An important feature of insects is that they can not change their overall size within the span of any one instar. Yet growth must occur. Air sacs, which are collapsible, allow for the growth of organs within the body. A third point also relates to body growth. Many insects appear to expand the tracheal system immediately after a molt, before new cuticle is completed.
Air sacs are also involved in thermoregulation in some insects.
In the dragonfly Aeschna air sacs in the thorax insulate the thorax from the abdomen. Similarly, abdominal air sacs in bumblebees thermally insulate the abdomen from the thorax. This is important in selective temperature regulation of the thorax during flight and of the abdomen during brood incubation.