Spiny-backed Orbweaver (Gasteracantha cancriformis)
This spider is quite common from the United States through some parts of South America and is highly variable in appearance. On Saint Martin, the most common coloration is red and yellow, although red and white and black and white variants are also seen regularly. Although this spider supposedly has six spines, the individuals on Saint Martin typically have four visible spines, while the other two spines may be so greatly reduced as to be unnoticeable. It would be quite interesting to know if this variation is to any degree locally specific. Either way, their bright coloration and jewel-like bodies make them a very attractive spider.
I commonly see these spiders in forested areas, although they are also visible at the margins of forests, or even in scrubland. They spin fairly large webs, usually about a foot in diameter and typically rest in the middle of them. I believe they are basically harmless, as I have often come back from hikes with them in my hair or on my clothing.