Hi Michael,
Using the small on-camera flash with most cameras can be a little bit hit and miss. In this instance, I think the effect has worked out well for you.
There are two things that I would have done to improve this photograph though.
The first would have been to move the camera ever so slightly to the left so you lose the branches of the tree in the background. By losing the branches you lose situation context; in many cases I try and keep this context that in this instance you are trying to create an element of mystery. If we are to lose the branches then this photograph could almost be taken anywhere.
Now, I have a hunch that what I am going to suggest next might not be an option for your camera, but if you your camera has the option of manual mode on this camera you could actually make the sky darker and even more dramatic.
How this works was rather simple. The flash on your camera has an exceptionally short duration and so aside from some called the maximum sync speed, the shutter on most modern cameras does not affect the actual flash exposure of the camera.
If you are in a similar situation again with a camera that has a manual setting, try making a flash photograph of the spider web while making adjustments to the shutter speed to compare the difference. The ability to separately control the flash exposure from the ambient exposure (the light that is about us most) can lead to some really creative photographs.
Cheers for now,
Anthony