When you type Trypophobia Trigger Images in google, you see a variety of images with irregular lumps and bumps among some more gory images.
Many people report that these images induce phobia like symptoms of anxiety.
Why do we get anxious when exposed to these images? What advantage is there to be had from this response?
I find the reasons like this ABC news report on ants and spiders. But still didn't get it any info from it.
Trypophobia is not a recognised specific anxiety disorder (Washington Post). It is worth mentioning that anyone can have a phobia to anything, this is merely a question of whether many people associate these spatial patterns with anxiety. Nevertheless, the response of individuals to these images can be quantified (Le et al., 2015). Ultimately the findings show that a response of trypophobia is not correlative with anxiety. Note that here we are discussing anxiety in a phobia response test. Typically anxiety manifests as sweating, dizziness, headaches, racing heartbeats, nausea, fidgeting, uncontrollable crying or laughing and drumming on a desk. This is not merely feeling uncomfortable.
One hypothesis was that these images had irregular spatial patterns that cause revulsion. A study found that in nature some animals and plants may use this patterning as a warning mechanism and that it is associated with poisonous animals (spatial pattern quantification of 10 poisonous animals versus 10 control animals p=0.03), and indeed spiders were among those that use irregular patterns (Cole & Wilkins, 2013). Note that this hypothesis was presented in a psychology journal so the evolutionary mechanisms remain, in my opinion, not fully explored and scrutinised.
Hover over the below yellow box to view a lotus seed head, which has typical irregular spatial patterning presented in the 2013 study.
This image is often reported as inducing trypophobia.
Answer: In summary, humans do not reliably feel anxious when viewing these images. It also remains unclear why some people do get anxious or uncomfortable when viewing these images. It is perhaps to do with an aversion to some potentially harmful animals, but evidence remains scarce.
As someone who is very disgusted by this kind of image, I think it is a caused by an association with maladies like burns, infections, and especially parasites. It is difficult for me to even describe this without feeling a bit nauseated, but it is hard for me to see things like that without picturing it being my, or someone else's, skin. Or that it is crawling with parasites. I don't have any particular source for this because it's from my own experience.
It isn't entirely unreasonable that this would be a biological defense because if you saw an animal with an area of perforated skin (also hard to even write) it would be a good idea to not touch it since this could indicate bacterial ulcers or a skin infection. If you saw a patch of ground with tiny holes all over it, it might not be a good idea to sit there as this would probably indicate burrowing insects such as termites were present.
Another thing that confirms this for me is that when trolls edit these patterns onto a human it has the worst effect, and nearly makes me vomit. As this article notes, also from its author's own experience, these types of images are some of the worst for Trypophobia. There have been some studies finding that the response is similar to our reaction to danger, rather that just disgust. From my own reaction to these images this seems to be accurate.
The "That's Odd" feeling is real. And now I understand the term "trypophobia" as a specific example of a more general aversion response which seems common in nature, and might even be hard-wired in the neural-circuits of many animals. The brightly coloured and mottled/spotted patterns found on various insects, amphibians and reptiles may represent more than just simple camouflage. I recall a large black salamander I found which was covered by yellow spots - I recognized it as a local species, not rare, but when I noticed it, despite my interest and curiousity, I felt a clear sense of "weird - ugh - yuck". Now this salamander lives under rocks, under leaves, in rotted-wood, and so on. The existence of the bright yellow spots are not likely related to helping it hide. Why would it evolve such a bright pattern, given its typical habitat? It may be the spot-pattern itself is a weapon, as it seems to provide the creature with a way to trigger a "yuck - leave that ugly thing alone" response in humans, who could be potential threats to the small, slow-moving and otherwise quite defenseless salamader. 
The pattern of spots on the lotus-seed head seem to be similar to the spot pattern on the salamander. The aversion-response in humans has almost certainly evolved as a protective mechanism to keep us from being poisoned, stung, bitten or burnt when young children. But it is also possible this response is not "hard-wired" at all, but is learned. Once stung by a brightly-coloured wasp or hornet, the child quickly learns to avoid handling this type of creature.
And the harmless salamander's bright yellow random-spaced spots simply mimic
the colour-coding of more effectively hostile agents.
It is possible the aversion to random spotted bumpy/lumpy surfaces may be related to a similar learning phenomenon. We encounter stinking, rotted flesh, and already have the evolutionary systems in place to detect that rotted meat should not be eaten - it smells bad. Really bad. And so, other creatures have an opportunity to evolve displays which - if they mimic the appearance of the rotted flesh - can benefit by the associative trigger of the visual appearance of the rotten flesh, and the chemical response from the smell. If the smell creates a powerful aversion response, the linkage that mammals - not just humans - might make to the visual appearance of the bad-smelling thing, might provide an opportunity for creatures to evolve mechanisms that can take advantage of this situation.
This linkage of stimulus-response, and the ability of the stimulus-response mechanism to be transferred to a different, associated response is well understood - Pavlov's dogs, for example, which learned quickly that the ringing bell meant feeding, and so would salivate when they heard the bell ring, without having to smell the food.
It is also possible the trypophobic response some humans feel, is simply learned, the way many phobias appear to be. The work of B.F. Skinner (creator of the "Skinner Box"), showed that behaviour could be "conditioned", and virtually any stimulus could be made to be associated with any response, given sufficient time, opportunity and intensity of the conditioning effort.
I suspect if a behavioural economist offered to pay $100 for each yellow spotted salamander a person could catch, the typical human would very quickly encode a different internal response to the appearance of the salamander's bumpy, spotted appearance, and it would certainly not be an aversion response.
