Adaptations of Insects to Subzero Temperatures

Abstract

Insects as a group have been especially successful in adapting to subzero temperatures. Typically, an integration of behavioral and developmental adaptations, as well as physiological and biochemical ones, are required to achieve overwintering success. In keeping with the tremendous adaptive radiation of the group, individual species often exhibit considerable variation in the particular set of adaptations by which they achieve the ability to survive subzero temperatures. Two fundamental physiological mechanisms are (1) freeze tolerance, adaptations that confer the abilty to survive extracellular ice formation, and (2) freeze avoidance (freeze resistance), adaptations that prevent freezing. Different populations of the same species, however, may exhibit different mechanisms, and even the same population may vary its overwintering mechanism from year to year. Feeze-avoidance adaptations usually involve production of antifreezes. These may be antifreeze proteins or colligative-type antifreezes, such as glycerol, which are often produced in molar concentrations. Antifreezes function not only to depress the freezing point, but also to extend the ability to supercool, an important factor since most species are protected by the cuticle from inoculation by external ice. Supercooling may also be extended by the removal of ice nucleators from the body fluids, either on an evolutionary-time scale or on a seasonal basis. Freeze-tolerance adaptations are likewise quite diverse. Most freeze-tolerant insects accumulate high levels of cryoprotectants, usually polyols (i.e., glycerol and sorbitol) and sugars (i.e., trehalose). The potential role of less traditional cryoprotectants, such as amino acids and methylamines, has not been properly investigated. Many freze-tolerant species produce extracellular ice nucleators, typically hemolymph proteins, which induce nucleation at fairly high subzero temperatures and thereby inhibit lethal intracellular ice formation. Some freeze-tolerant insects, however, have removed ice nucleators completely and supercool to - 50 to - 60circC, while other species require inoculative feezing by external ice at temperatures only a few degrees below zero. Other adaptations may involve an increase in unfreezeable water, recrystallization inhibition by antifreeze proteins, or vitrification (glass formation) of certain pools of body water. In spite of considerable progress in understanding subzero temperature adaptations in insects, it is certain that crucial adaptations remain unidentified, and a primary goal of future studies must be to elucidate these mechanisms. Emphasis should also be placed on providing an integrated understanding of the overall complement of adaptations used by particular species and how these adaptations impact, and are impacted by, the ecology of the organism. The great diversity of adaptations exhibited by different species should also be further investigated.