Differences in the physiological properties of the median and the lateral eyes and their possible meaning for the entrainment of the scorpion's circadian rhythm

Non-visual homing and the current status of navigation in scorpions

Within arthropods, the investigation of navigational aspects including homing abilities has mainly focused on insect representatives, while other arthropod taxa have largely been ignored. As such, scorpions are rather underrepresented concerning behavioral studies for reasons such as low participation rates and motivational difficulties. Here, we review the sensory abilities of scorpions related to navigation. Furthermore, we present an improved laboratory setup to shed light on navigational abilities in general and homing behavior in particular. We tracked directed movements towards home shelters of the lesser Asian scorpion Mesobuthus eupeus to give a detailed description of their departure and return movements. To do so, we analyzed the departure and return angles as well as measures of directness like directional deviation, lateral displacement, and straightness indices. We compared these parameters under different light conditions and with blinded scorpions. The motivation of scorpions to leave their shelter depends strongly upon the light condition and the starting time of the experiment; highest participation rates were achieved with infrared conditions or blinded scorpions, and close to dusk. Naïve scorpions are capable of returning to a shelter object in a manner that is directionally consistent with the home vector. The first-occurring homing bouts are characterized by paths consisting of turns about 10 cm to either side of the straightest home path and a distance efficiency of roughly three-quarters of the maximum efficiency. Our results show that neither chemosensation nor vision, but rather path integration based on proprioception, plays a superior role in the homing of scorpions.

Nonvisual photoreceptors in arthropods with emphasis on their putative role as receptors of natural Zeitgeber stimuli

In various insect and arachnid species, three different types of photoreceptors that do not serve image processing have been discovered and analyzed by means of neurobiological methods: They can be found for example: (1) as lamina and lobula organs (LaOs and LoOs) next to the optic neuropils in the optic lobes of holo- and hemimetabolous insects; (2) inside the last ganglia of the cord of the scorpion and a marine midge; and (3) as modified visual photoreceptors in metamorphosized larval stemmata and the lateral eyes of scorpions, which have been compound eyes in fossil scorpion relatives. Imnunocytology with various antibodies against proteins of the phototransduction cascade, the rhabdom turnover cycle and neurotransmitters of afferent and efferent pathways, was combined with light- and ultrastructural investigations in well-defined adaptational states, in order to study their photoreceptive function and neuronal wiring. Pilot chronobiological experiments with a newly developed twilight simulating lamp, behavioral studies, and model calculations provide evidence that these photoreceptors may well serve a role in the complex task of detecting time cues out of natural dawn and dusk.

Circadian rhythms and time measurement in locomotor activity of the scorpion Leiurus quinquestriatus (Buthidae)

The circadian rhythms of locomotor activity of the scorpion Leiurus quinquestriatus were examined under different light-dark cycles and in free-running conditions. The circadian rhythm is bimodal in LD 12:12 with alternating cycles of temperature (35 degrees-25 degrees C) with high intensity (1300 lux) or in LD 12:12 with constant temperature 35 degrees C with 300 lux. In LD 12:12 (1300 lux), in long or in short light spans with constant temperature, the bimodal pattern is slightly changed with the appearance of a third minor peak of activity. In free-running conditions, the bimodal rhythm of locomotor activity persists in DD with T about 24 hr, but in LL the rhythm becomes unimodal with T about 24 hr. Cosinor and power spectrum analysis showed the presence of more than one periodic component. It seems that there is a correlation between the range of light regimens, temperature, light intensity and the coincidence of these components. These components are independently entrained by the environmental light cycle. The mechanism of entrainment of components is discussed.