Summer is when many anglers pack up the fly rods and abandon the trout streams until the following spring, when mayfly hatches resume.

But not Doug Jones. And it's a good thing for him, for Jones is your classic trout fanatic. Fishing to him means trout fishing, period. If action fizzled on the trout steams in summer, Jones would be in a sad predicament. Forutnately, it doesn't.

Far from slacking off, Jones shifts into high gear at this time of year, travelling from his Arlington home almost every week to one or another of the better trout streams the area has to offer, such as Falling Spring, Big Spring, the Letort, Little Hunting Creek or the Rapidan.

And he catches lots of trout, many of them very good fish, despite what the traditional, Catskill-spawned trouting lore says. After all, the trout still have to eat, don't they? With the increased stream temperatures summer brings, trout must feed more, in fact, than they do in spring, fall and winter, because of their hyped-up metabolism.

And contrary to popular belief, the trout find plenty of food, even though it's not the classic mayfly. As Jones and other Summertime anglers are discovering, there's generally more food for the trout, and it's available for longer periods of the day than mayflyies were during their brief hatching period.

The food is terrestrials - insects born and bred on land that fall, get blown by wind, washed by rain or chased by larger predators into the watery domain of the trout. They include crickets, bluebottle flies, treehoppers, leafhoppers, ants, roaches, caterpillars, grasshoppers, beetles, true bugs, cicadas and hundreds of other strange six-legged creatures. Angling sage Charlie Fox applied the term "terrestrials" to this motley crew three decades ago, and it stuck.

A million or more different species of insects inhabit the planet, and some entomologists predict that ten times this number will eventually be discovered. Only 5 percent are aquatic; the other 95 percent are "terrestrial." Yet, on a nationwide basis, the average fly fisher's boxes probably contain 95 percent aquatic-insect imitations and 5 percent terrestrials.

Since our area is a prime terrestrial fishing belt, this neglect is not nearly so common among local trout fishermen. It helps, too, that most of the pathbreaking developments in terrestrial fishing occurred only a hundred miles away on the limestone creeks of south-central Pannsylvania.

It was here, on the Letort, that Vincent Marinaro, with close cooperation from Charlie Fox, developed the innovative theories presented in his classic work, A Modern Dry Fly Code (1950). In the preface to the 1970 Crown reissue of his book, Marinaro wrote, "The contents of my fly box, for fishing my home waters, would look rather strange to a fly-fisher of an older generation. He would see jassids, ants, beetles, crickets, grasshoppers, and the like. My box of mayfly imitations is reserved for those trips to blessed lands and waters where it is still possible to fish to a fine hatch of aquatics."

Where populations of fragile mayflies and caddis plummet, the hardy, dauntless terrestrials grow in numbers. They are tough and enduring, as any gardener will tell you.

Some beetles can eat arsenic and belladona without harm; others can bite through thick metal cables; one species of land insect can carry up to 270 times its own weight on its back - all signs of the enormous resiliency of these insects that thrive along trout streams while mayflies wane in numbers.

Fortunately for the angler, the trout seem in no way displeased with this switch in menu over recent decades. They gobble up ants and beetles and crickets with a relish many would have thought only mayflies could induce.

Better still, the calorie-rich land insects seem particularly appealing to big trout. Said fly fisherman extraordinaire Dave Whitlock, "Almost without exception most of the larger trout I've examined with stomach pumps have contained various terrestrials."

If you're going to fish terrestrials, there are several things you can do to insure quality sport this time of year.

First, pick water that still has a good head of trout in it. Basically, that means either a fish-for-fun stream in Maryland, Pennsylvania or Virginia, or one of the native brook trout streams in Shenandoah National Park. Sadly, few shocked streams with regular put-and-take rules have enough fish left to bother with during this part of the season.

Once you've selected a good place to fish, realize that the summer trout stream will run low and frighteningly clear. The fish will be skittish, to say the least. These two conditions dictate a stealthy approach and very fine tippets.

Wear drab clothes that blend in with your background, and keep a hunchbacked profile. Sometimes it's best to kneel or evne crawl, always staying as far back from the edge of the water as practical.

Leaders should measure at least nine feet; if you're fishing where long casts are possible, 10 to 12 feet is better. Tippets for terrestrial fishing should be 6X to 7X. Such leaders test only one to two pounds, but will handle most trout swimming in nearby waters if you fight them patiently. Use 6X for flies sized 16 or larger, 7X for anything smaller.

Recent years have seen a proliferation of novel terrestrial patterns. Some of them, such as Ed Sutryn's McMurray Ant, are remarkably effective on nearby streams. This particular fly is not easily found, but can be ordered direct from Sutryn at P. O. Box 104, McMurray, Pennsylvania 15317.

The basic terrestrials available at fly shops will also yield nice catches on area waters. A good selection should include black and cinnamon ants (sizes 16-22), Crowe beetle (12-16), Letort hopper (12-16), Letort cricket (10-14), clipped-hair inchworm (8-12) and a jassid or leafhopper (18-22).

Fish the smaller of these flies over sighted fish with as delicate a delivery as you can muster. The larger imitations can also be fished in this manner, but a more realistic delivery means dropping them with a tiny "splat" to stimulate the noise naturals make when they tumble off streamside preches into the water. This sound is frequently used as a key by trout in their summer feeding.

If you drop your fly with a plop to mimic this noise, the trout will often whirl around excitedly and furrow the water with a broad V-wake as they chase down the apparent floundering victim.

It's heady fishing, sure to match even the finest of springtime mayfly angling. CAPTION: Picture, SUMMER TROUT-FISHING: DULL-COLORED CLOTHES, A CROUCH AND THE PLOPI OF A FALLING BEETLE. By Gerald Almy.