An opera includes other things besides recitatives and arias. It will usually have an instrumental prelude and may have other purely instrumental interludes such as ballets. There may be a chorus.
Recitative was invented by the Florentine camerata, but aria is forever for aria is song. The division into recitative and aria is the difference between action and reflection. L'Orfeo of Monteverdi is the first true opera because he instinctively understood the need to break up the action oriented recitative with other more traditional musical numbers like dances, ensembles and arias.
The balance and emphasis has changed throughout opera's 400 year history. The great Italian tradition has always emphasized the aria. The form might change--ostinato in the early phase, florid da capo throughout the long Neapolitan style, the slow section followed by fast section form of bel canto, the free form of Puccini--but the opera exists for the arias.
The balance shifts from generation to generation. In Handel's time the lyric expression of the reflective arias had expanded to virtually smother the action. Gluck was part of the generation that searched for simplicity in art, though this trend began first in France with Rousseau. Because this urge for naturalness and the romanticized rural was felt first in France, Gluck's return to the primacy of action over reflection in opera resonated much more with the French. Gluck tried to hide the distinction between recitative and aria but didn't actually succeed in doing away with it.
Wagner subsumed all to the gigantic orchestral tone poem with singing, a style that seems to create endless, formless arias with no speechlike elements.
The recitative and aria dichotomy is a part of the words and music discussion--prima la musica, etc. The purpose of the invention of recitative was to rebalance the relationship of words and music toward word dominance. Music must conform to the requirements of the text. In aria the priorities are reversed. The words guide the expression, but it is the music that determines the form and contour of the aria.
Wagner destroyed the distinction, indeed he destroyed the entire phenomenon of recitative. He eliminated the text repetitions that tell you you are in an aria. He inflated the orchestra to the point that it carries the music much more than the singers. Everything is extended. Everything is aria. Unrelenting continuity replaced alteration of action and reflection.
In modern times composers can't seem to bear the inflated orchestra of Wagner, the slowing of time that drags his operas into four and five hour performances but retain his formlessness in a much more shrunken form that is closer to constant recitative than aria. There are still no text repetitions, and nothing is extended.
Richard Strauss' through-composed operas while seeming to imitate Wagner, often include extended patter recitatives, as for example most of the role of Baron Ochs. Strauss is still distinguishing action and reflection, recitative and aria, but the transitions are extremely subtle. Strauss is one of the last composers who still knows how to compose an aria.
It is the inability of modern composers to compose true arias that dooms most modern operas.
It is curious to think of Glass' opera Orphée in this context. It's almost as if he set out to create a sound track for a beloved film, in this case Jean Cocteau's Orphée, since he is already familiar with the process of writing for the movies. The French version of the film runs 112 minutes, a length that feels very similar to the opera. Then Glass decided to set the voices to music. There are alterations in the pacing between the film and the opera, but the overall pace is constant. The text brings lyrical moments, but the expected extension of the moment into aria does not come. This is from Philip Glass of all people. The artist who most wants to alter our perception of time does not.
It is the lyrical vocal outburst that makes it opera. Modern opera fails because the moment of extension, the moment of greatest emotion where the glory of the voice takes us to another realm never comes.
Thus the rage about John Adams' lousy librettos. John Adams is one of the few who can actually write a decent aria. But what good is reflection without action?
If I were giving advice, I would say write an aria for your opera. Think of it as something that will stand on its own outside the opera. Think of it showing off the voice of your singer. Golijov's best work is written for Dawn Upshaw. Aim high.