Torpor

A torpid animal is dormant: this could be anything from an overwinter hibernation to a temporary overnight slowing of their bodies. Torpor helps animals to survive periods of cold temperatures by conserving their energy.

Butterflies and other insects respond to cold weather by slowing down their heartbeat and respiration thus expending less energy to keep themselves alive in a period where they cannot obtain nutrients and when it is more difficult for them to become warm.

Cold-blooded is an outdated and somewhat misleading term. The current accepted terminology is poikilotherms which means "of varying temperature". This refers to creatures with an internal temperature that varies but often matches the ambient temperature outside of their bodies. Butterflies  can warm their flight muscles by vibrating their wings in place, giving them some control over their internal temperatures.

When it is cold,  insects caught outside will reach this state of torpor and remain remarkably still until temperatures rise. On cool mornings you can wander about a garden and see all sorts of creatures in this state of rest and get close enough to inspect their delicate details.

Photo: This fantastic picture of a frozen ladybug was taken by my friend Dwight Eric Cox in Chattanooga Tenn.