DOI: 10.5061/dryad.ngf1vhhz0
title: Orb-web, no web: unusual mating behaviours in an orb-web spider

description: 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.
description: 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).