How do ectothermic organisms regulate their body temperature?

Ectotherms employ various behavioral & physiological mechanisms to regulate their body temperatures. These mechanisms rely almost completely on the environment.

Ectotherms have no internal heat regulation mechanism like endotherms. Thus, they are heavily reliant on external heat sources to maintain their bodies at an optimum temperature for the body's functions.

These mechanisms can be classified into behaviour and physiological mechanisms.

1) Behavioral Mechanisms: To heat their bodies, ectotherms will often take advantage of a sunny day, and absorb its heat. They will especially opt to do this before or after energy requiring activities such as flying or swimming. To cool off they may take shelter from the heat, especially the hottest part of the day, by hiding under rocks, sitting in the shade or entering holes/caves.

This is why you see butterflies, reptiles, frogs, and other ectotherms bask in the sun with their body spread out to increase the surface area for more heat absorption. And when it's too hot, you see them hiding in the shade or near bodies of water.

Some animals exhibit group behavioral mechanisms. A good example is how honey bees huddle together in large groups to retain & generate heat. A similar example is how some gregarious caterpillars bask in the sun in large groups to cluster heat.

2) Physiological Mechanisms: These mechanisms function similarly but not identically to the mechanisms of heat regulation found in endotherms. They vary from molecular level mechanisms to organ level mechanisms, and body level mechanisms.

Molecular level example: Increase or decrease of cell phospholipid saturation to increase or decrease the melting point of the cell membrane and other cellular organelles.

Organ level examples: Heat exchange between the cold blood at the skin's surface with hot blood circulating from the body's core. Another example is the increased secretions of mucus on some amphibians' skins to cool the body by evaporation.

Body level example: Animals that engage in torpor (stillness, no activity) for different periods of time to conserve energy and heat. It can occur on a daily basis for short periods of time (lizards lie very still if you've ever observed them!) or up to several years (hibernation).