Wednesday, May 6, 2015

That's One Large Spider!

Zach H. over at the Zenopus Archives blog has done another of his excellent Holmes Manuscript series comparing the published Blue Book to the original document Holmes provided TSR. He's into the Sample Dungeon at this point and describes the giant spider as originally conceived:
He is Armor Class 3, plate mail, has 1 hit die and a poisonous bite.
The final text is significantly emended:
He is armor class 3 (plate mail), has 6 hit dice (31 hit points), and his bite causes 1-8 points of damage and is poisonous (-1 on saving throw dice because it is so strong)
For a first level group, that is straight-up nasty: 31 hit points takes 9 average hits to kill, and at AC 3 in Holmes that takes 36 attacks. It's quite likely to be a TPK unless the PCs just run from the encounter. Certainly a good lesson, but it points to the giant spider becoming standardized as a type.

The spider pictured above is a Brazilian wandering spider, reputedly the most venomous spider in the world. The largest is the giant huntsman spider, whose leg span reaches up to twelve inches. A Goliath birdeater tarantula only goes up to 6 ounces, and that's the heaviest spider by weight. So that gives us parameters for spiders.

Humans are the archetypal 1 hit die creature. At minimum, an adult human is about 4'10" tall and weighs 100 lbs. According to the square-cube ratio, if we took our maximum spider and made it 3' in width, that would be over 100 lbs already, and at 6' it would weigh almost 500. Compared to a person, I think that's enough to merit extra hit dice!

But Holmes's giant spider could be the more modest 3' wide spider. That's still a mean critter, but it is a sensible 1 hit die. We could scale up by a hit die per foot of leg span, with a 6' spider being 4 HD, and the monstrosity in the Blue Book at 8' and over 800 lbs. This gives us a good scale for giant insects in general, and allows for a flourishing variety. Maybe the spiders on your 3rd dungeon level have 3 hit dice. That would make them 5' wide and weighing as much as an NFL offensive lineman (over 330 lbs.). Similarly, we can change the effects of their bite, or even make them non-poisonous (if such a thing can be imagined).

Here's a chart based on some quick-and-dirty math:
1'6 oz.1 hp
2'14 lbs.1-2 hp
3'122 lbs.1
4'216 lbs.2
5'338 lbs.3
6'486 lbs.4-5
7'662 lbs.5-6
8'864 lbs.7-9
9'1094 lbs.8-10
10'1350 lbs.9+

From here, you can get an idea of the size and hit dice of any giant insect or arthropod you care to put in your game.

Spitder Image by Techuser, CC-BY-SA


  1. Thanks, Wayne. I've often thought that D&D would've benefited by keeping each of the Giant Animals/Bugs on a sliding HD scale based on how big they are, or at least a range for each rather than a single hit dice per type. After all, a 12 HD Giant Slug doesn't just spontaneously appear at that size.

  2. In 2010 I was thinking about giant spiders and I thought I would go right to the source and see if there was a spider forum I could ask. Here is my question and the answers:
    They had some great insights.

  3. You've got the 10' spider as 3600 times as heavy as a 1' spider. According to the cube law shouldn't it be only 1000 times as heavy, or 500lbs?
    In my sizing chart, weight only goes up with the 2.5th power, so it would only be about 300 times heavier. You'd only be human weight when it was 12' long!
    To check the 2.5 power versus 3 power, I looked for a smaller spider with a weight. The only info I could find was the Black Widow spider - given as 1.5'' long, 0.035 ounce, so 170 times lighter, but only 8 times bigger. 8^2.5=181, whereas 8^3=512. Hence the 2.5 power rule seems OK.
    I can't find anything comprehensive study of spider size versus weight, but I did find a paper on the web about bird wingspan versus weight,, and the formula it gives is a 2.616 power rule (close enough for me!)
    So using this rule, the numbers from my system for comparison would be:
    1' 6oz 0
    2' 2lb +5
    3' 6lb +8
    4' 11lb +10
    5' 23lb +12
    6' 32lb +13
    7' 45lb +14
    8' 64lb +15
    9' 90lb +16
    10' 125lb +17
    12' 180lb +18


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