Abstract
Orb-weaving spiders (Araneidae) are commonly regarded as generalist insect predators but resources provided by plants such as pollen may be an important dietary supplementation. Their webs snare insect prey, but can also trap aerial plankton like pollen and fungal spores. When recycling their orb webs, the spiders may therefore also feed on adhering pollen grains or fungal spores via extraoral digestion. In this study we measured stable isotope ratios in the bodies of two araneid species (Aculepeira ceropegia and Araneus diadematus), their potential prey and pollen to determine the relative contribution of pollen to their diet. We found that about 25% of juvenile orb-weaving spiders’ diet consisted of pollen, the other 75% of flying insects, mainly small dipterans and hymenopterans. The pollen grains in our study were too large to be taken up accidentally by the spiders and had first to be digested extraorally by enzymes in an active act of consumption. Therefore, pollen can be seen as a substantial component of the spiders’ diet. This finding suggests that these spiders need to be classified as omnivores rather than pure carnivores.
Highlights
Most spiders are generalist predators preying on a wide range of other arthropods [1,2], but there is evidence that resources directly provided by plants, such as pollen and nectar, may be an important dietary supplement [3,4,5,6]
Feeding experiment The number of pollen feeding events did not affect the gain in weight of the A. ceropegia spiderlings in the treatment group and treatment and control group did not differ
The feeding experiment with A. ceropegia delivered direct proof that orb-weaving spiders (Araneidae) feed on pollen captured in the sticky spirals of their webs and incorporate this into their body tissue, even when prey is available
Summary
Most spiders are generalist predators preying on a wide range of other arthropods [1,2], but there is evidence that resources directly provided by plants, such as pollen and nectar, may be an important dietary supplement [3,4,5,6].Orb-weaving spiders (Araneidae) take down and eat their webs at regular intervals, which enables them to recycle the silk proteins efficiently [7]. As the webs are a snare for arthropods and trap aerial plankton, orb-weaving spiders may feed on adhered spores and pollen when recycling their webs [4]. Smith & Mommsen [3] reported that spiderlings in a feeding experiment lived almost twice as long when fed with pollen compared to individuals fed with fungal spores or starved individuals, and about a third longer than spiders fed with aphids. They proposed that pollen might be the main food of juvenile orb-weaving spiders, with insects providing only a dietary supplement. Del Fiol et al [4] suggested that pollen grains are energetically important during spring when insects are rare
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