Abstract
Animals use adhesive secretions in highly diverse ways, such as for settlement, egg anchorage, mating, active or passive defence, etc. One of the most interesting functions is the use of bioadhesives to capture prey, as the bonding has to be performed within milliseconds and often under unfavourable conditions. While much is understood about the adhesive and biomechanical properties of the threads of other hunters such as spiders, barely anything is documented about those of the New Zealand glowworm Arachnocampa luminosa. We analysed tensile properties of the fishing lines of the New Zealand glowworm Arachnocampa luminosa under natural and dry conditions and measured their adhesion energy to different surfaces. The capture system of A. luminosa is highly adapted to the prevailing conditions (13–15 °C, relative humidity of 98%) whereby the wet fishing lines only show a bonding ability at high relative humidity (>80%) with a mean adhesive energy from 20–45 N/m and a stronger adhesion to polar surfaces. Wet threads show a slightly higher breaking strain value than dried threads, whereas the tensile strength of wet threads was much lower. The analyses show that breaking stress and strain values in Arachnocampa luminosa were very low in comparison to related Arachnocampa species and spider silk threads but exhibit much higher adhesion energy values. While the mechanical differences between the threads of various Arachnocampa species might be consequence of the different sampling and handling of the threads prior to the tests, differences to spiders could be explained by habitat differences and differences in the material ultrastructure. Orb web spiders produce viscid silk consisting of β-pleated sheets, whereas Arachnocampa has cross-β–sheet crystallites within its silk. As a functional explanation, the low tear strength for A. luminosa comprises a safety mechanism and ensures the entire nest is not pulled down by prey which is too heavy.
Highlights
Glowworms are the larval luminescent stage of certain species of keroplatid flies
This study provides the first description of the biomechanical properties of the fishing lines of Arachnocampa luminosa larvae in situ and details on their bonding strength to materials with different surface energies
The collected fishing line of A. luminosa displays a slightly, but not significantly, higher extensibility than dry threads; visual observations confirm that dehydration results in a loss of flexibility, as mentioned previously[17,31,33]
Summary
Glowworms are the larval luminescent stage of certain species of keroplatid flies. The genus Arachnocampa[10], contains the world-renowned “glow-worms” endemic to New Zealand and Australia[6,11,12,13,14] and known for its members’ ability to capture prey by means of adhesive threads in combination with bioluminescent lures[13,15,16]. A midge, Anatopynia debilis (=Tanypus debilis, Chironomidae), seems to be the main food source for glowworms in the Waitomo caves[17] Larvae of this midge live in the mud banks and streams within the caves and are present at high density from October to December after dawn[17]. Other larger insects, such as Hymenoptera (bees and wasps) and Coleoptera (beetles), are caught occasionally in the fishing lines[25]. Ex situ studies of the Arachnocampa tasmaniensis fishing lines indicate that beside the adhesive fluid changes the silk threads change their mechanical properties from high to low humidity conditions. In this study the authors used long-term stored dried thread fragments (1 cm length), which were exposed to high humidity conditions just prior to mechanical tests[33]
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