OPEN CIRCULATORY SYSTEM: the blood isn't always contained in vessels; it circulates through the body cavity and bathes tissues and cells.
The action of body muslces during movement may help circulate the blood.
A long muscular tube (pumping organ) under the dorsal surface of the insect may circulate the blood.
Blood from the body enters the heart via ostia and the heart pumps the blood to the head by peristalsis and the blood pours out of the heart into the body cavity towards the insects head.
Some, larger / more active, insects have open ended tubes attached to the heart that direct blood towards more active parts of the body such as the legs or wing muscles.
WHY DON'T ALL ANIMALS HAVE OPEN CIRCULATORY SYSTEMS?
Insects are small, the blood doesn't have to travel far.
Insect blood doesn't transport O2 or CO2 unlike the blood of larger organisms.
In an open circulatory system the blood travels at low pressure, flow is slow, this isn't sufficient to supply muscles in large/active organisms, many parts of the body wouldn't recieve sufficient O2…