"Terrestrial fishing. O.K., don't tell me. That must mean fishing on the Earth," said the nonfishermen with a laugh.

Close, but not quite. Terrestrial fishing is angling with imitations of land-based insects such as crickets, ants, bettles and the like, as opposed to aquatic insects.

It is a field of fly fishing that has blossomed in recent years, and now in the August heat we are in the prime terrestrial fishing season.

Alfred Ronalds in his classic Fly Fisher's Entomology (1837) gave dressings for many important land insects. After Ronalds, terrestrials took a back seat in fly fishing theory until 1950, when Vince Marinaro's classic Modern Dry Fly Code was published.

The code gradually has gained the reputation as the most significant American fly fishing book of the 20th century. Though much of Marino's work was devoted to aquatic insects, it is his contribution to terrestrial fishing, with the assistance of Charlie Fox, that is most important. Not only did these two innovative anglers bring out the predominance of these insects in the diet of trout they also described new and potent fly patterns such as the jassid, cinnamon ant an Japanese beetle.

Marino's work focused primarily on the gentle little Letort Spring Run in the Clumberland Valley of Pennsylvania. On this stream the insect-laden measows bristle with leafhoppers, crickets, ants, grasshoppers, true bugs and bettles, offering a constant supply of food for hungry brown and rainbow trout. The spring creeks of this region, easily reached from Washington on a two-hour drive, offer extraordinary terrestrial fishing sport.

One of the unfortunate side effects of Marino's concentration on the spring creeks, however, was that many readers erroneously assumed these flies were only effective on such meadow limestone waters.

Actually, terrestrial often represent a more important source of food on freestone trout streams than on spring creeks. The limestone waters typically offer an abundance of subsurface fare such as scuds, cress bugs and fresh water shrimp. Acidic freestone streams lack this richness of aquatic life. If anything, the fish in the mountain waters feed more avidly on the land insects than their limestone stream counterparts.

Both meadow and forest streches of trout streams support large numbers of land insects, some of which end up in the water. While meadows harbor greater numbers of land insects, forest waters offer a more diverse collection of terrestrial insects to lurking trout.

Some of the best terrestrial patterns for area waters include the cinnamon ant (No! 16-28), black ant (No. 14-20), crowe beetle (No. 10-16), jassid or leafhopper (No. 18-24), Letort hopper (No. 10-16). Inchworms are particularly devastating in sized No. 8-12, but the season for these larvae is about over for this year. Look for them on wooded streams next June.

One thing is obvious from the flies recommended above: terrestrial come in a wide variety of sized, from 28s up to 10s and even bigger. Fishing these flies falls into two basic methods.

The smaller offerings, No. 18 and smaller, are usually best delivered delicately and allowed to float drag-free over the trout. Some of the larger, denser flies, such as the bettles and crickets, can be cast hard, with a distinct "splat" on the water to bring savage strikes from alerted trout.

This is not sloppy fishing, though it may sound like it. The angler is imitating the natural entrace of such big, cumbersome bugs into the stream. They land with a plop in most cases, and it is the splash that draws the trout's attention.

Line and leader should not make a commotion on the cast - the fly should. The best way to do this is to use a dense bug - one made of cork or deer hair, for example - and stop your cast high above the water. The fly line will come down softly and the bug will fall with a subtle splat.

Stealth is vital to tessestrail fishing. Many of the heaviest trout in the stream wll be lying adjacent to the shortline, waiting for land insects to tumble or be blown into the water. The stream is usually shallow in these areas and the trout are particularly spooky.

It's wise to stay well back from the edge of the water and spend much of the day casting from kneeling and but you'll catch more and bigger trout that way.

While many anglers hang up their trout tackle after the major mayfly hatches of spring, the canny fisherman looks forward to the dog days of August and September for some of the finest topwater fishing of the year.

Terrestrial are the key to this intriguing late-season sport.