Further rise in temperature beyond optimum brings about decrease in metabolic rate, until it ceases at a temperature called maximum temperature thus, the favourable temperature range for any particular species is determined by the prevailing temperature at which normal physiological activities of the animals take place.
For example, rotifer Keratella procura is known to appear in the ponds of Pilani, Rajasthan (India), when the temperature is below 24rC and to disappear when temperature rises above 24°C. Its frequency of distribution reaches the peak during the months of October to March (Nayar, 1970).
The ability to withstand extremes in temperature varies widely among plants and animals, but there are temperatures above and below which no life can exist. A temperature of 52°C is about as high a temperature as any animal can withstand and still grow and multiply. However, larvae of chironomids and certain other Diptera are found to thrive at temperature near 55°C. Praying mantis is found to live on bare ground at a temperature of 62°C in deserts.
Likewise, among plants, some hot-spring algae can live in water as warm as 73°C under favourable conditions and some arctic algae can complete their life cycles in places where the temperature barely rise above 0°C.
Non-pathogenic bacteria inhabiting hot springs can actively grow at temperatures greater than 90°C Bott and Brock, 1969). Further, the eggs of the acanthocephalan Macracanthorhynchus hirudinaceus have been known to withstand temperatures from —10 to —45°C for about 140 days, and desiccation at temperatures upto 39°C for a period of 265 days.
Fig. 11.12. Effect of temperature on the metabolic rate.
The organisms which can tolerate very large fluctuations in temperature are called eurythermal organisms. For example, cyclops, toad, wall-lizard, grass-snake, man, etc., are the eurythermal animals. The organisms which can tolerate only a small variation in temperature are termed stenothermal organisms. The common stenothermal organisms are fishes, snails, coral reefs, etc.
In fact, most animals can toierate extreme temperatures only to a limited extent. Even with a very slow increase in temperature, there reaches an absolute upper lethal limit for the particular animal beyond which it cannot adjust to a further increase. Similarly, the absolute lower lethal limit is that point beyond which no further decrease in temperature is tolerated.
For example, when all organisms including insects are kept at unfavourable low temperature for a long time, they are killed. The death at the sub-zero temperature is caused by the freezing of the body tissues. The freezing point of most insects of cold regions is between — 10°C and —2°C when a cold-hardy insect is chilled below 0°C, its body do not freeze immediately but are super cooled beyond the freezing point.
Thus, cold-hardiness is the quality of an insect of a cold region to resist freezing at sub-zero temperatures at which tropical and subtropical insects would readily freeze and die. The tropical grain pests lack this capacity (undercooling, the capacity of cooling beyond the freezing temperature) and if subjected to low temperatures beyond 0°C, die without freezing.
The best example of cold-hardiness is furnished by mosquito larvae, which in Alaska normally pass the winter frozen in the ice of water pools and are known to tolerate repeated freezings and thawings. In these larvae about 90 per cent water freezes to ice at — 15°C. Many nematodes, rotifers, and tardigrades withstand cooling to — 272°C without ill effects.
Many lower organisms have been cooled in liquid to a temperature of — 183°C and in liquid helium to —269°C and have survived. In cold-hardy insects super cooling has some advantages. For example, Cephas survives super cooling to — 23°C to —30 C as compared with only —5 to —8°C by its less cold-hardy species.
During the super cooling of Bracon, an insectan parasite on sawflies to — 47-2°C, an increase in proportion of “bound” to “free” water was observed. According to Salt (1950) glycerol found normally in the haemolymph of insects like Bracon and Loxosiege plays an important role in increasing the super cooling. Glycerol lowers the freezing point of these cold- hardy insects and thus protect their tissues.
Vertebrates are limited in their tolerance of freezing. For example, arctic fishes do not survive total freezing but may withstand freezing of the body surfaces. However, deep sea fishes live in a permanent state of super cooling.
Upper limit of the range of lethal high temperatures varies from species to species and also within a species from season to season. The winter population would die at a much lower lethal higher temperature than the summer population. An insect with a cosmopolitan distribution will be more resistant to heat in those areas where the average temperatures remain higher.
A high body temperature is an adaptation to a warm climate. Many tropical mammals are genetically adapted to heat. So, heat-hardiness is the quality of an animal of the hot region to resist heating beyond its normal temperature without serious effects like heat shock (hyperpyrexia) or heat death. Desert insects and mammals (e.g., camel) are best examples of heat-hardy animals.