Humidity‐dependent mechanical and adhesive properties of Arachnocampa tasmaniensis capture threads

Abstract

Bioluminescent glow‐worms (Arachnocampa spp.) capture prey in glue‐coated silk capture threads hung from their nests on damp cave and wet forest substrates. In a dry environment, these animals are very susceptible to desiccation as their bodies can become life threateningly dry and their silk has been anecdotally observed to become non‐sticky. Water has a plasticizing effect on the structural proteins of several invertebrate silks, including those used in caddisfly nets, mussel byssus and spider webs. Moreover, water facilitates interfacial adhesion by spreading adhesive biomolecules in functionally analogous velvet worm slime and spider silk glue. We tested the effects of water on the mechanics and adhesion of Arachnocampa tasmaniensis capture threads sampled within damp caves. We found that threads tested at high humidity were three times more compliant and over 10‐fold more extensible than those tested at low humidity (30% RH). We also found the threads to be significantly more adhesive in high humidity with force at detachment increasing two orders of magnitude and work of adhesion increasing by five orders of magnitude compared to threads tested at low humidity. Our results unequivocally demonstrate that A. tasmaniensis capture thread functionality is dependent upon exposure to high humidity. Our results both confirm previous reports and indicate that the foraging habitat of these animals is restricted to caves and cave‐like environments, such as wet forests.

Biomechanical properties of fishing lines of the glowworm Arachnocampa luminosa (Diptera; Keroplatidae)

“…The collected fishing line of A . luminosa displays a slightly, but not significantly, higher extensibility than dry threads; visual observations confirm that dehydration also results in a loss of flexibility, as mentioned previously 17,31,33 .…”

“…The tensile strength in A . tasmaniensis is much higher (122.77 MPa at 90% RH; 159.04 at 30% RH) 33 than that measured for A . luminosa (from 1.9 MPa at RH = 100% to 4.6 MPa at RH = 60%).…”

“…The egg stalks have much higher extensibility (4–6 at RH = 100% and 2.5 at RH = 65% 39,40 ) and tensile strength (232 MPa at RH = 100% to 70 MPa at RH = 30%) 39 than A . luminosa (present study) and Arachnocampa tasmaniensis 33 , probably because “glow-worm silk is likely to have a substantially lower crystalline fraction, compared to either silkworm silk or lacewing egg-stalks” 31 . However, the differences in the strain and tensile strength between hydrated and dry C .…”

“…tasmaniensis were affixed for air transport with wood working glue on a paper frame, stored for a long time under dry conditions and segments of these samples were re-hydrated for a short period at 90% RH before the test series 33 . The authors, however, assure that their sample handling had no influence on the mechanical properties 33 . In the present study in A .…”

“…Ex situ studies of the Arachnocampa tasmaniensis fishing lines indicate that beside the adhesive fluid changes the silk threads also change their mechanical properties from high to low humidity conditions. Under dry conditions (<30%RH), the threads exhibit a higher Young’s modulus (18.38 GPa), while the breaking strain (from 0.02 to 0.6 ln (mm mm −1 )) and toughness (from 2.4 to 25.82 MJ m −3 ) both increase, when tested under high humid conditions (>90%RH) 33 . Unfortunely, 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 .…”