Same same but different: a stunning analogy between tracheal and vascular supply in the CNS of different arachnids

Described herein is an as yet unprecedented structural and functional analogy of both the tracheal supply of the prosomal ganglion in opilionids and the arterial supply of the prosomal ganglion in pulmonate arachnids. Within Arachnida, two different modes of respiration can be observed: the so-called book lungs, and the tube-like tracheae. These different respiratory modes always correlate with a specific setup concerning the complexity of the circulatory system. This fact has a particular influence on the supply of certain organ systems, such as the central nervous system. It has recently been shown that pulmonate arachnids possess a highly complex pattern of intraganglionic arteries. Here, we show that Opiliones (harvestmen) possess a complex tracheal system (which supplies the different organ systems with oxygen) and only a relatively simple vascular system, comprising a short heart and an anterior aorta that runs directly to the prosomal ganglion. Using a variety of modern and classical morphological methods, we studied the vascular, tracheal and nervous systems of different representatives from all higher taxa of Opiliones. We show that the prosomal ganglion is extensively supplied with intraganglionic tracheae. What is especially surprising is the high degree of correspondence between the pattern of these ganglionic tracheae in harvestmen and the pattern of arteries in the prosomal ganglion of pulmonate arachnids. We aim to provide mechanistic causal explanations of these analogous patterns by applying the concepts of role analogy and constructional analogy. We also aim to establish the circulatory system as a model organ system and hope that this may, in turn, provide a starting point for future research programmes.