The lessons of the wilds - from a biologist's point of view; In a Patch of Fireweed: A Biologist's Life in the Field, by Bernd Heinrich. Cambridge, Mass.: Harvard University Press. 194 pp. $18.50.

Henry David Thoreau went to the woods, he tells us in ''Walden,'' because he ''wished to live deliberately, to front only the essential facts of life, and see if I could not learn what it had to teach.''

Bernd Heinrich went to the woods because he had to. He and his family were forced to leave their home during World War II, and they lived for five years in the Hahnheide, a large reserve near Hamburg.

Of those years, Heinrich says: ''We had no work and hardly ever any money. The civilization that seals most of us off from the stark reality of existence had broken down. We were totally immersed in nature. Like most animals, our major concern was finding food.''

The experiences of those years in the forest led to the creation of a field biologist, too.

The bulk of Bernd Heinrich's ''In a Patch of Fireweed: A Biologist's Life in the Field'' is given to insects, his obvious love, even though Heinrich, a professor of zoology at the University of Vermont, knows the rest of the animal world well.

After a bit of autobiography, we move quickly to the wilds, and the study of honeybees, African dung beetles, whirligigs, ant lions, winter moths, bald-faced hornets, and the like.

Here's a typical Heinrich passage, drawn from a chapter on hornets:

''... All of these nest contents were typical. What was unusual, however, was that the nest contained not one but two egg-laying queens with swollen abdomens. Two queens for one nest had never been reported for these hornets and similar wasps. One could dismiss it as ''abnormal,'' but I found it an interesting observation. It shows what is within the realm of the possible in the real world. In biology there is no such thing as normal and abnormal, except in a statistical sense.''

No sentimentalist, Heinrich ''donated'' the nest ''to the ant colony in back of (his) cabin.''

Chapter after chapter of ''In a Patch of Fireweed'' deals with the close inspection of nature, and that inspection and frequent experimentation are rendered in language that the interested general reader can easily understand. Like Thoreau, Heinrich has learned what nature has to teach - and we benefit from the knowledge he shares.