The homology of the arachnid chelicera with respect to other head appendages in Panarthropoda has long been debated. Gene expression data and the re-interpretation of early transitional fossils have supported the homology of the deutocerebrum and its associated appendages, implying a homology between primary antennae (mandibulates), chelicerae (euchelicerates), and chelifores (sea spiders). Nevertheless, comparatively little is known about the mechanistic basis of proximo-distal (PD) axis induction in chelicerates, much less the basis for cheliceral fate specification. Here, we describe a new cheliceral teratology in the spider Tetragnatha versicolor Walckenaer, 1841, which consists on a duplication of the PD axis of the left chelicera associated with a terminal secondary schistomely on the fang of the lower axis. This duplication offers clues as to potential shared mechanisms of PD axis formation in the chelicera. We review the state of knowledge on PD axis induction mechanisms in arthropods and identify elements of gene regulatory networks that are key for future functional experiments of appendage development in non-insect model systems. Such investigations would allow a better understanding of PD axis induction of modified and poorly studied arthropod limbs (e.g., chelicerae, chelifores, and ovigers).

蜘蛛纲螯肢动物与泛节肢动物其他头部附肢的同源性长期以来一直存在争议。基因表达数据和对早期过渡化石的重新解释支持了后大脑及其相关附肢的同源性，这意味着初级触角（下颌）、螯肢动物（euchelicerates）和螯肢动物（海蜘蛛）之间存在同源性。然而，人们对螯合物近远端（PD）轴诱导的机制基础知之甚少，更不用说对螯合物命运规范的基础了。在这里，我们描述了蜘蛛Tetragnatha versicolor Walckenaer, 1841 的一种新的螯肢畸胎学，其由左螯肢 PD 轴的重复与下轴尖牙上的末端次级血裂相关。这种重复为螯肢中 PD 轴形成的潜在共享机制提供了线索。我们回顾了节肢动物 PD 轴诱导机制的知识状况，并确定了基因调控网络的要素，这些要素对于未来非昆虫模型系统中附肢发育的功能实验至关重要。此类研究将有助于更好地了解经过修饰且研究较少的节肢动物肢体（例如螯肢动物、螯肢动物和产卵动物）的 PD 轴诱导。