Is there correlation between photophobia and troglomorphism in Neotropical cave millipedes (Spirostreptida, Pseudonannolenidae)?

Absence of light, typical of the subterranean environment can lead to the loss of synchronizers of physiological and behavioral activities, mainly those related to photoperiods and light perception. Cave-dwelling species present phototactic responses that vary from photonegative to photopositive ones. There are few studies focusing on reaction to light in millipedes and the most studied species are considered photophobic. In this study, we analyzed the phototactic response of cave-dwelling and epigean millipedes of the genus Pseudonannolene (Spirostreptida, Pseudonannolenidae). For this, the chamber choice method was used. We related the phototatic reactions to morphological characters that indicate troglomorphisms (e.g., eyes and melanic pigmentation). The eight-studied operational taxonomic unit (OTU) showed photophobic reaction to all light intensities tested (150, 500 and 1000 lx) and photophobia can be considered a plesiomorphic character in the group. The morphological analysis showed that the light response is negatively correlated to the number of ommatidia and ocular area, that is, the larger the area and the number of ommatidia, the more negative is the light response (photonegative), suggesting that the eyes are the main sensory organs in the perception of light and, apparently, the integument of Pseudonannolene is not related to light sensitivity, opposite to that observed for other millipede species.