For months, the Dow Jones Industrials (^DJI -0.11%) seemed invulnerable to every potential threat, weathering the end of the Federal Reserve's economic stimulus and largely ignoring rising conflict in Ukraine and Iraq. Yet what has turned out to be the Dow's Achilles' heel is the energy market, with crude oil's recent 40% plunge having led to triple-digit drops in the Dow on three separate occasions this week alone. As crude added to its declines today, falling below the $59 per-barrel level, stocks followed suit, with the Dow trading down 200 points as of 11:50 a.m. EST.

As you'd expect, the Dow's energy stocks weighed on the average, with both ExxonMobil (XOM 0.02%) and Chevron (CVX 0.44%) posting greater percentage declines than the Dow overall. The latest estimates from the International Energy Agency on global demand for crude oil included a downward revision of 230,000 barrels per day, suggesting that even more attractive oil prices for buyers won't be enough to stimulate further consumption. Despite their falling share prices today, both Chevron and Exxon are more insulated from oil-price declines than smaller oil and gas exploration and production companies, and those smaller players have seen their stocks drop much more dramatically in recent weeks as a consequence.

Nevertheless, other stocks with more indirect ties to the energy markets proved to be larger drags on the Dow than the oil majors. Caterpillar (CAT 0.07%), for instance, led the Dow's decliners with a 2.5% drop, hitting its lowest level since January. In response to the slowing Chinese economy and the resulting lack of demand for construction and mining equipment, Caterpillar started relying more heavily on the energy and transportation industry for equipment sales in recent years. During the energy boom, that was a prudent strategy that helped Caterpillar sustain its domestic revenue. But with the plunge in oil prices, investors fear that Caterpillar could see the same drop in sales to the energy industry that it experienced with its mining-equipment business several years ago, when gold and silver prices fell so sharply that it left mining companies without any available cash to make major capital expenditures on heavy equipment.

In the long run, a sustained drop in energy prices will have positive impacts for some businesses and negative impacts for others. Right now, most investors are focused on the potential fallout of dropping energy prices, as the potential benefits won't be realized until further in the future. Given the energy industry's contribution to the health of the U.S. economy, it's not unreasonable to have concerns about how changing conditions could negatively affect economic growth. Yet long-term investors need to stay aware not only of the potential pitfalls for some companies, but also of the lucrative opportunities that will arise for companies that are heavy users of energy products, as well as consumer-facing stocks that could see greater demand from an American public with more cash to spend.