its weight in bulk water, insects with increased levels of body glycogen also have higher amounts of internal water. In general, insects selected for desiccation resistance also exhibit longer larval stages than those sensitive to desiccation. This increase in development time is likely a response to the environment, allowing larvae more time to accumulate glycogen, and therefore more water before eclosion.
Another possible source contributing to higher levels of initial body water in insects is hemolymph
volume. The insect equivalent to blood, hemolymph is the fluid found within the hemocoel, and is the largest pool of extracellular water within the insect body. In the fruit-fly Drosophila melanogaster
, flies selected for desiccation resistance also yielded higher amounts of hemolymph. Higher hemolymph volume is linked to an increase in carbohydrate
s, in particular, trehalose
, a common sugar found in many plants and animals with high desiccation resistance. Drosophila melanogaster flies selected for desiccation resistance show a 300% increase in hemolymph volume compared to control flies, correlating to a similar increase in trehalose levels. During periods of aridity, cells dehydrate and draw upon hemolymph stores to replenish intracellular water; therefore insects with higher levels of this fluid are less prone to desiccation.
Insects may also increase body water content by simply feeding more often. Because sugar is slowly absorbed into the hemolymph at each meal, increasing the frequency at which the insect ingests a sugar source also increases its desiccation tolerance. Furthermore, the crop
may also act not only to store food prior to digestion, but to provide an additional reservoir for water and sugar.
(respiration); and (3) excretion
, or waste products. The important feature in reducing water loss in land snail
s during inactivity is an epiphragm
.
or integument of insects acts as an impermeable, protective layer against desiccation. It is composed of an outer epicuticle, underlain by an procuticle which itself may be further divided into an exo- and endocuticle. The endocuticle provides the insect with toughness and flexibility and the hard exocuticle serves to protect vulnerable body parts. However, the outer cuticular layer (epicuticle) is a protein-polyphenol complex made up of lipoprotein
s, fatty acid
s and waxy molecules and is the insect’s primary defense against water loss. Many insect orders secrete an additional cement layer over their wax layer, likely to protect against the abrasion or removal of waxy molecules. This layer is composed of lipids and proteins held together by polyphenol
ic compounds and is secreted by the dermal
glands.
The rate of water loss in insects is generally low at moderate temperatures. Once a species-specific critical temperature (Tc) is reached, as temperatures continue to increase, rapid water loss occurs. The “lipid
melting model” is used to explain this sudden increase in the rate of water loss. The lipid melting model states that increased cuticular water loss is directly related to the melting of surface lipids. Insects already adapted to more arid environments have a higher Tc, that is, their cuticular properties change and lipid structures melt at a higher critical temperature.
In some insects, the rate of cuticular water loss is controlled to some extent by the neuroendocrine system. Immediately following head removal, decapitated cockroaches exhibit a large increase in transpiration
across the cuticle, leading to severe dehydration. Injection of brain hormone
s into freshly separated bodies results in a sharp reduction in cuticular water loss.
s, which shut when open respiration
is unnecessary and prevent water from escaping. Insects at a greater risk for water loss face the challenge of either a depleted oxygen supply or desiccation, leading to an adaptive increase in tracheal volume in order to receive more oxygen.
fruit flies, for example, will actively move to areas with higher atmospheric water content when placed in dry environments. Alternatively, the dung beetle
buries food in underground chambers, thereby ensuring water and energy sources during periodically dry conditions. Feeding location may also be altered to ensure body hydration. Some caterpillars preferentially feed on the underside of leaves, where microclimate has higher relative humidity. In a highly time-consuming activity such as feeding, these insects significantly reduce their chances of desiccation.
refers to the state of an organism that has no detectable metabolic activity, resulting from extreme and unfavorable environmental conditions; anhydrobiosis refers to the state of surviving the loss of (almost) all body water. Although this state is commonly observed in invertebrates, only one insect is known to be cryptobiotic (anhydrobiotic), the African chironomid Polypedilum vanderplanki
. Polypedilum vanderplanki undergoes anhydrobiosis, a cryptobiotic state wherein the body is completely dehydrated. The larvae of P. vanderplanki inhabit rock pools which commonly dry out completely. In response, P. vanderplanki larvae enter an anhydrobiotic state during which changes in body osmolarity trigger the production of large amounts of trehalose. Due to its capacity for water replacement and vitrification, the accumulation of trehalose prevents the death of the larvae from water loss.
Drought
A drought is an extended period of months or years when a region notes a deficiency in its water supply. Generally, this occurs when a region receives consistently below average precipitation. It can have a substantial impact on the ecosystem and agriculture of the affected region...
