Starving infecund widow spiders maintain sexual attractiveness and trade off safety for enhanced prey capture

Starving animals must balance their resources between immediate survival and future reproduction. False widow spiders, Steatoda grossa, inhabit indoor settings with scarce prey. Here, we investigated the effects of lengthy starvation on the physiology, web architecture, sexual signaling, and reproductive success of S. grossa females. Compared to well-fed females, starving females (1) lost body mass faster, (2) had lower survival, (3) produced more silk for prey capture than for safety, and (4) deposited less contact pheromone components on their webs but accelerated their hydrolysis to mate-attractant components. As starving females became infecund - but still attracted and copulated with males - they misguided males that would gain reproductive fitness by selecting fecund females. Whether starving females store sperm and potentially regain fecundity upon feeding is still unknown. Our study shows how prey shortage shapes sexual signaling, predation, and reproductive behavior of S. grossa females that seem to engage in deceptive signaling.

Non-targeted metabolomics aids in sex pheromone identification: a proof-of-concept study with the triangulate cobweb spider, Steatoda triangulosa

Targeted metabolomics has been widely used in pheromone research but may miss pheromone components in study organisms that produce pheromones in trace amount and/or lack bio-detectors (e.g., antennae) to readily locate them in complex samples. Here, we used non-targeted metabolomics-together with high-performance liquid chromatography-mass spectrometry (HPLC-MS), gas chromatography-MS, and behavioral bioassays-to unravel the sex pheromone of the triangulate cobweb spider, Steatoda triangulosa. A ternary blend of three contact pheromone components [N-4-methylvaleroyl-O-isobutyroyl-L-serine (5), N-3-methylbutyryl-O-isobutyroyl-L-serine (11), and N-3-methylbutyryl-O-butyroyl-L-serine (12)] elicited courtship by S. triangulosa males as effectively as female web extract. Hydrolysis of 5, 11 and 12 at the ester bond gave rise to two mate-attractant pheromone components [butyric acid (7) and isobutyric acid (8)] which attracted S. triangulosa males as effectively as female webs. Pheromone components 11 and 12 are reported in spiders for the first time, and were discovered only through the use of non-targeted metabolomics and GC-MS. All compounds resemble pheromone components previously identified in widow spiders. Our study provides impetus to apply non-targeted metabolomics for pheromone research in a wide range of animal taxa.

Widow spiders alter web architecture and attractiveness in response to same-sex competition for prey and mates, and predation risk

Female-female competition in animals has rarely been studied. Responses of females that compete context-dependently for mates and prey, and seek safety from predators, are ideally studied with web-building spiders. Cobwebs possess unique sections for prey capture and safety, which can be quantified. We worked with Steaoda grossa females because their pheromone is known, and adjustments in response to mate competition could be measured. Females exposed to synthetic sex pheromone adjusted their webs, indicating a perception of intra-sexual competition via their sex pheromone. When females sequentially built their webs in settings of low and high intra-sexual competition, they adjusted their webs to increase prey capture and lower predation risk. In settings with strong mate competition, females deposited more contact pheromone components on their webs and accelerated their breakdown to mate-attractant pheromone components, essentially increasing their webs' attractiveness. We show that females respond to sexual, social and natural selection pressures originating from intra-sexual competition.

Origin, structure and functional transition of sex pheromone components in a false widow spider

Female web-building spiders disseminate pheromone from their webs that attracts mate-seeking males and deposit contact pheromone on their webs that induces courtship by males upon arrival. The source of contact and mate attractant pheromone components, and the potential ability of females to adjust their web’s attractiveness, have remained elusive. Here, we report three new contact pheromone components produced by female false black widow spiders, Steatoda grossa: N-4-methylvaleroyl-O-butyroyl-L-serine, N-4-methylvaleroyl-O-isobutyroyl-L-serine and N-4-methylvaleroyl-O-hexanoyl-L-serine. The compounds originate from the posterior aggregate silk gland, induce courtship by males, and web pH-dependently hydrolyse at the carboxylic-ester bond, giving rise to three corresponding carboxylic acids that attract males. A carboxyl ester hydrolase (CEH) is present on webs and likely mediates the functional transition of contact sex pheromone components to the carboxylic acid mate attractant pheromone components. As CEH activity is pH-dependent, and female spiders can manipulate their silk’s pH, they might also actively adjust their webs’ attractiveness.

Pheromone communication among sexes of the garden cross spider Araneus diadematus

Chemical communication plays a fundamental role in many aspects of an animal’s life from assessing habitat quality to finding mating partners. Behavioural observations show that chemical communication likewise plays an important role in spiders, but the contexts and the substances involved are little explored. Here, we investigate the chemical communication in the garden cross spider Araneus diadematus (Clerck, 1757) between and within the sexes. Using choice trials, we demonstrate that males are attracted to odours of adult females, but not to those of subadult females. Our data further suggest that adult females avoid odours of conspecific adult females, possibly in order to reduce reproductive competition with other females. Cuticle and silk extracts as well as headspace samples of subadult and adult virgin females were analysed via GC–MS. Available candidate compounds for the female sex pheromone were tested via electroantennography on palps (electropalpography) of adult virgin females and on females in behavioural trials. We propose sulcatone (6-methyl-5-hepten-2-one) as a candidate substance for the female volatile pheromone and several long-chained alkanes and alcohols as candidates for contact pheromones. Apart from demonstrating that attraction of males to females depends on the latter’s developmental stage, our study suggests that pheromones can also play an important role between females, an aspect that requires further attention.

