Answer: Insects have a circulatory system but it is an open one, which is why much emphasis is not laid on it compared to other vertebrates e.g in human when it comes to respiration
Explanation: The circulatory system in vertebrates unlike in insect is closed, meaning that blood is transported all over the body through a confined blood vessle, blood contains hemoglobin (red blood cell) which carries oxygen which aids in respiration, while in
Insects they only have hemolymph (like a blood) which doesn't contain hemoglobin this means that the circulatory system in insect doesn't aid in respiration which is why it is not included in the respiratory system of insect. Oxygen is rather delivered through the trachea of an insect.