-like conditions. Plants and animals living in arid or periodically arid environments such as temporary streams or ponds may face the challenge of desiccation, therefore physiological or behavioral adaptation
Adaptation
An adaptation in biology is a trait with a current functional role in the life history of an organism that is maintained and evolved by means of natural selection. An adaptation refers to both the current state of being adapted and to the dynamic evolutionary process that leads to the adaptation....
s to withstand these periods are necessary to ensure survival. In particular, insects occupy a wide range of ecologically diverse niches and so exhibit a variety of strategies to avoid desiccation.
Desiccation resistance in insects is generally measured by the change in mass during dry conditions. The overall mass difference between measurements before and after aridity exposure is attributed to body water loss, as respiratory water loss is generally considered negligible.
Types of desiccation resistance
There are three main ways in which insects can increase their tolerance to desiccation: by increasing their total body water content; reducing the rate of body water loss; and by tolerating a larger proportion of overall water loss from the body. Survival time is determined by initial water content, and can be calculated by dividing water loss tolerance (the maximum amount of water that may be removed without resulting in death) by water loss rate.Increasing body water content
Insects with a higher initial body water content have better survival rates during arid conditions than insects with a lower initial body water content. Higher amounts of internal body water lengthen the time necessary to remove the amount of water required to kill the organism. The way in which body water content is increased may differ depending on the speciesSpecies
In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...
.
The accumulation of glycogen
Glycogen
Glycogen is a molecule that serves as the secondary long-term energy storage in animal and fungal cells, with the primary energy stores being held in adipose tissue...
during the insect larval stage has been linked to increased body water content and is likely a source of metabolic water during dry conditions. Glycogen, a glucose
Glucose
Glucose is a simple sugar and an important carbohydrate in biology. Cells use it as the primary source of energy and a metabolic intermediate...
polysaccharide
Polysaccharide
Polysaccharides are long carbohydrate molecules, of repeated monomer units joined together by glycosidic bonds. They range in structure from linear to highly branched. Polysaccharides are often quite heterogeneous, containing slight modifications of the repeating unit. Depending on the structure,...
, acts as an oxidative energy source during times of physiological stress. Because it binds up to five times its weight in bulk water, insects with increased levels of body glycogen also have higher amounts of internal water. In general, insects selected for desiccation resistance also exhibit longer larval stages than those sensitive to desiccation. This increase in development time is likely a response to the environment, allowing larvae more time to accumulate glycogen, and therefore more water before eclosion.
Another possible source contributing to higher levels of initial body water in insects is hemolymph
Hemolymph
Hemolymph, or haemolymph, is a fluid in the circulatory system of some arthropods and is analogous to the fluids and cells making up both blood and interstitial fluid in vertebrates such as birds and mammals...
volume. The insect equivalent to blood, hemolymph is the fluid found within the hemocoel, and is the largest pool of extracellular water within the insect body. In the fruit-fly Drosophila melanogaster
Drosophila melanogaster
Drosophila melanogaster is a species of Diptera, or the order of flies, in the family Drosophilidae. The species is known generally as the common fruit fly or vinegar fly. Starting from Charles W...
, flies selected for desiccation resistance also yielded higher amounts of hemolymph. Higher hemolymph volume is linked to an increase in carbohydrate
Carbohydrate
A carbohydrate is an organic compound with the empirical formula ; that is, consists only of carbon, hydrogen, and oxygen, with a hydrogen:oxygen atom ratio of 2:1 . However, there are exceptions to this. One common example would be deoxyribose, a component of DNA, which has the empirical...
s, in particular, trehalose
Trehalose
Trehalose, also known as mycose or tremalose, is a natural alpha-linked disaccharide formed by an α,α-1,1-glucoside bond between two α-glucose units. In 1832, H.A.L. Wiggers discovered trehalose in an ergot of rye, and in 1859 Marcellin Berthelot isolated it from trehala manna, a substance made...
, a common sugar found in many plants and animals with high desiccation resistance. Drosophila melanogaster flies selected for desiccation resistance show a 300% increase in hemolymph volume compared to control flies, correlating to a similar increase in trehalose levels. During periods of aridity, cells dehydrate and draw upon hemolymph stores to replenish intracellular water; therefore insects with higher levels of this fluid are less prone to desiccation.
