Music Composers Unite!
I see that this question has been treated rather in the vertical sense, however working horizontally divisi can be useful to make a difficult-to-play line easier to execute by the entire section, sometimes done with a common note overlap. Even though it might seem that only half the players are playing at any one time what they do play can be played with more weight
Richard Strauss is a great example to follow. Some of his tone poems have the best textures of strings; Also Sprach Zaratustra, Don Quixote, and Ein Heldenleben come to mind. There are portions that may divide the strings about twenty ways. Debussy also has sections in his Nocturnes or Images Pour Orchestre.
When analyzing a selection from Zarathustra, Strauss starts the passage with six soli; three celli, two viole, and a violin. He then very subtly adds the concert master's inside player, the section leader of the seconds, and a fourth cello all within about two measures. A few measures after that, Strauss has a total of ten parts while using less than half of the string sections. By the time that he adds everyone else at the buildup, he has somewhere around fifteen individual lines of strings. But as the crescendo nears the climax, he gathers the parts together to make them stronger at the cadence because he adds all six horns in there. Strauss does have the organ accompanying the strings the whole time and is essentially playing a reduction of all of the parts. This is probably the best way to add some color to the strings without rending their delicate web of texture when they are so thinly divided.
The thing with dividing the strings as much as Strauss did there, is that the more you divide them the more delicate the texture becomes. You will also notice a trend in pieces with a big orchestra that at the tutti sections or any dynamic marking above forte, all of the violins may share a single line, or the viole will join the seconds with the firsts an octave above, of course.
To answer the other question, every string section can benefit from divisi. When you have a large orchestra (16-16-12-12-8) you can use divisi to actually make a smaller orchestra within your normal orchestra, almost like a concertino in a concerto grosso.
Much so Travis, as regards Richard Strauss. In Metamorphosen, for 23 string players - he really was exploring all of the sonic, linear, and vertical potentials of a virtual constant divisi setup - somewhere between a traditional chamber and symphonic ensemble. So this really ties in with what you have said as regards the detail of string writing in Thus Sprach Zarathustra.
If anyone is inclined, I would much appreciate some help with the problem of string divisi on my own massive score - Ulysses, the 1st. Movement to Symphony in Indigo. I have only prepared this work as an audio score, thus far, and have yet to detail a realistic play score - part of which entails figuring out the string densities and divisis in this very dense score.
Anyone interested can listen to the audio at the following youtube site, because it won't fit anywhere here, and I can provide a PDF or Sibelius file from there:
Thanks and best wishes to you all,
i would suggest listening to Vaughan Williams' 5th Symphony, specifically the 3rd movement "Romanza". There is prolific use of divisi strings - in fact the opening utilises them to create a nice thick texture.
I'm not sure if it's too late to help, but anyone interested in divisi strings might find it interesting to study the full scores for some of Scriabin's orchestral music: particularly from the Symphony no. 3 ("The Divine Poem") onwards. Some pages in this, and also in "The Poem of Ecstasy" and "Prometheus: The Poem of Fire", show most elaborate structures based on very fine division of strings. Often, in certain passages, usually soft, and with a strong sense of atmosphere, he tends to have strings sections divided into 3 or 4 parts each, or sometimes even more, and some of the parts sustain notes of slowly moving harmony, while other parts have measured tremolos on pairs of notes duplicating the sustained notes. The tremolos tend to be slow in the lower parts of the chord - like just quavers, for instance - then they gradually get faster as you go up: triplets of quavers, semiquavers, triplets of semiquavers - and so on. Sometimes there are solo parts like a single violin playing an expressive melodic fragment combined with such passages. Sometimes the melody or counter-melody (Scriabin uses lots of counter-melodies in a kind of counterpoint) may be taken by one of the string groups, either not divided or divided less finely - or sometimes the melody or counter-melody are taken by other instruments.
Scriabin's music tends to follow the practice that I think most composers follow that finely-divided strings are used mainly in softer passages where rich harmony is wanted and few of the parts are of melodic or contrapuntal significance, and the parts tend to remain undivided in loud or vigorous passages where melodic continuity of the parts or strong counterpoint is more to the fore. In those cases, if rich harmony is desired, it is entrusted to other sections of the orchestra.
The first two symphonies include simpler examples of this sort of thing, but it's really from the 3rd Symphony that it gets very elaborate.
Some of Szymanowski's music also uses similar techniques, as well as composers like Liadov (the particular string-dividing method I mentioned seems to be a Russian thing of the late-19th and early-20th centuries).
Some of Schoenberg's early romantic music is also worth studying for such divisi techniques: e.g., "Transfigured Night", "Gurre-Lieder", or "Pelleas and Melisande". Not to mention Wagner's operas, particularly the later ones - which I get the impression were the original inspiration for this whole ultra-lush style of orchestration I have been talking about.
Some of Respighi's works are also interesting from this point of view: e.g., "Pines of Rome", "Fountains of Rome", "Roman Festivals", "Brazilian Impressions", and probably many other works too.
Almost all the works I've mentioned can be found on the I.M.S.L.P. web site (except some Szymanowski works - I think only the Symphony no. 4 ("Sinfonia Concertante") by him is there). Address is http://www.imslp.org .
I hope this helps a little.