> From: Keith Benson <[EMAIL PROTECTED]> > Date: Mon, 22 Sep 2003 17:36:49 -0400 > > Neal Grant wrote: > > >Just wondering, has anyone ever proposed a theory as to the > >"evolutionary advantage" of not having eyelids??? > > > >As a lot of geckos (and snakes I'm almost certain) live in warm, dry > >environments, and run about through rocks, brush, and trees, it seems > >like having an eye-lid would be pretty important... > > > They have eyelids - they are simply difficult to see, they are fused and > they do not move. Snakes have palpebrae (fancy word for eyelid) as > embryos, these fuse and form the spetacle. This is a clear covering > over the eye - excellent protection in the situations you describe > above. Geckos do the same (well, not the eublepharines) but it is > called a brille.
The really cool thing about this feature is that we can see how it developed by observing some skinks which are partway along in the process. They have transparent 'windows' in their lower eyelids, so they can see with their eyes closed! IIRC, these skinks live in deserts, and the adaptation helps them avoid damage from dust and grit. Eyelids that stay closed but don't impair vision are a real benefit for burrowing animals too, protecting the eye below ground, but allowing vision above ground. AFAIK, current opinion is that snakes developed from fossorial (burrowing) forms, and the most primitive snakes are tiny creatures that still live underground and have poorly developed eyes. Modern snakes had to 'reinvent' good eyes from these forms, and as a result use a different method of focusing than other vertebrates - instead of using muscles to change the shape of the lens, they pull the lens closer to or farther from the retina. BTW, an anatomy question. I sometimes have a baby leo that doesn't get all the shed skin off its face without help. It sometimes seems that they have a membrane over the eye surface that comes off with the shed, but I'm not sure what it's attached to. They usually have the eye partly closed and dull-looking, but when I remove the shed they open the eye wide and it looks clear and wetter than usual. Could this be the skin from the underside of the eyelid covering the cornea? _______________________________________________ Global Gecko Association http://www.gekkota.com Classifieds http://www.gekkota.com/cgi-gekkota/classifieds.cgi gecko mailing list [EMAIL PROTECTED] http://lists.gekkota.com/mailman/listinfo/gecko
