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So throughout all, or almost all, editions of Dungeons and Dragons, there is a creature called the Displacer Beast whose primary ability is to make itself appear as if it were a few feet from where it actually is making it very difficult to hit. What I'd like to know is two-part:

  1. Are there any real creatures with a similar ability? I'm assuming that if there is, it would be aquatic in nature and using either an odd property of light or lives deep enough that it is able to do so in a non-visible section of the spectrum.
  2. If there are real creatures that have a similar ability that I can make use of, how would such a feature work, on land or in the water?
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I'm not aware of anything that actually does this, but you could get the effect you are looking for by throwing out a blob of slime that is sufficiently different from the surrounding water to refract the light away from you.

This is way more complicated than putting out a simple cloud of ink that hides you just as well, but I could see it developing from something like hagfish protective slime.

A land based creature is a bit harder — it isn't so easy to make a lens that floats in air. A suitably shaped bubble might work, but I can't think of anything reasonable that it might develop from.

A creature that stays in one area might work though — it hangs sheets of refractive slime all over the place, creating lots of invisible hiding places where it can wait for its next meal to wander in.

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  • $\begingroup$ Yeah, DBeasts are thoroughly constructed based on magic, though technically a creature might develop something with a negative refractory index that would naturally operate similar to what the OP is talking about... $\endgroup$ – Isaac Kotlicky Mar 22 '15 at 21:29
  • $\begingroup$ I think the slime would work, especially if it behaved more like circus mirrors and the creature can alter its shape. $\endgroup$ – Brad Mar 27 '15 at 15:00
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Dragonflies do this, more or less. It is called motion camouflage.

Basically they use an optical illusion to hide their motion, which causes their target to underestimate their speed which quite naturally results in the target estimating their position wrong.

The downside is that it only really works when you are moving fast to attack the target. So it is not really defensive weapon. Although obviously it does make any counter-attack more likely to fail.

A closer range and lower speed version might be possible for a species that can produce patterns of light on its otherwise dark skin. If you are in a dark environment attacked by something that has bright spots on, you will naturally take the motion cues from the spots because they have higher contrast than the actual outline. If the spots are bright enough you might not be able to even see the outline.

The simplest method might be to have those light producing spots at the ends of tentacles. This would allow the assumed position be out of line with the actual position and extend the motion camouflage to defensive benefit.

The main benefits would still be hiding the attack and possibly illuminating the target so that the predator can attack visually.

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  • $\begingroup$ Light producing spots on the ends of tentacles sounds remarkably like what some police officers are trained to do. When in a very dark area, hold your flashlight out at arms length from the side of your body facing forward. from the welts I gained playing airsoft with my kids, I can tell you this is effective, to a point. $\endgroup$ – Paul TIKI Apr 20 '17 at 18:19
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It depends upon how fast the other creatures that you're facing think.

Insects are a good example of this. Flies,some species of jumping spider and Springtails are the best. Can you remember all the times that you swung a newspaper at a housefly, only to find that it had somehow already taken off and was flying somewhere behind you? That's because the fly experiences time and can move its limbs far, far faster than we can. Watching a fly walk over any given surface is like watching a stop-motion film, its legs seem to disappear from one location as reappear at the next.

Jumping spiders and springtails are similar. When they leap they expend enough energy to clear their own body distance before the human eye/brain can catch up, appearing to leap from one spot to another without covering any of the intervening space.

Of course: This is an optical illusion caused by the difference between how fast they move and how fast our eyes work, so scaling the effect up to a larger creature could become tricky, and would cause other effects such as displaced air or (depending upon the size of the creature) damage to the ground where it pushes off. Some larger creatures like lizards almost manage to do it with their tongues though, which neatly links back to flies.

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  • $\begingroup$ "the fly experiend time and can move its limbs far, far faster than we can. " we generally don't move the fly's limbs at all! $\endgroup$ – JDługosz Nov 6 '15 at 11:21
  • $\begingroup$ @JDługosz Hooray for syntactic ambiguity! Also: I seem to be experiencing real trouble with my possessive apostrophes at the moment. This is what, the third post you've corrected me on it in? $\endgroup$ – Joe Bloggs Nov 6 '15 at 12:56
  • $\begingroup$ I don't know about ambiguous so much as that a software engineer will parse it exactly as it was written and laugh, while mundane folk will autocorrect a more approximate parse, like a web browser swollowing so-called xhtml that won't validate. Posessives: I pronounce them distinctly, so "they're" is clearly "they (a)re" run together. But "there" and "their" sound the same, but "taste"different in writing... I don't know how to explain, just that I get more sense frommthe word when reading. $\endgroup$ – JDługosz Nov 6 '15 at 15:27
  • $\begingroup$ The ambiguity comes in the implicit extension of the sentence to 'the fly experiences time and can move it's limbs far faster than we can (experience time and move our/it's limbs)'. From a software engineering perspective the sentence is incomplete! I never said what we could do :D $\endgroup$ – Joe Bloggs Nov 9 '15 at 10:06
  • $\begingroup$ "Its limbs" (possessive pronoun: his/hers/its form a set and have similar appearance), not "it is limbs" which does not scan. Implicit extension and DWIM grammar reminds me, a long time ago, on a writer-heavy pre-Internet forum, I coined "English++" with tongue-in-cheek rules like polymorpic words allowed to act over compounds. It earned a mention in "The Devil's DP Dictionary." 's 1995 update, I discovered via Google. $\endgroup$ – JDługosz Nov 9 '15 at 13:48
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In DND, the displacer beast is using magic. The magic in particular is appearing to be where it was a second ago, meaning that where it is right now is a different spot. So, if you are willing to accept magic in return for a magical question, it is appearing behind in time a second or two. In this unvoted answer's defense, you didn't say that wasn't a legal answer. Real creatures, if you have a magical mind, as some here do, may very well implement fun magicks.

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Cephalopods has defensive mechanism of ejecting ink in water surrounding them - it can trick some animals into thinking that ink blob is actually the cephalopods.

Link to youtube video of cuttlefish ejecting ink

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  • $\begingroup$ This is mentionee in the accepted answer, you know. It might be better to contribute your link as a comment or edit to the old answer. $\endgroup$ – JDługosz Nov 13 '16 at 20:17

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