Organism, which present in tropical regions are called megatherms.

Temperature gradient over the earth's surface is $6.4-6.5^{\circ} C$ per $1000\, m$ altitude or $10^{\circ}$ latitude. Therefore, there is lowering of mean temperature from equator to poles. Tropical, sub-tropical, temperate and arctic organisms living in these zones are respectively called Megatherms, mesotherms, microtherms and hekistotherms