Mammals are "warm-blooded" and develop at a constant
rate regardless of the environmental temperature,
because they are able to maintain an internal temperature
that allows their biochemical reactions to progress normally.
Insects, which are "exothermic" (the same temperature as
their environment; there is no such thing as "cold-blooded"),
do not generate body heat, and are therefore limited in their
development to periods of favorable external temperature.
Below a certain temperature, which varies among species,
the insect's biochemical reactions cannot proceed, and
development stops. This temperature is known as the
insect's developmental threshold ordevelopmental base.
By charting the ambient temperature, it is possible to keep
track of insect development, which is directly proportional to
the amount of time accumulated above the developmental
threshold (up to some maximum not often reached during
the season). We arbitrarily divide this time into heat units,
or degree-days (DD)