Been wondering about these for a while. Student pilot working on my PPL. When preflighting my Cessna 172 the other day, I noticed these holes just in front of the elevator. What are they for? The image shows the crease between the elevator (white) and the horizontal stabilizer (red) on the left side of the fuselage. 
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14$\begingroup$ Speed holes. They make the plane go faster. $\endgroup$– NijSep 19, 2016 at 4:23
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2 Answers
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Those are called "Lightening Holes". The name refers to the weight reduction brought about by removing part of the material—lightening—and should not be confused with anything to do with electrical discharge—lightning.
From the FAA's AMT Airframe Handbook, in Chapter 4, page 4-82:
Lightening holes are cut in rib sections, fuselage frames, and other structural parts to decrease weight. To avoid weakening the member by removal of the material, flanges are often pressed around the holes to strengthen the area from which the material was removed.
The particular part depicted in your photo is the left hand rear spar assembly for the horizontal stabilizer, No. 32 in this figure from the 1975 Cessna 172 Illustrated Parts Catalogue (IPC):
As you can see, these lightening holes are present throughout much of the spar and rib structures in the horizontal stabilizer, as well as the vertical stabilizer, wings, and fuselage.
In addition to the gains in weight reduction, lightening holes also serve as openings to allow the routing of electrical wiring, control cables, hydraulic tubing, and other system components. If you look at the same location on the right side of the horizontal stabilizer, you should see the elevator jack screw passing through one of the lightening holes, in the location marked "C" in the Cessna IPC figure above.
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1$\begingroup$ And from what I was told they provide an excellent nesting place for small birds $\endgroup$– TomMcWSep 19, 2016 at 3:23
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4$\begingroup$ At first I thought "lightening" was a typo for "lightning", and was confused. $\endgroup$ Sep 19, 2016 at 3:42
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1$\begingroup$ @Lyall Yes, this general technique pre-dates aviation by a good margin and is found in many vehicular technologies. $\endgroup$– J WSep 19, 2016 at 15:53
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2$\begingroup$ Note that in beams, the majority of force is concentrated along the top (in compression, assuming it's holding something up) and bottom (in tension). This is why I-beams, H-beams, I-joists, etc, are all shaped the way they are. $\endgroup$– Nick TSep 19, 2016 at 21:20
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That is one of the horizontal stabiliser spars that holds the plating together. It has holes in it so that it would be strong and still lightweight.
answered Sep 19, 2016 at 1:45
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$\begingroup$ Interesting, thanks! I thought it might have something to do with weight, but they don't go into the innards of a 172 in Jeppeson! Any chance you'd have more documentation on that? $\endgroup$ Sep 19, 2016 at 1:47
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2$\begingroup$ The technical name is
lightening holes. The nomenclature is due to the weight reduction, not to be confused with electrical discharge. $\endgroup$– J WSep 19, 2016 at 1:48 -
$\begingroup$ Well, I found this video about repairing the tail spar of a C172. $\endgroup$ Sep 19, 2016 at 1:53