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Spiders are diverse, predatory arthropods that have inhabited Earth for around 400 million years. They are well known for their complex venom systems that are used to overpower their prey. Spider venoms contain many proteins and peptides with highly specific and potent activities suitable for biomedical or agrochemical applications, but the key role of venoms as an evolutionary innovation is often overlooked, even though this has enabled spiders to emerge as one of the most successful animal lineages. In this review, we discuss these neglected biological aspects of spider venoms. We focus on the morphology of spider venom systems, their major components, biochemical and chemical plasticity, as well as ecological and evolutionary trends. We argue that the effectiveness of spider venoms is due to their unprecedented complexity, with diverse components working synergistically to increase the overall potency. The analysis of spider venoms is difficult to standardize because they are dynamic systems, fine‐tuned and modified by factors such as sex, life‐history stage and biological role. Finally, we summarize the mechanisms that drive spider venom evolution and highlight the need for genome‐based studies to reconstruct the evolutionary history and physiological networks of spider venom compounds with more certainty.

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Experimental work in the early 20th century showed that background albedo experienced by larvae of the fire salamander (Salamandra salamandra) induce a durable morphological modification of the postmetamorphic color pattern, which needed confirmation due to the controversies regarding Paul Kammerer's experiments. Such a carry‐over effect would be relevant as the black and yellow pattern of the alkaloid‐containing adult fire salamanders has been suggested to serve as an aposematic signal. Hence, we hypothesized that (a) adult coloration is conspicuous to potential predators under light conditions at night, given the nocturnal activity of this species, and (b) a condition affecting the salamander's coloration pattern would also affect its toxicity to maintain a quantitatively honest aposematic signal. To test the first hypothesis, we used spectrometry to model the vision of potential avian and snake predators and confirmed that fire salamander's black‐and‐yellow pattern is contrasting enough against the forest leaf litter to be considered conspicuous at night. To test the second hypothesis, we first confirmed the background carry‐over effect on black and yellow proportions in the dorsal skin of experimentally reared fire salamanders, using a rigorous experimental design. Then, we calculated the conspicuousness and determined the alkaloid profiles of these individuals. We did not find a correlation between conspicuousness and toxicity at the intrapopulation level. Moreover, there was no background carry‐over effect on the alkaloid profile. We discuss our results in a physiological, ecological, evolutionary, and historical context.