The stigma

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Stigmas, or spiracles, are the 10 pairs of respiratory openings located on the two lateral walls of the thorax (3 pairs) and the abdomen (7 pairs). They pass through the pleurites* of the exoskeleton and branch into tubules or tracheae, allowing O₂/CO₂ gas exchange to occur directly at the tissue level, without diffusion through the hemolymph.

In humans, O₂ is transported from the lungs to the tissues by red blood cells circulating in blood vessels. The mechanics of inhalation and exhalation can be directly observed when a bee is motionless on the landing board: its abdomen moves back and forth like a piston, recalling the inspiratory–expiratory movements of the human thorax.

*The tergite (or notum) is the dorsal arch of each abdominal segment;
  the pleurite (or pleuron) is located on the side;
  the sternite (or sternum) corresponds to the ventral part.

These plates are articulated with one another by an interscleritic joint membrane.

The bee larva also has 10 pairs of spiracles, but gas exchange does not benefit from a ventilatory mechanism as in the imago and occurs solely by passive diffusion.

For further reading :

► Internal anatomy: general aspects