Introducing Problem Based Learning in Higher Education

Introductio

Fog-basking behaviour and water collection efficiency in Namib Desert Darkling beetles

Background: In the Namib Desert fog represents an alternative water source. This is utilised by Darkling beetles (Tenebrionidae) that employ different strategies for obtaining the fog water. Some dig trenches in the sand, while others use their own bodies as fog collectors assuming a characteristic fog-basking stance. Two beetle species from the genus Onymacris have been observed to fog-bask on the ridges of the sand dunes. These beetles all have smooth elytra surfaces, while another species with elytra covered in bumps is reported to have specialised adaptations facilitating water capture by fog-basking. To resolve if these other beetles also fog-bask, and if an elytra covered in bumps is a more efficient fog water collector than a smooth one, we examined four Namib Desert beetles; the smooth Onymacris unguicularis and O. laeviceps and the bumpy Stenocara gracilipes and Physasterna cribripes. Here we describe the beetles' fog-basking behaviour, the details of their elytra structures, and determine how efficient their dorsal surface areas are at harvesting water from fog. Results: The beetles differ greatly in size. The largest P. cribripes has a dorsal surface area that is 1.39, 1.56, and 2.52 times larger than O. unguicularis, O. laeviceps, and S. gracilipes, respectively. In accordance with earlier reports, we found that the second largest O. unguicularis is the only one of the four beetles that assumes the head standing fog-basking behaviour, and that fog is necessary to trigger this behaviour. No differences were seen in the absolute amounts of fog water collected on the dorsal surface areas of the different beetles. However, data corrected according to the sizes of the beetles revealed differences. The better fog water harvesters were S. gracilipes and O. unguicularis while the large P. cribripes was the poorest. Examination of the elytra microstructures showed clear structural differences, but the elytra of all beetles were found to be completely hydrophobic. Conclusions: The differences in fog water harvesting efficiency by the dorsal surface areas of beetles with very different elytra surface structures were minor. We therefore conclude that the fog-basking behaviour itself is a more important factor than structural adaptations when O. unguicularis collect water from fog

The night-time temporal window of locomotor activity in the Namib Desert long-distance wandering spider, Leucorchestris arenicola

Even though being active exclusively after sunset, the male Leucorchestris arenicola spiders are able to return to their point of departure by following bee-line routes of up to several hundreds of meters in length. While performing this kind of long-distance path integration they must rely on external cues to adjust for navigational errors. Many external cues which could be used by the spiders change dramatically or disappear altogether in the transition period from day to night. Hence, it is therefore imperative to know exactly when after sunset the spiders navigate in order to find out how they do it. To explore this question, we monitored their locomotor activity with data loggers equipped with infrared beam sensors. Our results show that the male spiders are most active in the period between the end and the beginning of the astronomical twilight period. Moreover, they prefer the moonless, i.e. darkest times at night. Hence, we conclude that the males are truly—and extremely—nocturnal. We further show that they are able to navigate under the very dim light conditions prevailing on moonless nights, and thus do not have to rely on the moon or on moon-related patterns of polarised light as potential compass cue

Use of local cues in the night-time navigation of the wandering desert spider Leucorchestris arenicola (Araneae, Sparassidae)

Adult male Leucorchestris arenicola can walk round-trips of several tens of meters in search of females. Most excursions end with the spiders returning to their burrow. For small animals homing over distances of several meters is theoretically impossible without the aid of external cues. It was investigated, whether the spiders use local cues or they rely solely on global cues. Individually marked male spiders were captured during their excursions and displaced several meters inside an opaque box. Ten out of twelve displaced spiders returned to their burrows. This shows that the male L. arenicola are using local cues during their homing, as the comparatively small displacement distances could not be detected by means of global, e.g. celestial cues. In order to test whether the spiders could be using olfactory guidance, the burrows were displaced by 2m while the spiders were out on their journeys. In 12 out of 15 experiments, the spiders did not find their burrows. These results show that the burrows do not function as olfactory beacons for the homing spider