Know your foe: synanthropic spiders are deterred by semiochemicals of European fire ants

Many ants prey on spiders, suggesting that web-building spiders may avoid micro-locations near ant colonies or frequented by foraging ants. Here we tested the hypothesis that ant-derived semiochemicals deter synanthropic spiders. To generate stimuli, we exposed filter paper for 12 h to workers of European fire ants, Myrmica rubra, black garden ants, Lasius niger, or western carpenter ants, Camponotus modoc, and then offered select urban spiders in three-chamber olfactometer bioassays a choice between ant-exposed filter paper and unexposed control filter paper. Semiochemical deposits of M. rubra, but not of L. niger or C. modoc, had a significant deterrent effect on subadults of the false black widow, Steatoda grossa, the black widow, Latrodectus hesperus, and the hobo spider, Eratigena agrestis, as well as a moderate (but statistically not significant) deterrent effect on the cross spider, Araneus diadematus. The deterrent effect caused by semiochemical deposits of M. rubra may be attributable to the aggressive nature and efficient foraging of M. rubra in its invaded North American range, exerting selection pressure on community members to recognize M. rubra semiochemicals and to avoid micro-locations occupied by M. rubra.

Ultrastructure of chemosensory tarsal tip-pore sensilla of Argiope spp. Audouin, 1826 (Chelicerata: Araneae: Araneidae)

While chemical communication has been investigated intensively in vertebrates and insects, relatively little is known about the sensory world of spiders despite the fact that chemical cues play a key role in natural and sexual selection in this group. In insects, olfaction is performed with wall-pore and gustation with tip-pore sensilla. Since spiders possess tip-pore sensilla only, it is unclear how they accomplish olfaction. We scrutinized the ultrastructure of the trichoid tip-pore sensilla of the orb weaving spider Argiope bruennichi-a common Palearctic species the males of which are known to be attracted by female sex pheromone. We also investigated the congener Argiope blanda. We examined whether the tip-pore sensilla differ in ultrastructure depending on sex and their position on the tarsi of walking legs of which only the distal parts are in contact with the substrate. We hypothesized as yet undetected differences in ultrastructure that suggest gustatory versus olfactory functions. All tarsal tip-pore sensilla of both species exhibit characters typical of contact-chemoreceptors, such as (a) the presence of a pore at the tip of the sensillum shaft, (b) 2-22 uniciliated chemoreceptive cells with elongated and unbranched dendrites reaching up to the tip-pore, (c) two integrated mechanoreceptive cells with short dendrites and large tubular bodies attached to the sensillum shaft's base, and (d) a socket structure with suspension fibres that render the sensillum shaft flexible. The newly found third mechanoreceptive cell attached to the proximal end of the peridendritic shaft cylinder by a small tubular body was likely overlooked in previous studies. The organization of tarsal tip-pore sensilla did not differ depending on the position on the tarsus nor between the sexes. As no wall-pore sensilla were detected, we discuss the probability that a single type of sensillum performs both gustation and olfaction in spiders.

Multimodal and multifunctional signaling? - Web reduction courtship behavior in a North American population of the false black widow spider

Males of widow spiders courting on the web of females engage in web-reduction behavior which entails excising a section of the web, bundling it up, and wrapping it with their silk. Males of the false black widow spider, Steatoda grossa, in European populations also produce stridulatory courtship sound which has not yet been studied in their invaded North American range. Working with a North American population of S. grossa, we tested the hypotheses that (1) web reduction by males renders webs less attractive to rival males; (2) deposition of silk by courting males has an inter-sexual (male-female) signal function that enhances their likelihood of copulation; and (3) stridulatory sound is a courtship signal of males. Testing anemotactic attraction of males in Y-tube olfactometer experiments revealed that reduced webs (indicative of a mated female) and intact webs (indicative of a virgin female) were equally attractive to males. Recording courtship behavior of males with either functional (silk-releasing) spinnerets or spinnerets experimentally occluded on the web of virgin females showed that males with functional spinnerets were more likely to copulate with the female they courted. Although males possess the stridulatory apparatus to produce courtship sound, they did not stridulate when courting or copulating on the web of females. Our data support the conclusion that web-reduction behavior of S. grossa males in their invaded North American range has no long-range effect on mate seeking males. Instead, web-reduction behavior has an inter-sexual signaling function that seems to be linked to functional spinnerets of the courting male. The signal produced by a male likely entails a volatile silk-borne pheromone, but may also embody a gauge of his endurance (the amount of time he engages in web reduction causing web vibrations).