When I was in grade school in the 1970s, I saw a movie on TV. This would have been during the "Movie of the Week" heyday, and the movie was shown on prime time network TV (in the US).
I'll describe the few scenes I can recall clearly.
Early in the film, a blond woman is chatting up a guy in the parking lot of a bar or nightclub. There's a creepy sound effect and some camera shenanigans, and the guy starts screaming as he flees from the lady (but we don't know why he's running from her). We - the viewers - see him through her "eyes" (the camera) as he runs in terror.
The next scene (that I can recall, but it had to be sequentially near the one described above) is the police (central characters, I think?) inspecting the parking lot/crime scene (and the body?). One cop is looking up, at the top of a brick wall obscured in darkness/unlit. His partner asks what he sees, and the staring cop mumbles some sort of nonsensical "I'm not sure" sort of response.
Through the course of the movie, we discover that...
...the woman has a birthmark just below her navel which is the red hourglass of a black widow spider.
We also learn that...
...she is literally transforming into a HUGE black widow spider to devour the men she's...? Having sex with? maybe? dating?
And later we learn...
...the woman had a daughter who ended up with foster parents. This may have been due to the fact the mother was a sex worker, but was later discovered to be the black widow shapeshifter (?)
Then the big climax takes place...
...and there's a battle against the monster in a house/building/warehouse/whatever. Someone lights the big black widow aflame and we see the huge thing spinning and burning (the legs didn't move; it wasn't the most stunning special effect you've ever seen, but it stuck with me as a kid).
Finally, in the closing scene...
...the daughter is shown playing on the beach in not far from her caretakers/guardians, and the camera zooms in tight to reveal she has the telltale red hourglass just below HER navel, too.
Some other things I recall:
- The woman may have been a prostitute or sex worker of some kind, but I can't say for sure
- The daughter possibly didn't know her mother at all; she may have been in foster care from (near) birth.
- The movie was set in a coastal city; it was not a rural setting but an urban sprawl where the events take place.
I'm hoping this is enough for someone to identify the movie. Every time I search for something in Google I get Marvel/MCU hits. It's also all I have; this was 40 years ago and nothing else of note is in memory.