Spiders just got cheaper.

The annual expenses on the popular Standard & Poor's Depositary Receipts (SPY:Amex - news) -- commonly known as Spiders -- have been reduced to a maximum 0.12%, or 12 basis points, from the current 0.18%.

The move by State Street Bank & Trust, trustee for the Spiders, appears to be a pre-emptive strike aimed at Barclays Global Investors.

In a matter of weeks, Barclays is expected to introduce a slew of its own exchange-traded funds, one of which will compete directly with the Spider.

Barclays must come up with a way to attract attention to its own rival products, and one area of battle will obviously be cost. Barclays, however, has yet to disclose the expenses on its upcoming exchange-traded funds.

State Street is capping the expenses at 12 basis points for two years, according to a filing with the Securities and Exchange Commission.

"It could go lower," says Gus Fleites, head of exchange-traded products at State Street Global Advisors. "We're just ensuring it's not going to go higher."

As part of the move, State Street cut its trustee fees, a major component of the portfolio's expenses.

Spiders track the benchmark S&P 500 index, like many index mutual funds, but shares trade just like stocks.