Haemoglobin carries oxygen in humans, but in insects haemoglobin does not exist. Then how does the flow of oxygen happen?

Answer 1: 

Image source: en.wikipedia.org

Blood vascular system in insects is open type, consisting of a large cavity called Haemocoel. Haemocoel is filled with blood, in which all the organs bathe. The blood is white in colour due to the absence of the respiratory pigment `haemoglobin'. Its absence is compensated by a special system called `tracheal system'.Tracheal system consists of small white shining tubes called `trachea'. They open to the exterior through a pair of openings called `spiracles' in every segment of thorax and abdomen. These spiracles are valved. The trachea divides and re-divides to form fine tubules which end as `tracheole cell' near the tissues. Tracheole cell has distal tube like projections called the `tracholes', which innervate the tissues. Tracheole cell is filled with tracheolar fluid (mainly consisting of water) and plays a vital role in respiration.In insects the oxygen from environment is directly carried to tissues.Oxygen continuously enters the trachea passively through thoracic spiracles, but carbon-dioxide is pumped out actively at regular intervals through abdominal spiracles.

Answer 2: 

Water in equilibrium with atmospheric air at 37{+o}C dissolves about 0.46 volumes per cent of oxygen and this is adequate only for tissues with low rates of metabolism. In arthropods, to which insects belong, there are no small blood vessels or capillaries connecting the arteries with veins. Nets of minute sinuses are found. Hence the circulating fluids bathe the major organs and tissues. Tracheal system, characteristic of insects helps in exchange of gases. The air from outside enters the tracheal tubes by the stigmata and conveyed to the tissues directly. The alternate contraction and relaxation of the insect abdomen brings the ingress and egress of the air. Exchange of gases takes place directly without the involvement of blood or blood pigments.

Source: thehindu.com