Masquerading predators deceive prey by aggressively mimicking bird droppings in a crab spider

In aggressive mimicry, a predator accesses prey by mimicking the appearance and/or behavior of a harmless or beneficial model in order to avoid being correctly identified by its prey. The crab spider genus Phrynarachne is often cited as a textbook example of masquerading as bird droppings (BDs) in order to avoid predation. However, Phrynarachne spiders may also aggressively mimic BDs in order to deceive potential prey. To date, there is no experimental evidence to support aggressive mimicry in masquerading crab spiders; therefore, we performed a field survey, a manipulative field experiment, and visual modeling to test this hypothesis using Phrynarachne ceylonica. We compared prey-attraction rates among BDs, spiders, and control empty leaves in the field. We found that although all prey combined and agromyzid dipterans, in particular, were attracted to BDs at a higher rate than to spiders, other dipterans and hymenopterans were attracted to BDs at a similar
rate as to spiders. Both spiders and BDs attracted insects at a significantly higher rate than did control leaves. As predicted, prey was attracted to experimentally blackened or whitened spiders significantly less frequently than to unmanipulated spiders. Finally, visual modeling suggested that spiders and BDs can be detected by dipterans and hymenopterans against background leaves, but they are indistinguishable from each other. Taken together, our results suggest that insects lured by spiders may misidentify them as BDs, and bird-dropping masquerading may serve as aggressive mimicry in addition to predator avoidance in P. ceylonica.

在侵略性拟态中，捕食者通过模仿无害或有益模型的外貌及/或行为来接近猎物，以避免被猎物正确识别。蟹蛛属中的 Phrynarachne 常被引用为伪装成鸟粪（BDs）以避免捕食的经典实例。然而，Phrynarachne 蜘蛛也可能通过侵略性地模仿 BDs 来欺骗潜在的猎物。迄今为止，尚无实验证据支持伪装蟹蛛的侵略性拟态；因此，我们进行了现场调查、操纵性现场实验和视觉建模，利用 Phrynarachne ceylonica 来验证这一假设。我们在野外比较了 BDs、蜘蛛和控制空白叶片对猎物的吸引力。我们发现，尽管所有猎物，尤其是Agromyzid双翅目昆虫，对 BDs 的吸引力高于对蜘蛛的吸引力，但其他双翅目昆虫和膜翅目昆虫对 BDs 的吸引力与对蜘蛛的吸引力相似。蜘蛛和 BDs 吸引昆虫的频率显著高于控制叶片。正如预期的那样，实验中变黑或变白的蜘蛛被吸引的频率显著低于未处理的蜘蛛。最后，视觉建模表明，蜘蛛和 BDs 在背景叶片中可被双翅目昆虫和膜翅目昆虫检测到，但它们之间无法区分。综合来看，我们的研究结果表明，被蜘蛛诱捕的昆虫可能会误将它们识别为 BDs，而在 P. ceylonica 中，鸟粪伪装可能除了作为捕食者回避策略外，还充当了侵略性拟态的作用。