One of the interesting things about the human mind is that although we tend to be fascinated by the diversity in Nature, we endlessly try to simplify what we see, to control Nature by reducing it into a smaller number of units. We value simple explanations over complex ones and even codify this preference in science with Occam’s Razor. This has various formulations, but Frustra fit per plura quod potest fieri per pauciora at least comes from Occam’s writings and fits this posting: ‘It is futile to do with more things that which can be done with fewer’.
It is winter now, if a rather mild one so far, but still not the best time for observing living arthropods. However, a friend has posed a challenge – how many living arthropods can you find during winter in Alberta? So, I’ve been wandering around the house taking bad pictures of tiny insects and far too large spiders and trying to coerce my wife into taking better ones. However, exams and marking reign at the moment and she suggested: why not see what is living in the soil of our house plants? Brilliant idea! – at least 21 species in the first extraction, 2/3rds mites, but also 7 species of Collembola – springtails. As I was perusing said springtails I noticed that the large entomobryid and isotomids all were missing the distal segements of their antennae.
Although Tennyson’s “Tho’ Nature, red in tooth and claw, With ravine, shriek’d against his creed [i.e God's]” is most colourfully exemplified by vertebrate predators at their dinner, you can get an idea of what life in the soil is like for springtails by viewing Jeffrey Newton’s entertaining video. The spring in the springtail is one way these animals avoid being eaten, as are the slippery scales on some (as in the SEM above) and the chemical defenses of others. Apparently, shedding the ends of the antennae is another (good news is that they can grow back – as one victim with half-regrown antennae demonstrated).
In my house plant ecosystem, the antenna-snapper seems to be a species of Holaspulus. These are rather large predatory mites (Mesostigmata, Parholaspididae) more typically found in tropical areas. But then, I guess the greenhouses these plants came from do all they can to mimic the tropics including, it seems, hosting tropical mites.
As you can see from the image above (this mite is from Queensland), Holaspulus have very long chelicerae, almost half the length of their body. These chalicerae can be shot out to snap and grab prey, such as springtails. I know this is true through my own observations, but I also know this for the other chelicerae of the very different mites illustrated above and several other unrelated genera. Just as springtails have a diversity of means of escaping becoming dinner, so do their predators of capturing them.
And just to keep things clear as mud, I know of many mites with much less impressive chelicerae that also seem to have no problem catching springtails, e.g. the parasitid mites in Jeffrey’s video. Another example, species of Podocinum have fairly typical chelicerae, not obviously enlarged at all, but are seemingly efficient predators of springtails.
Such cheliceral modesty is true of many mesostigmatans that prey on springtails – they lack the snap-trap type of chelicera. But some such as Podocinum species do have extraordinarily attenuated legs.
I’ve watched Podocinum fishing for springtails. They are slow-moving, stately mites, but as they wander the front legs dangle in front exploring their world. When a springtail is encountered by the light touch of the long distal setae, the mite pauses, and then quickly scopes their prey in towards their chelicerae: another springtail become mite fodder.
Epicriid mites carry this long-legged fishing one step further. The long ventral setae on the tips of the front legs have blobs of glue with which they capture their collembolan prey (see Alberti 2010*). So we have yet another way to scarf-up a springtail, but why such diversity in such a simple pursuit?
As far as I know, no one has studied the feeding behaviour of the mite above, but I suspect springtails are part of its diet. For those who like a challenge, try naming the unnamed cheliceral images. A genus would do and you have five such unlabelled images to guess upon.
*Alberti, Gerd. 2010. On predation in Epicriidae (Gamasida, Anactinotrichida) and fine-structural details of their forelegs. Soil Organisms Volume: 82 ( 2); 179-192. (Open Access – search for the title and the pdf is yours)