On the back of their worst week since November, U.S. stocks are looking listless today, with the S&P 500 (INDEX: ^GSPC) and the narrower, price-weighted Dow Jones Industrial Average (INDEX: ^DJI) down 0.15% and 0.24%, respectively, at 10:05 a.m. EDT.

Earnings: The week ahead
Each day of this week will see one Dow component report its results, beginning with Caterpillar (NYSE: CAT) today (see below), followed by AT&T, Procter & Gamble, ExxonMobil, and Chevron. Also note that Apple -- oddly, not part of the Dow -- reports tomorrow.

Caterpillar: When you're in a hole, stop digging
Dow component Caterpillar reported its first-quarter results before the opening bell this morning, and while the shares are responding well, up 0.3%, the numbers are hardly encouraging for the broad market. As the Financial Times' James Mackintosh neatly summed up in a tweet:

Caterpillar is the world's largest manufacturer of construction and mining equipment. Due to the nature of its customer base, the company's health is a barometer of global economic growth. In the first quarter, Caterpillar earned $1.31 per share, falling short of the consensus earnings-per-share estimate of $1.40. Revenue of $13.2 billion came in below the $13.7 billion estimate. Worse yet, the company indicated that full-year EPS for 2013 would be at the bottom end of the $7 to $9 range it had previously provided. Through last Friday, Caterpillar is the worst-performing stock in the Dow, having badly lagged the broad market during that period:

CAT Chart

CAT data by YCharts.

Caterpillar blamed a decline in demand for its mining equipment for the first-quarter miss. With the prices of multiple metals (both precious and industrial) having taken a fall since the end of the first quarter, that trend may well continue. While those factors are specific to Caterpillar, I believe the company is representative of the S&P 500 in one critical respect: Earnings estimates will have to come down. Anyone who thinks operating EPS for the S&P 500 will rise 13% this year is living in cloud cuckoo land.

Despite its troubles, Caterpillar is still the market share leader in an industry in which size matters, and its quality products, extensive service network, and unparalleled brand strength combine to give it solid competitive advantages. Read all about Caterpillar's strengths and weaknesses in The Motley Fool's brand-new report. Just click here to access it now.

Fool contributor Alex Dumortier, CFA has no position in any stocks mentioned; you can follow him on LinkedIn. The Motley Fool has no position in any of the stocks mentioned. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.