Orb-web, no web: unusual mating behaviours in an orb-web spider
Willmott, Nikolas (2022), Orb-web, no web: unusual mating behaviours in an orb-web spider, Dryad, Dataset, https://doi.org/10.5061/dryad.ngf1vhhz0
The evolution of the orb-web was associated with a major radiation in spider diversity. The major functions of orb-webs are prey capture and as substrates for courtship and mating. However, the use of orb-webs has associated costs, and the modification and loss of orb-webs has evolved multiple times. While variation between species is evident, such as reductions in or loss of the orb-web for foraging, this kind of variation within species is rarely seen. Here, I describe laboratory observations of foraging and mating without an orb-web in a typical orb-weaving spider, the Australian garden orb-weaver (Hortophora biapicata). I discuss these behaviours, which are likely cases of opportunistic plasticity, in an ecological and evolutionary context. Further investigation of these rare and unusual behaviours may provide unique insights into the function and origin of important traits associated with the orb-web, and the evolution of extended phenotypes.
Over the course of several experiments between 2016 and 2022, I have maintained laboratory populations of H. biapicata. I kept juvenile and adult spiders in inverted plastic cups (diameter 9cm, height 15cm) and Perspex frames (25 cm × 25 cm × 10 cm for juveniles, 58 cm × 58 cm × 15 cm for adults) lined internally with a strip of masking tape, to allow silk attachment for web building. Some spiders were reared in cups from hatching until maturity, at which point females were transferred to Perspex frames for web building. Other spiders were collected as late-stage juveniles (1 – 4 instars away from maturity), and kept in cups until maturity.
I fed juvenile spiders with vinegar flies (Drosophila melanogaster), and adults with house flies (Musca domestica) and house crickets (Acheta domesticus). Adults did not construct webs when kept in cups due to insufficient space for web construction, and were instead provided with live crickets, live adult house flies, or live house fly pupae. Crickets were hand-fed to spiders by grasping the crickets with forceps and gently pushing them into the chelicerae of the spiders. In some cases, a live cricket was placed in the cup rather than being hand-fed to the spider. Pupae were placed on the shelf underneath the cup, such that the adult flies would emerge from the pupae and be captured by the adult spiders. Adult females usually constructed complete orb-webs when placed in a Perspex frame, at which point they were fed by live crickets, live adult house flies, or live house fly pupae that were placed in the web.
To facilitate mating, I allowed a mature female to construct a web in a large Perspex frame, then introduced a mature male into the bottom corner of the frame. Males then identified the web of the female and initiated the courtship procedure. During a mating experiment in early 2022, 12 / 38 (32%) females did not construct webs in their frames over a two-month period, despite the introduction of live flies to stimulate web building. These females instead constructed errant strands of silk, mostly in the top half of the frame (Fig. 1). Regardless, for these females, I introduced a male into one bottom corner of the frame and recorded the subsequent courtship behaviours – time from introduction to first physical contact between male and female, time from first physical contact to initiation of copulation, and duration of copulation. For each of these response variables, I constructed general linear models in R (R Core Team 2017) with four predictor variables: web status (whether the female had constructed an orb-web or not), difference in body mass between male and female, exposure to artificial light at night or not, and diet quality (high or low). Across all mating trials, there was no difference in female age between web and no web females (t-test: P = 0.2).