A Study on Spiders

Do female Araneus diadematus spiders exhibit selective predation?

European Garden spider - Araneus diadematus

On the easterly tip of a small reservoir next to the university campus down here in Cornwall, a mass of knotted Bramble creates a habitat to a range of invertebrate species. At least 5 Shieldbug species (Pentatomidae) have been spotted, Thistle Tortoise beetles, as well as a host of solitary bees, bumbles, wasps and hoverflies. The arachnid contingent are also present with numerous Tertragnathidae sp., Wolf spiders (Lycosidae sp.), Crab spiders (Misumeninae) such as the beautiful misumena vatia, as well as Nursery Web spiders and European Garden spiders (Araneus diadematus).

College Lake

Gorse Shieldbug - Piezodorus lituratus 

Nursery Web Spider - Araneus diadematus

Misumena vatia

The last species in this list, the European Garden spider, is a fairly large spider - female body size varying in my study between 6mm and 13mm. They exhibit sexual dimorphism with the males being significantly smaller. Males are smaller due to the fact they have to rove long distances to find a mate and also potentially to look less appetising to females who are occasionally cannibalistic if the mood takes them. The more sedentary females build large orb webs between the bramble stems and sit in the middle, waiting for their prey to unwittingly join them.

Female Araneus diadematus

Interestingly, I noticed that although the spider was present in the web, the majority of webs had a number of prey items that had been left untouched. My mind mulled over this observation for a few days and eventually came up with this thought: does the amount of prey items left untouched vary with spider size and whether the spiders display a form of selective predation.

Selective predation is present in large apex predators such as Wolves and Jaguars. The predator will elect a prey individual that is weaker than the others, meaning the predator will expend the minimum amount of energy catching it.

A female A. diadematus preying on a Diptera sp. fly

Female Garden spiders need to obtain enough nutrients to ensure that they can produce enough proteins needed in reproduction. The females will produce 300-900 eggs and will continue to care for them until they hatch and balloon away. This is a semelparous (one large reproductive event in life history) event and so all of the female’s energy goes into providing her young with enough nutrients. She dies shortly after her children have floated off.

To begin to answer my question, I chose female spiders which had an intact web and were of a range of sizes, measured from tip of the chelicerates to the spinnerets. For each of the spiders found I would measure their body size before counting the number of prey items (predominantly Midges) left untouched in the web. As the graph shows, there was strong correlation between the size of the spider and the number of prey items left untouched. Smaller spiders left fewer prey items (avg 3.1) while larger spiders left far more (avg 24.2). I also measured the web size of every few spiders to see if that had an effect but that value seem to have very little correlation with that of spider size, the smallest spiders having a similar web size to the largest ones.

Araneus diadematus wrapping up a Chorthippus parallelus

While watching the spiders, a few of the small midges flew into the webs but the spider would ignore them entirely. However, on one occasion a Meadow Grasshopper (Chorthippus parallelus) hopped accidentally into the web of a large female A. diadematus, she darted across the web with great pace and proceeded to rapidly encompass the poor grasshopper in her silk. Once wrapped, the paralysed grasshopper was left to contemplate his fate.

Trapped Meadow grasshopper Chorthippus parallelus

The prey type that I found most frequently wrapped in webs seemed to be hoverflies or diptera sp. flies.

My study seems to suggest that the reason that more food is left untouched in larger A diadematus webs is down to energy content. A 1mm midge always contains a set amount of energy but as a percentage this energy is worth different amounts to spiders of different sizes. If the midge is 10% of the energy content a 6mm spider needs in a day then it is 5% of what a 12mm spider needs. As the spider requires energy to produce the silk needed to wrap the prey and also to produce and secrete the digestive enzymes, if the prey item is below an energy percentage threshold then it will not be worthwhile for the spider to consume.

The spiders studied seemed to exhibit selective predation, choosing which prey items they consumed based on their energy content. This selective predation would be a great advantage to the female spider as less energy would be lost and, therefore, more is available for reproductive purposes.

Further studies are definitely needed to test my 'energy content of prey items' hypothesis (i.e measuring energy content of midges and working up to other, wrapped, prey items).

Potential implications of this study include more efficiently utilising spiders as biological pest controls. Assuming spiders of all sizes catch a similar amount of prey (due to similar web sizes) the smaller spider’s webs may be more effective as they remain ‘invisible’ for longer as the spider removes more obvious prey items than the larger spiders. 

But for now, the spiders are left to go about their normal lives. Peacefully inhabiting a Cornish bramble thicket.