Insects may also increase body water content by simply feeding more often. Because sugar is slowly absorbed into the hemolymph at each meal, increasing the frequency at which the insect ingests a sugar source also increases its desiccation tolerance. Furthermore, the crop
Crop (anatomy)
A crop is a thin-walled expanded portion of the alimentary tract used for the storage of food prior to digestion that is found in many animals, including gastropods, earthworms, leeches, insects, birds, and even some dinosaurs.- Bees :Cropping is used by bees to temporarily store nectar of flowers...
may also act not only to store food prior to digestion, but to provide an additional reservoir for water and sugar.
Reducing rate of water loss
Another strategy used to reduce the risk of death by dehydration is to reduce the rate at which water is lost. The three main ways through which insects can lose water are (1) the surface of the body (integument); (2) the tracheaeInvertebrate trachea
The invertebrate trachea refers to the open respiratory system composed of spiracles, tracheae, and tracheoles that terrestrial arthropods have to transport metabolic gases to and from tissues....
(respiration); and (3) excretion
Excretion
Excretion is the process by which waste products of metabolism and other non-useful materials are eliminated from an organism. This is primarily carried out by the lungs, kidneys and skin. This is in contrast with secretion, where the substance may have specific tasks after leaving the cell...
, or waste products. The important feature in reducing water loss in land snail
Snail
Snail is a common name applied to most of the members of the molluscan class Gastropoda that have coiled shells in the adult stage. When the word is used in its most general sense, it includes sea snails, land snails and freshwater snails. The word snail without any qualifier is however more often...
s during inactivity is an epiphragm
Epiphragm
An epiphragm is a temporary structure which can be created by many species of shelled, air-breathing land snails, terrestrial pulmonate gastropod mollusks. It can also be created by freshwater snails when temporary pools dry up....
.
Integument
The exoskeletonExoskeleton
An exoskeleton is the external skeleton that supports and protects an animal's body, in contrast to the internal skeleton of, for example, a human. In popular usage, some of the larger kinds of exoskeletons are known as "shells". Examples of exoskeleton animals include insects such as grasshoppers...
or integument of insects acts as an impermeable, protective layer against desiccation. It is composed of an outer epicuticle, underlain by an procuticle which itself may be further divided into an exo- and endocuticle. The endocuticle provides the insect with toughness and flexibility and the hard exocuticle serves to protect vulnerable body parts. However, the outer cuticular layer (epicuticle) is a protein-polyphenol complex made up of lipoprotein
Lipoprotein
A lipoprotein is a biochemical assembly that contains both proteins and lipids water-bound to the proteins. Many enzymes, transporters, structural proteins, antigens, adhesins, and toxins are lipoproteins...
s, fatty acid
Fatty acid
In chemistry, especially biochemistry, a fatty acid is a carboxylic acid with a long unbranched aliphatic tail , which is either saturated or unsaturated. Most naturally occurring fatty acids have a chain of an even number of carbon atoms, from 4 to 28. Fatty acids are usually derived from...
s and waxy molecules and is the insect’s primary defense against water loss. Many insect orders secrete an additional cement layer over their wax layer, likely to protect against the abrasion or removal of waxy molecules. This layer is composed of lipids and proteins held together by polyphenol
Polyphenol
Polyphenols are a structural class of natural, synthetic, and semisynthetic organic chemicals characterized by the presence of large multiples of phenol structural units...
ic compounds and is secreted by the dermal
Dermis
The dermis is a layer of skin between the epidermis and subcutaneous tissues, and is composed of two layers, the papillary and reticular dermis...
glands.
The rate of water loss in insects is generally low at moderate temperatures. Once a species-specific critical temperature (Tc) is reached, as temperatures continue to increase, rapid water loss occurs. The “lipid
Lipid
Lipids constitute a broad group of naturally occurring molecules that include fats, waxes, sterols, fat-soluble vitamins , monoglycerides, diglycerides, triglycerides, phospholipids, and others...
melting model” is used to explain this sudden increase in the rate of water loss. The lipid melting model states that increased cuticular water loss is directly related to the melting of surface lipids. Insects already adapted to more arid environments have a higher Tc, that is, their cuticular properties change and lipid structures melt at a higher critical temperature.
In some insects, the rate of cuticular water loss is controlled to some extent by the neuroendocrine system. Immediately following head removal, decapitated cockroaches exhibit a large increase in transpiration
Transpiration
Transpiration is a process similar to evaporation. It is a part of the water cycle, and it is the loss of water vapor from parts of plants , especially in leaves but also in stems, flowers and roots. Leaf surfaces are dotted with openings which are collectively called stomata, and in most plants...
across the cuticle, leading to severe dehydration. Injection of brain hormone
Hormone
A hormone is a chemical released by a cell or a gland in one part of the body that sends out messages that affect cells in other parts of the organism. Only a small amount of hormone is required to alter cell metabolism. In essence, it is a chemical messenger that transports a signal from one...
s into freshly separated bodies results in a sharp reduction in cuticular water loss.
