I have a friend, an exceptional naturalist, who has traveled this country widely from Iriomote-jima to Hokkaido, yet who swears that he will never visit the Ogasawara Islands.

Now that is a strange aversion for a naturalist, because those particular islands, 1,000 km or so south of Tokyo, have a rich, unusual flora; they are home to many sea birds difficult to see elsewhere in Japan and they also have the unique Bonin Islands honeyeater. Why, then, would a serious naturalist not want to go there?

The answer: toads.

I have come across plenty of people who are hysterical about snakes; arachnophobes are common too, and I have even come across, much to my amazement, at least two people who have a morbid fear of butterflies. So far however, I have only met one person who has such a deep loathing for toads.

My friend chooses not to visit the Ogasawara Islands, then, because there are so many toads there. Introduced to the Ogasawara Islands as someone's brainchild, perhaps for some form of biological control, they have spread. My memory is of dead and dying toads in deep roadside channels, and though the smell of dying toads is not one of my favorites, it wasn't a smell that was sufficient to put me off the islands.

My antitoad friend can't always choose to avoid toady encounters, though. Driving with him through a semi-desert region of northern Morocco, we had reason to stop suddenly at the roadside. A flock of buntings had flown up into nearby trees and I was eager to show him his first cirl bunting, a European and North African specialty. Toads were furthest from our minds; after all, the countryside seemed dry and barren for kilometers around.

Yet right where I stopped, when my friend opened the passenger door, the first thing he saw was a desiccated toad on the roadside.

I suppose when you have a phobia about such things you do tend to notice them more readily than anyone else, but it seemed a hilarious irony, given the kilometers of toadless roadside in that country, that we should have parked right next to one!

Despite his aversion to them, my friend would have to agree that toads and other amphibians are interesting creatures. If evolution worked along linear lines we could easily think of amphibians as early terrestrials, precursors to reptiles and mammals, though of course it was actually their ancient ancestors that fulfill that role. Modern amphibians are as well evolved to their existing habitats as any other species.

Amphibians are exothermic (cold-blooded) vertebrates. Like fish, they can live in water, and like reptiles, many of them can also live on land. Fossil evidence indicates that it was the amphibians that were the first vertebrates to switch from an aquatic habitat to a terrestrial one.

Better known than toads are the frogs, so let's clear up the differences between them before we go any further. The first point of course, is that my naturalist friend detests toads, whereas he has no such aversion to frogs, which is a clear pointer that, to naturalists at least, there is a noticeable distinction between them. Amphibians in general have soft, thin, very permeable skins, making them dependent on freshwater for some or all of their life cycles.

Frogs are typical amphibians in many ways, though among their nearly 4,000 species there are certainly some very bizarre lifestyles. The nearly 300 species of toads tend to be rather more terrestrial in their habits than frogs. Toads are often encountered farther away from water and in drier habitats than frogs are -- witness the poor beast I parked next to in Morocco.

Unlike frogs, which have a moist skin, toads have rather dry, warty skin, and it seems to be that feature that my friend objects to most of all. Whereas frogs range from the gaudy, such as the Ishikawa frog of Japan's Nansei Shoto, which is lime green and black, to the positively repellent such as the blue and black poison arrow frogs of the Amazon, with some as brilliant as the golden frogs of Costa Rica, the majority of course are more cryptically colored in greens and browns.

Toads, however, are almost always much plainer and much duller, ranging from gray to brown.

Frogs are also distinguished by their long hind limbs, adapted for leaping, whereas the more terrestrial toads have shorter rear legs, better adapted for crawling and hopping.

Toad activity is dependent on the ambient temperature. In temperate climates, as the autumn weather cools, toads become less and less active, eventually entering a state of torpor. Conversely in spring, as the days warm, so toads wake and become increasingly active. This seasonal shift in temperature seems to trigger breeding.

At this season, toads migrate back to water, returning to the ponds where they hatched, which often have been used for many generations. They are at great risk at this time in some regions because as masses of these slow-moving creatures cross roads they are in danger of being crushed by cars. In parts of England I have even seen warning road signs alerting drivers to the presence of toads on the road.

Having returned to the vicinity of a pond or lake, male toads call to attract a mate, and having found a mate, they fertilize the eggs externally as soon as they are laid.

Toads lay strings of gelatinous eggs attached to underwater vegetation. Tadpoles, which are fat-bodied feeding machines equipped with tails, hatch out from the eggs and spend several weeks or months growing steadily before acquiring limbs, absorbing their tails and finally metamorphosing into the adult toads which my friend dislikes so much.

Tadpoles feed on freshwater plankton, simple plants and single-celled animals. Adult toads are armed with a sticky tongue that they can shoot out of their mouths to ensnare a wide range of invertebrates. Ants and beetles are favorites; they also eat earthworms, slugs and snails. Effective predators of smaller creatures themselves, they have a trick or two up their sleeve (or at least in their skin) that keeps them safe from larger predators: poison-secreting glands in the skin of their backs. These are effective deterrents even against animals the size of foxes.

They are also actors. Depending on the fact that size is important, they inflate themselves and straighten their legs to make themselves look larger. This is enough to repel most potential predators, though snakes and hedgehogs are known to eat some toad species.

Many amphibians secrete substances from their skin for various purposes, from a chemical form of signaling to potent chemical warfare. If in doubt as to whether the creature you are observing is a frog or a toad, don't be tempted to pick it up: You might just be allergic to its chemical arsenal. A skin rash or worse could result.

And if, like my friend, you have a phobia for toads, don't even think of going to the Ogasawara Islands.