Phenology |Morphology | Behavior| Physiology

The most severe problems arthropods living in high elevations have to face are cold temperature, large fluctuations in microclimate and the atmospheric aridity which increases with altitude. Snowcover, however, is much appreciated by arthropods since it protects them from cold and desiccation. Adaptations or strategies evolved to survive in this harsh environment can be classified as physiological, phenological, behavioural or morphological. Any adaptations usually occur in combination.
 

Phenological adaptations:

Extended development to more than one year. While most temperate and tropical terrestrial arthropods have one or more generations per year, alpine species often require two or more years to complete their life cycles. The difficulty here is, that unlike univoltine species where cold hardiness is normally restricted to a single instar (egg, one larval instar, pupa or adult), in species with elongated development more than one instar has to have this ability.

Arthropods with univoltine life cycles show adaptations like increased development rate or a reduced number of instars.
 

Morphological adaptations:

Reducing body size. Small size makes it easier to seek shelter in microhabitats and may be an adaptation to reduced food resources as well. Furthermore, compact bodies may favour conservation of moisture in windy environments.

Melanism. There is a general tendency for insects to be darker at higher altitudes than in the lowland. The advantage of darker colours is probably to raise body temperature by absorption of heat. But, especially in larger insects, melanism may also involve the risk of overheating. Melanism can further be involved in camouflage and protection against harmful UV-radiation. In carabids, red colours often replace the black. Rufinism fulfils the same function as melanism. Furthermore, red pigments are able to absorb more infrared radiation.

1 - Melanistic scree grasshopper (Russalpia albertisi) (Photo: K. Green; 117K)

2 - Wingless alpine grasshopper (Miramella alpina) (Photo: B. Knoflach-Thaler; 115K)

Wing antropy. About 60% of insects above the treeline are wingless or short-winged. Many alpine species live in litter and dense vegetation, where wing antropy enhances movement. But the most important selection pressure for wing antropy is the wind. Flying insects are easily taken by the wind and the loss of the ability to fly will reduce the risk of being carried away to unsuitable habitats.

3 - Wingless snow-fly (Chionea sp.) (Photo: B. Knoflach-Thaler; 99K)

Pubescence. Increased density of hairs and bristles presumably give better insulation at higher altitudes.

4 - Hairy bagworm moth (Psychida sp.) (Photo: B. Knoflach-Thaler; 113K)

Behavioural adaptations:

Cold avoidance. The choice of favourable microhabitats is of great importance for terrestrial alpine arthropods. They find shelter under rocks, in vegetation, in the soil or may migrate to lower altitudes in winter.

Sun basking. Exposing the body to sunlight to be heated by radiation is a common behaviour in alpine insects. Especially rock surfaces and the corolla of flowers are often frequented to bask in the sun.

Nocturnal activity. The adaptive advantage of nocturnal activity may be to reduce the risk of overheating, to maintain water balance and to avoid predators (since birds as the main predators are particularly diurnal). Predator species like many aranids are nocturnal because their major food source, the arthropod fallout carried by winds from lower elevations,is higher at night than in daytime. The non-adapted prey species are immobilized by the cold of the snow while predators are able to walk around and eat them.
 

Physiological adaptations:

Regarding their cold hardiness, terrestrial arthropods usually fall into one of two categories: Freeze tolerant species, that can survive the formation of ice within their tissue and freeze susceptible species, that depend on their ability to avoid ice formation by supercooling. In all species, whether freeze tolerant or freeze susceptible, there is a lower temperature limit and the duration of exposure is important for survival.

Supercooling capacity. Supercooling is known in most terrestrial arthropod orders. Two mechanisms are important in lowering the minimum temperature to survive without freezing (=supercooling point). Cessation of feeding and clearing the gut to avoid ice nucleating agents in the body, and the accumulation of polyols, sugars and antifreeze proteins. A high concentration of polyols and sugars (mainly glycerol) lowers the supercooling point because water molecules are bound to the surface of glycerol molecules and therefore less water is available for ice nucleation. Antifreeze proteins stabilize the supercooled state by adhering to the surface of seedling ice crystals and inhibiting further growth.

5 - Spiders (Pardosa oreophila) often show high supercooling capacities. (Photo: B. Knoflach-Thaler; 125K)

Freeze tolerance. Freezing may take place within the cells or in the surrounding hemolymph. Intracellular freezing is usually lethal because metabolic processes are disturbed and osmotic swelling may damage the cytoskeleton and cell membranes. In contrast to freeze susceptible species, for freeze tolerant species it is important to have ice nucleating agents in their hemolymph to avoid intracellular freezing. Glycerol may also depress the lower lethal temperature by reducing the amount of ice formed in the tissue.

5 - Distribution of ice nucleating agents (INAs) in body fluids of freeze-susceptible and freeze-tolerant species.

During the summer, INAs are present in the gut or other internal compartments (open symbols) of both categories. In winter, freeze-susceptible arthropods are free of INAs, while many freeze-tolerant species have INAs in their hemolymph (closed symbols). (Graph after Zachariassen)

Anaerobiosis. In several alpine habitats the upper layer of the soil freezes to a compact ice cover. Overwintering arthropods can become enclosed in ice so that an oxygen deficiency arises. Some beetles have been shown to survive more than six months enclosed in ice at temperatures slightly below 0°C. As an end product of anaerobic metabolism, lactate is accumulated.

Resistance to dessication. To avoid dessication, many arthropods seek shelter in protected microhabitats such as under rocks or in vegetation. But there are also species that survive in the open. The resistance to dessication of some grasshoppers is comparable to that of desert insects.

Sømme L (1989) Adaptation of terrestrial arthropods to the alpine environment. Biol Rev 64:367-407.

29 August 2011
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