I have a creature or more accurately creatures that I have a problem with. I can't figure out why it is afraid of water.
They are land based tiny tiny little piranha like organisms that work in unison. Each individual organism is about the size of phytoplankton and plankton. They work together almost like a hive mind but have no evil queen to rule them.
Each individual creature is not intelligent and acts more on instinct and signals from others. Think of eusocial insects using chemicals and pheromones. Because you never see one alone and only in large clusters, they always appear like a low-lying mist or fog. Let's call a cluster of these creatures the Mistraille.
Mistraille can consume all living plants and animals by entering the victim through a cut/wound/tear etc. and leave behind a desicated husk (I believe they consume everything, bones, muscles, all plant matter except the outer covering/skin but this can be changed if necessary). The plant/animal can still be alive when the Mistraille enters. After a time the Mistraille can actually move the animal around like a 'meatsuit' (this could allow isolated cases of Mistraille in areas not normally associated with it).
A small amount of Mistraille enters the body, reproduces while consuming the body and emerges in greater numbers. The Mistraille therefore leaves a fairly desolate empty environment behind them. They appear to live in deserts — which came first, Mistraille or the desert? (rhetorical question).
I didn't want an entire desert wasteland of a planet, so I have tried to limit Mistraille range by giving it two major weaknesses; light and water. Mistraille normally take cover during daylight hours, either by hiding deep in the desert sands, deep shadows (like basements, caves, inside empty shoes etc.) or in their latest victim. They are also physically constrained by large water bodies and can be temporarily repelled by water sprinkled at them (more on this later).
EDIT I had already figured this would also limit their climate range as well. i.e. it would be limited to the dry, 'desert' mid latitudinal regions of the planet. Extreme variations in the day/night cycle, rain, humidity, and even soil moisture would hinder them. Equatorial regions would probably be too wet. High latitudinal areas can have deserts. If the Mistraille had managed to use a 'meatsuit' or darkness to travel there (by accident) it would then have to cope with the longer summer day cycles (only a brief dry night cycle) and the 'wetter' winter cycles. I still don't think high latitudes like tundra would be the best location. Same for the polar desert regions - 'eternal' day is not what this little critter is all about! EDIT
Two minor restrictions are they cannot hide in or travel through rock/solid surfaces. Porous rock yes, solid rock no. Mistraille also cannot consume a victim from the outside. It has to get inside the body first. So any cuts, wounds, open sores are a way in. To further limit this I feel that natural orifices like mouths, noses, ears, bums, skin or plant pores should be off limits to entry (reasoning not completely worked out yet).
I need limiting factors to stop the spread and range of said Mistraille as well as some way to keep it at bay at night by 'intelligent' creatures.
Extra information in case it influences the answer
I have figured out several constraints and reasoning behind their light weakness.
Mistraille actually are in deadly danger to any length of exposure to the UV spectrum of light.
Due to their limited exposure to daylight they have an unfounded fear of the entire visible spectrum of light (almost a Pavlovian response).
They can be repelled at night by artificial light sources such as fire, or modern day torches (it is the light not the heat that they avoid). This won't prevent the Mistraille from crossing the barrier of light if it really decides to go at you.
I have figured that the parts of the plants that photosynthesize during the day would be off limits. Therefore Mistraille can only hideout during the day in deep shadows, animals and in the more solidish parts of the plants E.g. tree trunks. (I know that there are light and dark phases of the photosynthesis cycle, but hopefully you get my reasoning).
Related information to question
In working out the details on water I have figured out some constraints but not the reasoning.
I want to limit their spread, so I made them unable to cross large bodies of water; oceans, seas, rivers, and perhaps marshes. Mistraille could possibly create 'bridges' to cross little trickles or very tiny streams. E.g. ants working together to cross a small river but this might involve sacrificing those in the water.
At night or in the deep shadows you can temporarily 'repel' Mistraille by 'sprinkling' water at them. A deluge would also work, but you are in a desert so you don't want to use all the water at once.
What I can't figure out is why they are afraid of water! They go inside animal and plant bodies that are made up of a large proportion of water. How can I reasonably explain the fear of the one but not the other?
My question is: why are my creatures, Mistraille, afraid of water outside their victims but not when they are inside their victims?
I've been thinking that maybe the water would interfere with their hive mind communication, isolating each creature from the cluster. But how would they then communicate inside their victims?
Maybe something in the chemical makeup of the victims body fluids could trigger a delayed transformation to a form more adapted to the fluid environment. But with that reasoning it can't be related to the freshness or saltiness of the fluid and can't be plant /animal species specific.
Hopefully you see my conundrum. I'm not looking for idea generation per se, just a somewhat valid reasoning on how I can explain my creatures actions. And if necessary alternate solutions.
EDIT BASED ON ANSWERS & COMMENTS GIVEN SO FAR
These are very tiny creatures, so their body structure would be fairly basic and not too complex. (I don't think they would be similar or based on felides. Sorry!).
I had been toying with a caste structure but now think I will definitely need one. Highly eusocial creatures are defined as having a job role bases on the creatures body characteristics from birth. No changing a worker into a queen or a queen onto a worker. I believe Mistraille would be 'primitively eusocial' - their job roles aren't defined from birth, they can have multiple generations, shared care for the young, etc.
So using that basis I believe I can make Mistraille have different sets of body characteristics based on the surrounding environment.
When they are traveling outside any victims they could be fairly similar to each other. Each simple creature communicating via pheromones and the larger collective consciousness a sort of hive mind with no queen directing any actions. These outside Mistraille move by either crawling/hopping or 'flying' on very basic proto-wings. No swimming. The best image I have, in my head, is the classic sediment transport image by bouncing, rolling, dragging, and suspension etc. It is this 'outside' form that would be the most vulnerable to light and water (a holdover from their 'inside' forms?).
When they encounter a victim, they enter and then this would trigger a process of transformation to occur. This transformation would result in a better adaptation for 'liquid' interiors but the reasoning so far of narrow osmosis range/required toxin would still hold (or a similar reason not yet mentioned). They could transform into several specialised body types suited to what they are munching on E.g. animal or plant AND bone, muscle, organs, plant fibers etc. They would also lose any proto-wings they have and develop 'claws', 'razor sharp teeth' and 'swimming' abilities. The 'nests' where the Queens and child-rearing occur would be in locations safe from light (so in plants, in the protected areas not involved in photosynthesis).
As I mentioned before, this transformation requirement from 'travel' mode to 'attack' mode (as @Drenzel called it) would explain why they can't eat a victim from the outside. It could also explain why natural orifices are off limits.
They would still communicate by chemicals and pheromones but might be directed by several Queens. I don't believe the hive mind is as developed as each creature knowing and experiencing the others experiences (as these are very simple creatures) but rather each individual creature influencing and been influenced by the collective consciousness.
THANK YOU VERY MUCH FOR THE ANSWERS SO FAR I'll be taking a little bit from most of them.