Insects outnumber humans by quadrillions, compete with them for food, cause damaging that costs U.S. agriculture billions of dollars a year and prompt use of long-life pesticides that area a major environmental problem.

Yet most humans do not know their formicids (ants) from their isopterans (termites).

For those around the country who want to know what is bugging them, the official word comes from a group of government insect detectives who work in cramped offices at 10th Street and Constitution Avenue NW, surrounded by 58,000 drawers containing 24.5 million insect specimens. So far.

Hunched over microscopes trained on intimate bug parts, these entomologists, employed by the Department of Agriculture, each year between one-third and one-quarter of a million insects sent to them by farmers, plant inspectors, scientists and random citizens.

Their clients might include a cotton grower in California who wants to know if he has boll worm problem. The entomologists' answer can mean millions of dollars for the grower in quarantine measures.

An FBI lab man may want to know how long a corpse has been dead. Entomologists figure it out based on the successive cycles of insects that attack decaying matter.

They also identify insect fragments in food for the Food and Drug Administration, and frequently they receive "urgent" requests, they say, from agricultural port inspectors for word on whether the stowaways in a cargo of fruit flies the cargo must be destroyed.

The entomologists play a game of awesome numbers, not only in terms of the dollars at stake but in terms of what is known and knowable about insects, according to Dr. Ronald W. Hodges, chief of the entomology lab.

"There are perhaps 700 kinds of birds in the area so that you could take a book, go into the field and become fairly competent," he said. "But there are more than 50,000 kinds of insects described from North America north of Mexico alone . . . and much information about insects is not in any book yet.

Entomologists throughout the world have identified about 1 million kinds of insects. Hodges said scientists expect eventually to identify 10 million species.

Since most insects go through four distinct life cycle stages each insect virtually has four different identities.

That results in 4 million different forms to distinguish, and more are being discovered every day, Hodges said.

Hodges and the others spend much of their time peering at slides of insects sex organs. The genitalia provide some of the most valuable clues in this peculiar kind of sleuthing, he said.

"The sex organs are, speaking very roughly, comparable to the human hand in their complexity and in the fact that they can manipulate. Because of this differentiated from one species to another than, say, a wing pattern or other features, and so we are able to identify the animals," Hodges said.

Unknown to tourists who pass nearby, all but four of the entomologists are tucked away in a backwater of offices in the Smithsonian Institution Museum of Natural History.In the museum rotunda stands the famous stuffed African bush elephant - "largest animal of the modern world." For lack of space at the museum, four of the entomologists work at Agriculture Department offices in Beltsville.

They work with and add to the National Collection of Insects, the largest such collection in the world. Its 24.5 million specimens are increasing at a rate of about 500,000 specimens each year, according to Hodges, and it's a big job just keeping them in (what else?) moth balls.

The collection enables scientists to classify insects, providing the basis for identifying them.

The collection and insect studies were born in the 1880s, Hodges said, because farmers needed to know what was eating their crops. The organization "still has a strong bias toward biological, rather than chemical, control of insects," he said. Biological control uses insects and other organisms to feed on other, unwanted insects.

Chemicals tend to create insect populations resistant to chemicals, he said, adding that the agency is working more toward integrating the two methods, in order to minimize the need for spraying chemicals.

The entomologists are "a breed apart," who actually enjoy looking at those crawly things all day, aciohding to an Agriculture Department spokesman who has worked with them.

One of the entomologists retired from the USDA more than 20 years ago. Now 83, Carl F. W. Muesebeck continues to arrive at his desk in the museum every day at 6 a.m. He works until 1 p.m., seven days a week.

"This is work I want very much to finish before I die," he said. The work involves classification of groups of small parastic insects to be used in biological control of insects injurious to man.

Lab chief Hodges says he is amazed that he can be paid for doing something he enjoys so much. As a boy in Michigan he collected butteflies and moths. In college, he stuck with insects against the advice of his professors. "They told me there would be no way to earn a living at it," he said.

Among his duties, Hodges is working with several other entomologists on a manual about moths, his particular specialty. The manual pictures and describes only moth species found in North America north of Mexico. He expects the project to take 25 or 30 more years and eventually fill 120 to 150 volumes.

He picked up one of the few completed volumes and leafed through page after page of green-hued wings that, to a layman's eye, looked very much alike. "Now someone can pick this up and start from there and go further," he said. "If I die, I've at least wrapped this bit of information."