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This (how the pupa gets O2) is a pretty basic anatomical question that MUST have been looked into.

Respiration. The last larvae of Odonata, Plecoptera and Megaloptera always leave water prior to the larval/adult or larval/pupal ecdysis and at the same time they start breathing with an open (usually hemipneuistic) tracheal system. Moreover, a layer of air is eventually distributed below the apolysed larval cuticle and, consequently, there is no need for the already terrestrial pharate adults or pupae to use larval gills for respiration. Another situation obtains in the Trichoptera. There the pharate adult is fully aquatic and has to be considerably active while cutting its way out of the pupal case or cell and swimming to the surface where eclosion takes place. Although a layer of air is then already present between the pupal cuticle and the cuticle of the pharate adult which is already capable of breathing with open spiracles, it is possible that diffusion through the fine cuticle of pupal/pharate gills provides an additional and for some species necessary source of oxygen for the short period of intensive imaginal aquatic activity.

the fine cuticle of pupal/pharate gills provides

I can imagine, in some otherworld
Primeval-dumb, far back
In that most awful stillness, that only gasped and hummed,
Humming-birds raced down the avenues.

Before anything had a soul,
While life was a heave of matter, half inanimate,
This little bit chipped off in brilliance
And went whizzing through the slow, vast, succulent stems.

I believe there were no flowers then,
In the world where humming-birds flashed ahead of creation.
I believe he pierced the slow vegetable veins with his long beak.

Probably he was big
As mosses, and little lizards, they say, were once big.
Probably he was a jabbing, terrifying monster.

We look at him through the wrong end of the long telescope of time,
Luckily for us.

The insect respiratory system consists of simple tubes that penetrate the body and branch into smaller tubules, which allows only for diffusion of air into the body to major organs.

Gonzo, I've seen a few wasps "pumping" their abdomens on occasion, usually on very hot days, but I don't think most insects have much means of forcing air into their tracheal system in anyway near the same way that creatures with internal, muscle-walled lungs have.

Recently, rapid cycles of tracheal compression and expansion have been observed in some insects using X-ray videoing. Movements of the hemolymph and body could not explain these cycles, which appear to be a new mechanism of gas exchange in insects. In addition, large or dialated tracheae may serve as an oxygen reserve when spiracles are closed. In very active insects, especially large ones, active pumping movements of the thorax and/or abdomen ventilate (pump air through) the outer parts of the tracheal system and so the diffusion pathway to the tissues is reduced. Rhythmic thoracic movements and/or dorsi-ventral flattening or telescoping of the abdomen expels air, via the spiracles, from extensible or partially compressible tracheae or from air sacs. Co-ordinated opening and closing of the spiracles usually accompanies ventilatory movements and provides the basis for unidirectional air flow that occurs in the main tracheae of larger insects. Anterior spiracles open during inspiration and posterior ones open during expiration. The presence of air sacs, especially if large or extensive, facilitates ventilation by increasing the volume of tidal air that can be changed as a result of ventilatory movements.