MadSci Network: Zoology

Re: What colors can the common green iguana percieve?

Date: Fri Oct 27 10:18:24 2000
Posted By: Janet Hoff, Staff, Unit for Laboratory Animal Medicine, University of Michigan
Area of science: Zoology
ID: 971804411.Zo

Lizards are members of the class Reptilia and the order Squamata, along 
with snakes, worm lizards, and the Tuatara, Sphenodon punctatus.  The 
lizard suborder Sauria, with 3750 species, is the most successful group of 
living reptiles.  The Sauria are further divided into four infraorders, 
Iguania, Gekkota, Scincomorpha, and Anguinomorpha, each containing severla 

The Eye
The repitile iris contains striated, not smooth, musculature, and common 
mydriatics have no effect.  (the pupil does not dilate)  
The green iguana has a well-developed parietal (relating to the wall of 
any cavity) eye found on the dorsal midline of the head.  It is a 
degenerate eye containing a lens and retina.  It connects neurologically to 
the pineal body and plays a role in hormone (especially reproductive) 
production, thermoregulation, and regulation of the amount of time a 
lizared basks in sunlight.  It does not form images.  (They can absorb 
light - I would assume including colors-  but do not get images to the 

Here's something really interesting and may explain why the eye is so 

Certain horned lizards (Phrynosoma cornutum, P. coronatum, and P. solare) 
can squirt an alarming amount of blood from their eyes in response to 
threats from predators.  They rarely do this in response to humans but will 
do so when molested by dogs, coyotes, and foxes.  If the blood spray enters 
their mouth, canines shake their heads and salivate as if it were 
distasteful.  When threatened, the horned lizard arches its back and closes 
its eyes, which become swollen.  A fine stream of blood then shoots out 
from the margins of the eyelids of one or both eyes for a distance of up to 
4 feet.  The hemorrhage lasts 1 or 2 seconds and may be repeated two or 
three times.  The lizard may repeat this performance after a brief rest if 
another predator threatens.  The amount of blood lost is considerable but 
the lizards recover rapidly with no visible signs of harm.

Reference;  Reptile Medicine and Surgery, Douglas R. Mader, M.S., D.V.M.

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