Abstract

Eukoenenia spelaea is a troglobiont palpigrade found in caves of the European Alps. These small animals have a maximum body length of 1.5 mm without the characteristic terminal flagellum. They lack eyes and breathing organs but have unique sensory organs. Detailed morphological studies of Palpigradi date back the late 19th and early 20th century. The placement of Palpigradi within a morphology based phylogeny of Euchelicerata is difficult. Data on microscopic anatomy and comparative morphology is incomplete, they show numerous plesiomorphic features, and they are so small, resulting in reduction, simplification and loss of organ systems. In this study, I analyze the microscopic anatomy of Eukoenenia spelaea, present evidence that progenesis is the developmental mechanism resulting in a paedomorphic adult morphology and miniaturized size, and discuss the results in a phylogenetic framework. I used serial sectioning for light microscopy as well as transmission and scanning electron microscopy to describe the microscopic anatomy. The morphological analysis comprised all organs and external structures. Several new morphological features were described which are unique for Eukoenenia spelaea. For other structures I provide evidence that allow for new interpretations in the light of evolutionary morphology. (1) The prosoma is dorsally divided into two prosomal shields, the pro- and metapeltidium. (2) The ventral plate is probably an osmoregulatory organ. (3) The prosternum consists of the fused sternites of segments 2–4. (4) The frontal organ and trichobothria were found to have a morphology which is unique within euchelicerates. (5) The esophagus is enveloped by the supraesophageal ganglion associated with the chelicerae. (6) The heart lacks ostia, a pericard, a nerve, and well developed musculature. (7) The rostrosoma is not associated with chelicerae or pedipalps. (8) The midgut is simple and sac-like, a hindgut is missing. (9) The coxal gland (tubule and glandular section) has no lumen. (10) Females have an unpaired ovary with only few large eggs. (11) The aflagellate sperm has a prominent vacuole. The morphological results were fed into a phylogenetic analysis. Several autapomorphic characters were recognized to characterize Palpigradi (i.e. the dorsal division of the prosoma into pro- and metapeltidium, the frontal organ, and the rostrosoma with no association with chelicerae or pedipalps). A phylogenetic sister group relationship was found with Acaromorpha with which they share the morphology of the leg musculature and joints, the opening of the coxal organ on leg 1, the lack of a postcerebral suction pump, a myogenic heart, and vacuolated sperm. This phylogenetic position suggests that the common ancestor of Palpigradi and Acaromorpha was already small. The small body size of Eukoenenia spelaea (and probably all Palpigradi) resulted in reduction or even loss of structures like musculature, and the complete lack of breathing organs and Malpighian tubules. Some organs showed a typical paedomorphotic morphology which was interpreted as the result of progenetic development, e.g. the microscopic anatomy of the heart, the brain, and the midgut. These findings suggest that E. spelaea is miniaturized.