Abstract

The opisthosoma of pseudoscorpions carries 12 dorsal and 11 ventral sclerites. The external genital opening is between first and the second ventral sclerite, a position that contrasts to the arachnid body organization in which the reproductive organs always open on/behind the second opisthosomal segment. The mismatch of dorsal and ventral opisthosomal sclerites, and the unconventional position of the genital opening traditionally have been explained by the loss of the ventral sclerite of the first opisthosomal segment, so that the morphologically second sclerite becomes the first visible on the ventral side of the opisthosoma, and a mismatch between dorsal and ventral sclerites emerges. We study the serial pattern of opisthosomal musculature to analyze the segmental (re)organization of the opisthosoma in two species of pseudoscorpions. We use micro-computed tomography (?CT) and light microscopic serial sections to reconstruct the segmental musculature. We test five explicit hypotheses about changes in the pattern of musculature that are predicted from the possible changes in sclerite morphology. By analyzing the serial pattern of dorso-ventral and intersegmental musculature including their origin and insertion, we document borders between body tagmata and segments, and assign dorsal and ventral sclerites to specific opisthosomal segments. The results of the study offer an interpretation of the muscle topography that is in contrast to the current textbook paradigm, i.e., the first opisthosomal sternite being reduced. In contrast, our results on the segmental musculature of the opisthosoma support the idea that the first opisthosomal segment carries dorsal and ventral sclerites. The second opisthosoma segment carries a dorsal sclerite, but its ventral sclerite is internalized and forms the cuticular wall of the genital atrium. The third opisthosoma segment and all following segments possess dorsal and ventral sclerites.