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I think you're out of luck. Current version appears to require at least Android 2.2.
Perhaps you should look in the device-specific parts of this forum, and see if there's a custom ROM for your device that is based on a higher version of Android.

I need Instagram for my Samsung Galaxy GT-I5500, that has currently installed Android OS 2.1.
Btw, can i upgrade the OS of this phone to 2.2?
Ooo, i saw today earlier in internet Instagram for Android OS 2.1 but link of Mediafire was broken, in comments all the users thanked the author for that cool stuff. I mean, and i was thinking that there is no Instagram for Android OS 2.1 but after i saw that post i was a little bit fulfilled with positive energy![]()
It won't matter even if you find a version of Instagram that supports 2.1
Instagram image editing renders uses OpenGL for the graphics, so it requires a device with a graphics chip that has OpenGL capabilities. Your phone (GT-I5500) doesn't even have a graphics chip to begin with.

You are wrong man, i upgraded my Samsung GT-I5500 to Android OS 2.2 and Instagram works prefectlly![]()
As I said, your phone's complete specs were probably unlisted.
I'm not understanding you what do you wanna say with unlisted?
Instagram's programming requires that you need an OpenGL ES capable chip for formatting to work. This means a graphics chip. Otherwise it would crash. Ever tried running a high end game on a netbook without a graphics chip? Kind of like that.
On GSMArena's database, and other places I look, your phone model does not say it has a graphics chip. So basically, its not listed on databases if your phone has a graphics chip or not. But if Instagram works, it probably has one.