Traveling on a small Caribbean airline? Remember your toothbrush

On a map of the Caribbean, most of the dots that are luxurious holiday spots have tiny planes on them, signifying airstrips. The scale is appropriate: The equipment of Prinair (Puerto Rico International Airlines) Air BVI (British Virgin Islands) and LIAT (Leeward Island Air Transport) - the airlines that do the hopping from island to island - is not large.

Pan American, American, and Eastern Air Lines have direct flights to many Caribbean islands. But a few are just too tiny for jets. To get to Virgin Gorda , a five-minute flight from Tortola's Beef Island Airport, which the big carriers fly to, you have to climb into one of Air BVI's six-seaters. You watch the dials over the shoulder of the pilot as he takes off. All around you, the islands aren't specks anymore, but more like big, frond-tufted pieces of furniture arranged on the silver floor of the Caribbean. These trips put the thrill back in travel; some airlines ask passengers' weights in order to decide where to seat them, and my carry-on luggage was weighed and tagged.

If you want to add another island or two to your itinerary, it's the small local airlines, and sometimes ferries or mail boats, that will take you there. For $180, Prinair will fly you to any of its destinations, over a period of 30 days. This means you could fan out south and east from Puerto Rico (San Juan, Mayaguez, and Ponce) to hop around the US Virgin Islands and dip south to St. Kitts-Nevis or skip east to St. Martin's. From those islands, there are various air shuttles and ferries to explore the islets around them.

BWIA (British West Indian Airlines) offers a 21-day excursion fare that includes two stopovers on the way to any of its destinations. They fly to Kingston, Jamaica, San Juan, Puerto Rico, and Antigua - and can then get you to the southern end of the loop that is the Lesser Antilles: Barbados, Port of Spain, Trinidad, and Saint Lucia. Traveling south to Trinidad on this fare is $ 500 round trip.

You can also submit an itinerary of your own to an agent, who will figure out a fare based on distance and other factors. BWIA has a frequent shuttle service (hourly, at least) between Trinidad and Tobago. For other, smaller islands, BWIA connects with LIAT.

The Virgin Islands Seaplane Shuttle, which flies among the US and British Virgin Islands, has been described as ''fun and reliable,'' which makes it a standout among small airlines. The seaplanes fly from downtown to downtown, carrying about 20 passengers. Buzzing and splashing between Saint Thomas and Tortola in the mornings and afternoons (with service between Saint Croix and Saint Thomas every 25 minutes) seaplanes are good for day trips. The fare from Saint Thomas to Saint Croix, the longest distance, is $78.

Whether you're just getting from airport to hotel or you plan to set foot personally on every little landfall and survey - every little white, wave-lapped cove - you should be ready for a wholly Caribbean experience on these little airlines. Travel writers and old Caribbean hands Marsha Vickery and Margaret Zellers both advise voyagers to carry on toothbrushes and a bathing suit when flying the smaller airlines. ''That way,'' says Margaret Zellers philosophically , ''You can at least go swimming if they lose your luggage.''

Unfortunately, luggage doesn't always make all the connections you do. Mine, for example, dropped out of a trip from Virgin Gorda to Boston after the five-minute flight from Virgin Gorda to Tortola. I continued alone through San Juan to Boston, meeting up with it the next morning, when it was delivered to my door by a friendly Eastern Airlines representative.

''Refuse to budge till you see your luggage go from one plane to the other with your own baby blues,'' was Marsha Vickery's comment. This would have been possible, since I went from the airstrip, through customs, and back out onto the airstrip to another plane without all the tubular walkways that insulate one from bag handlers in bigger airlines. A man was loading luggage right next to the stairway as I climbed on, but I politely didn't ask about mine.

If you find yourself in a similar situation, talk to that baggage loader. Or just take a look in the hold. It may seem rude to you, but the people I encountered after I lost my luggage were so affable and accustomed to the problem that they must be really easy to get along with when no one has even lost anything yet.

Or, just carry all your luggage with you on the plane. This is another reason to heed Ms. Zellers's advice in ''Fielding's Caribbean '84'':

''Travel light with all baggage - mental as well as physical. . . .'' Another key, says Ms. Vickery, is to stay within an ''orbital'' pattern. Stay in your own island system, venturing out from a home base.

''Small airlines in the Caribbean each have their own little personality,'' she says, adding that ''Air Martinique and Air Mustique are very high right now, '' in terms of service. She felt LIAT offered very poor service, but Margaret Zellers counters this in her book.

When worst comes to worst, there are always air taxis.

So there you have it, travelers. When flying around the Caribbean, you have a responsibility. Beware of overweight mental baggage and keep your light physical suitcase on your lap - or keep asking about it. Keep a sane schedule. Stick to your island group.

And one more thing: Don't forget to look out the window.