 Dear students, in this topic, we shall discuss the temperature relations of heterotherms in detail. Heterotherms are the animals which are intermediate between pure ectotherms and endotherms. These animals do not generally regulate their body temperatures within a narrow range. They mostly depend on external heat source like ectotherms but are also capable of endothermic heat production due to their muscular activity. This feature is like endotherms. Such heterotherms include certain insects, certain reptiles and certain fishes. First we shall discuss the heterothermic insects which include some flying insects like locusts, beetles and arctic flies. They are strictly ectotherm when they are inactive but they can raise and regulate their temperature of the thoracic parts that is thoracic muscles before flight. Without this warm-up, the flight muscles contract slowly and produce insufficient power for flight. During flight, thoracic muscles keep on producing heat and maintain the temperature of these muscles at about 40 degrees centigrade. This level is the endothermic level of heat production and on this level, insects have to employ even heat dissipating mechanisms to prevent overheating. Thoracic flight muscles are considered to be the most active metabolically tissue. Now we shall discuss heterothermic reptiles which can produce muscle-generated heat. This heat generation is increased with declining ambient temperature. This heat is used to provide warmth for the group of eggs which are being brooded. This student now we shall discuss heterothermic fishes. Some fishes like a tuna or maco sharks can raise the temperature of their core bodies above the ambient temperatures through intensive metabolic activity. They have specializations for generating and retaining heat in some parts so they are regional heterotherms. For example, they can raise the temperature of their muscles or brain or eyes up to 10 degrees centigrade or more above the ambient temperatures. Dear student, these fishes have certain anatomical features which render them heterothermic. 1. The deeper location of aerobic swimming muscles in the core body. 2. They can retard the escape of heat through skin and gills by a counter-current heat exchange system. 3. This is used for heat conservation which makes them successful as a heterotherm.