Mites (Acari)

Arthropoda -> Chelicerata -> Arachnida -> Acari

Mites are a sub-class of Arachnids (e.g., Spiders, Scorpions, Pseudoscorpions), but are significantly more diverse in terms of life strategies and distribution than all the other Arachnid groups. Most Arachnids are predatory, however mites have developed to exploit almost every niche available - whether it’s as predator, parasite, detritivore, saprophage, bacterivore, fungivore or even beneficial symbiote to other animals. Their diversity in size and lifestyles has allowed them to colonise almost every habitat on earth, from the driest deserts to the hair follicles on your face! And mites aren’t just immensely diverse, they’re also incredibly numerous; easily the most abundant arthropod in soils all around the world. I won’t get into the taxonomy of mites much, because it’s so complicated that it’s frankly horrifying, but generally they’re split into a number of sub-orders which I’ll run through below with photos. If you want to dive into mites further, I’d recommend following the Soil Biodiversity UK group on Facebook (it’s the only reason I’m on the accursed app - trust me it’s worth it though) and the Acarological Society of America. If you’re feeling brave enough to learn mite taxonomy, then a really handy resource is the Soil Biodiversity UK website, which also has handy reference photographs for many species.

‘Moss’ or ‘Beetle’ mites (suborder Oribatida)

These heavily armoured (or ‘sclerotised’) mites are mostly detritivores or fungivores, playing an important role in regulating the breakdown of dead plant material into soil organic matter. Because of their diets, they’re particularly abundant and diverse in litter-rich forest soils, as well as in peatlands. Oribatids have what ecologists call a ‘K-selected’ life strategy - which means they have long and slow life cycles and low reproduction rates - in other words, they believe that slow and steady wins the race. The tortoise and the hare analogy works rather well here, given their hard shells. Because their lifecycle makes them slow to recover from changes to the soil environment, Oribatid communities are very good indicators of disturbance and changes in organic matter inputs to soil. They’re also simply wonderful to look at - having developed a range of remarkable anti-predation body structures and behaviours. Young Oribatid nymphs will often wear their moulted exoskeletons as armour, accumulating them like inverse Matryoshka dolls. And because the adults need to stay alive for a longer time to reproduce than most mites, they have evolved all kinds of weird and wonderful ways to avoid being someone else’s lunch, including hinged flaps and hatches, spurs, waxy exudates, camouflage, armour plates and retractable legs. They’re also slow-moving, so rather easy to photograph!

‘Predatory’ mites (order Mesostigmata)

Most Mesostigmatid mites are free-living soil predators, rushing around the soil in search for prey to suck the fluids from. I find that many mites can be anthropomorphised fairly easily, but not these - they really often do just seem like tiny, armoured death robots - think the ‘warrior bugs’ from the film Starship Troopers (now I’m showing my age). That’s not to say that Mesostigmata aren’t complex and interesting in their own right; they have developed some amazing solutions to the curve-balls that life in soil throws at you, for example being able to enter a fasting state to get through periods of low food availability. They’re also masters of dispersal (or ‘phoresy’), hitchhiking rides on larger animals to get to greener pastures. For example, Poecilochirus mites will ride on Sexton beetles to find fresh corpses, where they can disembark and feed on fly larvae. Some Mesostigmata are instead parasites or symbionts to a wide range of vertebrates and invertebrates, whilst a select few are specialised to feed on fungi and plants. One thing is for sure - they don’t hang around for long, so photographing them can be a challenge!

‘Sucking’ mites (suborder Prostigmata)

I’m going to get in trouble for casually calling them ‘sucking mites’, but it does work - as all Prostigmatid mites are fluid feeders in some form. But that’s not to trivialise them - they are the most diverse group of mites, with many species living as terrestrial and aquatic predators, and many others as plant and animal parasites. There are fungivorous species and even some saprophagous species, feeding on dead and decaying organic matter. Many species of Prostigmatid mite start off life as parasitic nymphs, before adopting predatory or other feeding strategies as adults. The Prostigmatid mites that folks may be most familiar with are the charismatic ‘red velvet’ mites (family Trombidiidae) and the ‘whirligig’ mites (family Anystidae), who endlessly rush around the soil surface looking for prey. In fact, a species of Anystidae is the fastest recorded terrestrial animal, capable of running 322 body lengths per second! Most Prostigmata are soft-bodied (weakly sclerotised), many have eyes, they can be quite colourfully pigmented and are a bit front-heavy in terms of body shape (no judgement there). Size is also incredibly variable between all the different species of Prostigmata, ranging from about 0.3 - 12 mm. One of my favourite tiddlers is the white slug mite Riccardoella oudemansi, which are only around 0.5 mm in size, and have taken the unenviable evolutionary path of spending their entire lives living and feeding on slugs. They even breed in the slug’s lungs. Isn’t nature amazing?

‘Dust’ or ‘feather’ mites (cohort Astigmatina)

If the Oribatid mites are analogous to tortoises, then the Astigmatina are definitely the hares of the mite world. These freewheeling and fast-breeding ‘r-strategists’ have short development times, rapid reproduction rates and very good dispersal abilities - enabling them to quickly take advantage of new resources and disturbed habitats. Mite taxonomists don’t really seem sure on where to place the Astigmatina; they used to sit beside the Oribatids and Prostigmatids, but now they’ve been demoted to cohort level within the Oribatida based on their body plan and some complicated reproductive behaviour that I won’t get into here (because I don’t understand it). Many species of Astigmatina are parasites on vertebrates (including humans - scabies, for example, is caused by the Astigmatid species Sarcoptes scabiei), however there are also saprophages, fungivores and bacterivores, and in many soil-based habitats Astigmatid mites can be the dominant mite groups. As I mentioned, these mites are expert dispersalists - you might sometimes see very small mites attached like limpets onto the bodies of other invertebrates, including other mites - well, those are probably the special juvenile life stage of Astigmatina called a ‘deuteronymph’ or ‘hypopus’. In this form, the young mite metamorphoses special new features such as a sucker pad to help it attach firmly to a host’s cuticle, whilst simultaneously committing to a foodless life on the road by ‘losing’ its mouth and gut functions. It’ll hang around for a while, before dropping off in pastures new and reverse-metamorphosing back into a feeding form. Sounds like a lot of effort to me, but it clearly works for them!

Ticks (suborder Ixodida)

Perhaps ticks aren’t soil biodiversity per se, but they’re mites all the same, and if you’re into rummaging around the ground in fields and forests then you’re (unfortunately) going to meet a tick or two at some point. Ticks are purely parasitic creatures, and depend on blood meals to complete their lifecycles, which is where the problem comes in - they’re notorious disease vectors. In the UK, the most common tick-borne disease is Lyme Disease, a bacterial infection carried by ticks which have previously fed on an infected animal. It’s important to read up on how to safely remove ticks and monitor for any signs of disease after being bitten, as well and knowing the risks in your particular region. I hope not to have too many opportunities to photograph ticks, but I’ll add more photos of these much maligned creatures as the chance arises. In the meantime, Andy Murray has a page with lots more interesting information and macro photos of ticks over on his website here, do go pay him a visit!