Sombke Andy, Klann Anja E, Lipke Elisabeth, Wolf Harald
1University of Vienna, Department of Integrative Zoology, Althanstrasse 14, 1090 Vienna, Austria.
2Department of Forensic Molecular Genetics, University Medicine Greifswald, Institute of Legal Medicine, 17489 Greifswald, Germany.
Zoological Lett. 2019 Aug 2;5:26. doi: 10.1186/s40851-019-0137-z. eCollection 2019.
Arachnids possess highly specialized and unorthodox sense organs, such as the unique pectines of Scorpiones and the malleoli of Solifugae. While the external morphology, numbers, and shapes of sensory organs are widely used in taxonomic studies, little is known about the internal anatomy of these organs and their associated processing neuropils in the central nervous system. Camel spiders (Solifugae) possess pedipalps and first walking legs heavily endowed with sensory structures, as well as conspicuous malleoli located ventrally on the proximal fourth walking legs. Malleoli are fan-shaped organs that contain tens of thousands of presumptive chemoreceptor neurons, but mechanoreceptive structures are absent.
Here, we examine the organization of the synganglion based on microCT analysis, 3D reconstruction of serial paraffin sections, and backfill preparations to trace the malleolar pathway. The projection area of malleolar afferents is intriguingly located in the most anterior ventral nerve cord, located in between the pedipalpal neuromere hemispheres. However, malleolar axon bundles are separated by a thin soma layer that points to an anteriad projection of the fourth walking leg neuromere. A conspicuous projection neuron tract that may receive additional input from pedipalpal sensory organs connects the malleolar neuropil with the mushroom bodies in the protocerebrum.
Arthropod chemosensory appendages or organs and primary processing neuropils are typically located in the same segment, which also holds true in Solifugae, although the malleolar neuropil is partially shifted towards the pedipalpal neuromere. A comparison of the malleoli in Solifugae and the pectines in Scorpiones, and of their primary processing neuropils, reveals certain similarities, while striking differences are also evident. Similarities include the ventral arrangement of peg-shaped sensory structures on the respective segmental appendage, exposing dense arrays of chemoreceptive sensilla, and projections to a primary processing neuropil with glomerular subdivision. Differences are, e.g., the lack of mechanoreceptive afferents and an associated processing neuropil.
蛛形纲动物拥有高度特化且非传统的感觉器官，如蝎目的独特栉器和避日目的听毛。虽然感觉器官的外部形态、数量和形状在分类学研究中被广泛应用，但对于这些器官的内部解剖结构及其在中枢神经系统中相关的处理神经纤维网却知之甚少。骆驼蜘蛛（避日目）的触肢和第一对步足大量分布着感觉结构，并且在第四对步足近端腹侧有明显的听毛。听毛是扇形器官，包含数万个推测的化学感受器神经元，但没有机械感受器结构。
在此，我们基于显微CT分析、连续石蜡切片的三维重建以及反向填充制备来追踪听毛通路，研究神经节的组织结构。听毛传入纤维的投射区域位于最前端腹神经索，处于触肢神经节半球之间。然而，听毛轴突束被一层薄的体细胞层隔开，这表明第四对步足神经节有向前的投射。一条明显的投射神经元束可能从触肢感觉器官接收额外输入，将听毛神经纤维网与前脑的蘑菇体相连。
节肢动物的化学感觉附肢或器官以及初级处理神经纤维网通常位于同一节段，避日目动物也是如此，尽管听毛神经纤维网部分向触肢神经节偏移。对避日目的听毛和蝎目的栉器及其初级处理神经纤维网进行比较，发现了某些相似之处，同时也存在显著差异。相似之处包括在各自节段附肢上钉状感觉结构的腹侧排列，暴露密集的化学感受性感器阵列，以及向具有肾小球细分的初级处理神经纤维网的投射。差异例如缺乏机械感受器传入纤维和相关的处理神经纤维网。
