The "spiderweb" is like a frame. The spider is symmetrical, little bit left of the middle: the "12" in row 11 - part of two legs and body - starts in column 5. Think about a fat ugly spider in your garden or house than you can complete it ;-)
Man...I probably wouldn't have needed to use the hints if I just remembered that spiders have 8 legs instead of 6. Oh well. Thanks for getting me there, Eirin. LOL!
Very hard. After long guessing drawing, I got a wrong answer. So, besides a not logical puzzle, it has multiple solution. At least two. Well, after some changes got finally the right one. I couldn´t give up after all... I do not recommend.
For people who want more hints:
First row black boxes are in positions 4, 7-10, 17 and 23
Third row are in 1, 3, 6, 12, 15, 21.