This topic comes up every now and again...
I found a passage in the manual for my Nikon SB-26 flashes that reads:
"DO NOT FIRE FLASH NEAR THE EYES: Firing the flash light very close to any person's eyes can injure the retina, thereby weakening eyesight or causing blindness.
DO NOT TOUCH THE FLASH HEAD WHEN FIRING THE SB-26: The flash head generates significant heat during normal operation, which may cause burns. Also, when using the flash, keep delicate materials away from the flash head."
I don't think that passage leaves much doubt that the light coming from the flash is a significant event if you are doing macro photography close to the subject. But that being said, I have been taking such photographs for years, using TWO such flashes mounted on the sides of the camera and a third one suspended directly above the subject. Under such circumstances after reading the above warnings, one would expect that after taking the photographs, I would have only the charred remains of the snake sitting there. This certainly has NOT happened, so take it with a grain of salt.
Moderation is the watchword. If you take strings of photographs of the same animal, look at it from the animal's perspective. Those constant flashes in the eyes are probably getting real old, so do the animal a favor and take a break for a while. Normally nocturnal animals probably HATE flashes, so wait for the proper time to take your photographs and keep them down to a minimum. If at anytime you wonder what the animal thinks about all this, take the flash and hold it the same distance from your own eye to get an idea of what you are putting that animal through.