They are mostly CO2 with some water (probably only a few percent). When the spider paper eventually gets published (it has been accepted) you can read in detail.

Away from the south polar regions the temperature is too high for the CO2 ice - so there are no spiders currently forming (emphasis on currently), but other forms do occur.

BUT water is required or one would not get the reverse dendritic pattern. I think the ice forms on the outside of the vent as it chills and grows narrower as it restricts the flow which causes the tube to burst (a 'very' little bit like the toothpaste tubes in pillow basalts). We get a hollow tubes because the ice does not revert to water under current martian atmospheric condtions (I am starting to believe the physics is flawed as water is stable on mars for very short periods of time)

The spiders are often elevated because they grow preferably on basalt or other rocks and avoid certain objects (some types and parts of craters). In looking at these things remember a few fine details:
1) the gravity of mars is 1/3 earth,
2) there is often a central plug,
3) the idea of science is not to prove your belief (so one needs an open mind - if it disagress with your pet theory chuck the theory not the data) - if you cant do that it is not science it is religion.