I wanted to connect my Raspberry and the Spider controller board with Arduino
via i2c.
On websites like
http://neophob.com/2013/04/i2c-communication-between-a-rpi-and-a-arduino/
http://blog.retep.org/2014/02/15/connecting-an-arduino-to-a-raspberry-pi-using-i2c/
I read that I can simply connect both as long as Raspberry is the master.
But there is some important issue, if Raspberry and Arduino stay connected:
if you do nothing in the void loop() (like the blink example) and/or if you do
not set digital pin D20 and D21 to 0, these pins are at 5V per default!
I never set these pins and they were on 5V out of the box.
Then I tried a Arduino script setting these to 0, was ok.
But as soon as you put a script onto the Arduino which doesn't do that,
the SCL and SDA are on 5V again.
Even if you set the pins to 0 in void setup()
and then use the script on
http://neophob.com/2013/04/i2c-communication-between-a-rpi-and-a-arduino/
the two pins are on 5V afterwards!
Luckily I measured this before connecting the Raspberry trusting the above
websites.
THIS MAY GRILL YOUR Raspberry!
You should be sure you didn’t enable the
internal pull-ups on your arduino.
Or just use a 3.3V Arduino 
Or just use a 3.3V Arduino
If there is some 3V3 version
If there is some 3V3 version of the Spider Controller ?
Solved: solution for 3V3 Raspberry stuff and 5V Arduino stuff
I had a solution here afair but it seems to be deleted.
So I simply point to another posting which I found afterwards:
https://www.robotshop.com/letsmakerobots/running-both-5v-and-33v-devices-i2c
which is nearly identical to my solution.