Rolling in money spiders

On a Spring stroll through the local park, the arachnophobe will often recoil at the sudden arrival of a leggy speck of black on their cardigan, coat, hoody or whatever obscure outerwear they favour. Fear not, it is but a money spider!

Money spiders can be minuscule.

Money spiders, of the spider family Linyphiidae, comprise over 4,300 species globally, landing them second only to jumping spiders in the competition for most speciose spider family. In the British Isles though, they would certainly take home the prize for largest spider family, comprising around 290 of the approximate 700 British spider species1.  Not only this, but they are also vastly abundant, with populations of up to 2,000,000 predicted per acre of grassland2.

Despite their great diversity and abundance, money spiders are best known for drifting through the air via a technique termed “ballooning”.

Ballooning involves finding a breezy spot, casting a long line of silk into the wind and drifting off into the horizon. These silken sails can carry a spider as high as 4 km into the sky3 and take them hundreds of kilometres4 and even across oceans, given optimal meteorological conditions5,6. More recently, it has even been discovered that ballooning can be triggered by natural electric fields which enable ballooning7. Like many humans though, this wanderlust wanes in later life and ballooning is most prevalent in juveniles.

The silk of money spiders is not, however, purely purposed for ballooning. The webs of money spiders are typically horizontal sheets and they can vary wildly in size and location, mostly between species8,9. For example, spiders such as Erigone atra will often build small webs close to the ground which they abandon regularly to hunt on the ground, whilst other spiders like Tenuiphantes tenuis produce larger webs higher from the ground10.

Webs are effectively an investment of nutrients, so web sites (yes, we are calling them that) are typically areas where prey are highly abundant11. If this abundance is a particular type of prey, money spiders can adjust the structure of their webs to suit their target by attaching the silk to different surfaces11. Just as envious neighbours in the heart of suburbia might envy one another’s beautiful garden of tidily-kept hydrangeas and lawn ornaments, spiders suffer web-envy and territorial contests can ensue8,12–14.

Not only are money spider webs highly functional, their delicate and beautiful nature has inspired many arachnologists to prose. Robert Burton described their webs as “bright ephemeral lines – all length and no thickness” whilst William Bristowe described money spiders as hanging from “dew-covered hammock webs of exquisite delicacy that glisten in every hedge and even amongst the close-cut grass of a garden lawn… fine days stir their multitude into restless activity, leaving a matted sheet of intercrossing drag-lines to cloak the fields with a mantle which shimmers in the evening sunlight like a silver sea”.

Money spider webs are particularly beautiful when laced with dew.

What is it that money spiders catch in these ephemeral hammocks? I don’t think it’s much of a surprise that flies commonly feature in these still-life (or rather void-of-life) galleries. Spiders are, however, polyphagous, meaning they feed on different types of food, with very few exceptions (including one monophagous species which only feeds on a specific species of termite15). Other than flies, money spiders are known to eat springtails, flies and aphids, as well as one another16–18.

Money spiders hunt their prey using a range of techniques, including sitting and waiting for their prey to come to them (often on webs), and actively hunting them12,19. Of these, money spiders typically employ the former, sitting on their sheet webs for much of their lives, but some species are known to stretch their legs from time to time, scooping up what prey they find on the ground. Despite often waiting for their prey to come to them, there’s no rest for the wicked; spiders forage both diurnally and nocturnally to make best use of the activity periods of different prey species14.

Whilst all of this paints a picture of eternally-hungry spiders roaming the country in search of their next meal, this isn’t entirely accurate. Spiders can in fact survive very long periods of time without food by reducing their metabolism dramatically. Even when they do catch prey, they don’t necessarily eat them! Spiders employ “superfluous killing” which is effectively a nicer way of saying “killing everything and deciding what to eat later”. Once a spider kills its prey, it digests it externally by pumping it with enzymes to break it down and will sometimes never return, leaving a trail of dead insects leading to a very busy predator12,20. Not only this, but abandoned webs can remain, catching and trapping invertebrates, for some time after the spider has moved on20,21, much like a smaller-scale equivalent of “ghost-fishing”.

Ultimately, spiders consume more biomass per year than that of all humans, according to a conservative estimate2. Much of this is comprised of sometimes-beneficial insects, so you may be wondering “are spiders the terrible monsters our press and childhood stories made them out to be?” Well, this seemingly-mindless consumption of invertebrates can actually be beneficial!

Given their skills as predators, money spiders have been considered as biological control agents of cereal crops for decades12,20,22 since they eat many pests such as aphids10,23–26, planthoppers27,28, psyllids29, medflies30, lepidopterans31–33, and weevils 34. Money spiders are the most abundant cereal crop spiders26,35, which can reach densities of 200-600 per square metre in Britain26,36. In areas where these spiders are less abundant, such as North America, pest populations tend to thrive more37.

Hidden amongst crops, money spiders are natural enemies of many pests.

Whilst many predators are disrupted by crop cycling, spiders are ready for action when pest populations are first establishing themselves since they can sustain themselves on other prey12,35. Given their superfluous killing, spiders are an efficient means for eliminating many pests and by ballooning, spiders can quickly populate crop fields, providing additional biocontrol6,38,39. So the strange traits and tricks of money spiders make them the ultimate defenders of our crops!

So, what should you do next time a money spider lands on you?

Well, according to folklore, money spiders are a symbol of good fortune and you should either throw it over your shoulder, whirl it around by the web or put it in your pocket (not sure how the spider feels about this). If it dies (probably from being whirled around by you at tremendous speeds), apparently you should carry its corpse in your shoe2. Regardless of obscure money-making traditions and biocontrol potential, try to regard the invader of your summertime stroll with fascination, as this marvel of nature may have travelled an immense distance just to land on you.

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