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, 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 conditions^5,6. More recently, it has been discovered that ballooning is triggered by natural electric fields⁷. Like many humans though, this wanderlust wanes in later life, ballooning being 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 differing between species^8,9. For example, spiders such as Erigone atra will often build small webs close to the ground which they can abandon regularly to hunt on the ground, whilst other spiders like Tenuiphantes tenuis produce larger webs slightly further 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 a particular type of prey is abundant, 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 covet one another’s beautiful gardens of tidily-kept hydrangeas and lawn ornaments, spiders suffer web-envy and territorial contests can ensue^8,12–14.

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”.

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 and aphids, as well as one another^16–18.

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 them^12,19. Of these, most will 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. The spider will sometimes never return, leaving a trail of dead insects to a very busy predator^12,20. Not only this, but abandoned webs can remain in place, catching and trapping invertebrates for some time after the spider has moved on^20,21, much like a smaller-scale equivalent of “ghost-fishing”.

Ultimately, spiders consume more biomass per year than that of 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 biological control agents for cereal crops for decades^12,20,22 since they eat many pests such as aphids^10,23–26, planthoppers^27,28, psyllids²⁹, medflies³⁰, lepidopterans^31–33, and weevils ³⁴. Money spiders are the most abundant cereal crop spiders^26,35, which can reach densities of 200-600 per square metre in Britain^26,36. In areas where these spiders are less abundant, such as North America, pest populations tend to get off lightly, at least relatively speaking³⁷.

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 prey^12,35. Given their superfluous killing, spiders are an efficient means for eliminating many pests and, by ballooning, spiders can quickly populate crop fields, increasing their biocontrol potential through these parachuting reinforcements^6,38,39. 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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