Tracheae
In general, insects adapted to arid environments also have an impermeable cuticular membrane which prevents water loss. Therefore a majority of water lost to the atmosphere occurs via the air-filled tracheae. To help reduce water loss, many insects have outer coverings to their tracheae, or spiracleSpiracle
Spiracles are openings on the surface of some animals that usually lead to respiratory systems.-Vertebrates:The spiracle is a small hole behind each eye that opens to the mouth in some fishes. In the primitive jawless fish the first gill opening immediately behind the mouth is essentially similar...
s, which shut when open respiration
Respiration (physiology)
'In physiology, respiration is defined as the transport of oxygen from the outside air to the cells within tissues, and the transport of carbon dioxide in the opposite direction...
is unnecessary and prevent water from escaping. Insects at a greater risk for water loss face the challenge of either a depleted oxygen supply or desiccation, leading to an adaptive increase in tracheal volume in order to receive more oxygen.
Excretion
Following feeding, most insects retain enough water to completely hydrate their bodies, excreting the remainder. However, the amount of water excreted differs between species, and depends on the relative humidity and dryness of the environment. For example, Tsetse flies maintained at a high relative humidity, and thus non-arid conditions, excrete fecal matter with approximately 75% water content whereas Tsetse flies maintained at a low relative humidity, and thus dry conditions, excrete fecal matter with only 35% water content. This adaptation helps minimize water loss in unfavorable conditions and increase chances of survival.Tolerating greater water loss
Most insects can tolerate a 30-50% loss of body water; however insects adapted to dry environments can tolerate a 40-60% loss of body water. Initial body size also plays a large role in how much water loss can be tolerated, and in general, larger insects can tolerate a larger percentage of body water loss than smaller insects. The female beetle Alphitobius diaperinus, for example, is larger than its male counterpart and can thus tolerate 4% more water loss. It is hypothesized that larger insects have increased lipid reserves, preventing dehydration and desiccation.Behavior modification
In addition to physiological adaptations which increase desiccation resistance, behavioral responses of insects to arid environments significantly decrease dehydration potential. Drosophila melanogasterDrosophila melanogaster
Drosophila melanogaster is a species of Diptera, or the order of flies, in the family Drosophilidae. The species is known generally as the common fruit fly or vinegar fly. Starting from Charles W...
fruit flies, for example, will actively move to areas with higher atmospheric water content when placed in dry environments. Alternatively, the dung beetle
Dung beetle
Dung beetles are beetles that feed partly or exclusively on feces. All of these species belong to the superfamily Scarabaeoidea; most of them to the subfamilies Scarabaeinae and Aphodiinae of the family Scarabaeidae. This beetle can also be referred to as the scarab beetle. As most species of...
buries food in underground chambers, thereby ensuring water and energy sources during periodically dry conditions. Feeding location may also be altered to ensure body hydration. Some caterpillars preferentially feed on the underside of leaves, where microclimate has higher relative humidity. In a highly time-consuming activity such as feeding, these insects significantly reduce their chances of desiccation.
Cryptobiosis (Anhydrobiosis)
CryptobiosisCryptobiosis
Cryptobiosis is an ametabolic state of life entered by an organism in response to adverse environmental conditions such as desiccation, freezing, and oxygen deficiency. In the cryptobiotic state, all metabolic procedures stop, preventing reproduction, development, and repair...
refers to the state of an organism that has no detectable metabolic activity, resulting from extreme and unfavorable environmental conditions; anhydrobiosis refers to the state of surviving the loss of (almost) all body water. Although this state is commonly observed in invertebrates, only one insect is known to be cryptobiotic (anhydrobiotic), the African chironomid Polypedilum vanderplanki
Polypedilum vanderplanki
Polypedilum vanderplanki or the sleeping chironomid, is a dipteran in the family Chironomidae. It occurs in northern Nigeria and Uganda where its larvae grow in temporary pools of water that frequently dry out during the lifetime of P. vanderplanki larvae. Under these conditions, the larvae's body...
. Polypedilum vanderplanki undergoes anhydrobiosis, a cryptobiotic state wherein the body is completely dehydrated. The larvae of P. vanderplanki inhabit rock pools which commonly dry out completely. In response, P. vanderplanki larvae enter an anhydrobiotic state during which changes in body osmolarity trigger the production of large amounts of trehalose. Due to its capacity for water replacement and vitrification, the accumulation of trehalose prevents the death of the larvae from water loss.