Harvestmen possess several chemical substances as a defence against predators. Among these, gonyleptidine has been one of the most studied and best characterized. This substance, produced by the harvestman Acanthopachylus aculeatus, is composed mainly of noxious substances such as benzoquinone. However, little is known about the secondary effects caused by the ingestion of this substance. Web-building spiders are an appropriate group to test the effects of several neurotoxic compounds, since the effects of these substances are directly reflected in their behaviour, notably in web construction. The aim of this study was to analyse the influence of the ingestion of gonyleptidine on spider web building, using as a model the orb weaver Araneus lathyrinus. We designed two experimental groups composed of subadult specimens. In one we offered larvae of Tenebrio molitor as prey for the spider; each mealworm was contaminated with 2.0 µl of gonyleptidine. In the control group the spiders were fed with uncontaminated T. molitor larvae. We found adverse effects on the group of spiders contaminated with the gonyleptidine, such as the construction of irregular webs, and loss of predatory capability. These results suggest that, when ingested, the gonyleptidine deteriorates the spider's coordination, similar to the effect of neurotoxic substances, like some pesticides. Our findings indicate that gonyleptidine has not only a repellent—defensive function but also a toxic effect on predators.