Respiration in insects is called direct because

1. the cells exchange O2/CO directly with the air in the tubes
2. the tissues exchange O2/CO2  directly with coelomic fluid
3. the tissues exchange O2/CO2 directly with the air outside through body surface
4. tracheal tubes exchange O2/CO2 directly with the haemocoel with tissues which then exchange with tissues

(a) Insects have a network of tubes (tracheal tubes) to transport atmospheric air with in the body. these openings lead to trachae. the cells exchange O2/CO2 directly with the air in the spiracles present on insect body.