Right, first ants and roaches need oxygen to breathe. There's no oxygen in the Mars atmosphere, and very little atmosphere either - it's a near vacuum. So we don't expect to find them there. Also we have analogues of Mars on Earth,
Microbial Oasis Discovered Beneath the Atacama Desert - Astrobiology Magazine
These places are so dry that there's no higher life at all, only microbes.
So, we expect only microbes on Mars also, for most of the habitats, even if higher forms of life evolved there in the past. That's by analogy with Earth life, and because Mars is more inhospitable to life than even our harshest deserts.
Microbes can also form biofilms, and mats, so those also could occur, often a mix of many different microbes working together symbiotically.
However there are some lichens that may be able to grow on Mars. As they also can grow on Earth in places like that - in Antarctica, so long as there is enough moisture vapour in the air - and on Mars the air reaches 100% humidity at night because of the huge temperature differences - for 200 days of the year even in the equatorial regions - their "tropics" it gets so cold at night that CO2 condenses out of the atmosphere as dry ice. In the morning as it warms up then it is still 100% humidity and you get ice left momentarily on the surface as frost (it gets trapped in the dry ice) - well when it's 100% humidity, lichens like this may be able to grow on Mars. They do okay in Mars simulation chambers on Earth if partially shaded from the UV radiation - metabolize and photosynthesize slowly - so might be able to do okay on Mars in semishade, e.g. "huddling" in cracks in a boulder.
Lichen on Mars - Astrobiology Magazine
As for higher life - well lichen is a combination of a fungus and an algae - and the fungal component is able to survive in these simulation chambers because it gets oxygen from the algae component, just enough to get by.
There are also minute lifeforms that can manage without oxygen at any point in their life cycle. Species of Loricifera
They look like miniature jellyfish in a protective shell, are a mm in size, and never use oxygen at any point in their life cycle
First oxygen-free animals found
So - it's possible to have tiny animals that don't use oxygen at all. So - I would say that animal lifeforms not ruled out on Mars. There are many different habitats there.
One especially interesting one for me: the flow like features in Richardson's crater.
The interesting thing about this habitat, if it exists, is that according to one of the two most likely seeming models, it starts as fresh water at 0 C (most ideas for water on Mars involve very salty water), originally in a thin layer up to a couple of cms thick below the ice at the top of this slope. Perhaps a few tens of cms below a surface layer of clear ice. It forms briefly in late spring, spreads out and takes up salt and other materials and flows down the slope. Could this be exploited by something more complex than microbes and perhaps lichens?
We don't know if that's the true situation there. The alternative model is that it forms due to liquid on ice / rock boundaries only a few microns thick. But the cms thick layers form in Antarctica below the ice in similar conditions, so should form on Mars if it does have clear ice. Whether Mars has layers of clear ice is the big question here. A similar process causes the Martian geysers with explosions of gas from below thin clear layers of dry ice, so it does have clear areas of dry ice, most are agreed. The question is whether it also has clear areas of water ice later in the season in the debris from those dry ice geysers.
There are many other potential habitats on Mars. It could have subsurface water heated by geothermal heating, maybe with hydrogen sulfide, which life can use, maybe hydrogen, eaten by methanogens explaining the methane plumes. It could have underground caves with various complex chemical environments different from the surface. So those also might have forms of life unique to the cave type.
It's a very complex diverse planet and now that we can image it on sub-meter scales from orbit, we have found many seasonal changes. So it will take a long time before we explore it thoroughly.
So yes, I think it's at least possible that there are tiny animals on Mars like the ants and roaches - but most likely living in water as it would be very hard for them to survive running around on the surface with no air to breathe as the atmosphere is almost non existent, equivalent to a laboratory vacuum.
As for detecting them - well - we would need to go right up to the habitats they live in. The problem there is sterilizing our spacecraft well enough so that they can contact liquid water without introducing Earth life to it. That's a major challenge. So far we have not yet managed to sterilize any spacecraft 100%. So it's done on the basis of probability. I'm not sure how this will play out - we don't currently have any plans to send robots to investigate any of these proposed habitats close up.
ExoMars I think will be the most carefully sterilized spacecraft since Viking, and does have as a goal to search for present day life, not just past life. But not sure if it is sterilized to the level where it could explore an actual liquid habitat - that's a huge challenge.
It's only looking in the equatorial regions. These are amongst the most unlikely places to look on Mars, though even there, there is liquid water - now pretty much confirmed. Curiosity found evidence for water beneath the sand dunes as it drove, from l0oking at atmospheric humidity. Most think this liquid water is not habitable because at various times of day it is either too salty, or too cold (reaches a habitable salinity and temperature but only seperately, not both at once), but one investigator, Nilton Renno, thinks that biofilms could ptoentially modify it to make it into a habitable environment.
It could potentially find present day life, and is designed to look to see if it is there, although its primary goal is past life. But this is amongst the least likely places to find multicellular life, except just possibly lichens.
There are ideas for rovers to go to more habitable regions of Mars
Our robots could certainly see ants and suchlike if there were any moving around on the surface close to where they are - and have seen nothing - but nobody expected they would for the reasons I've just given.
We couldn't see them from orbit, our instruments are not quite that high in resolution.
You can find out about the habitats on Mars here:
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