Robert B. Suter

Courtship and the assessment of virginity by male bowl and doily spiders

Early in cohabitations of pairs of bowl and doily spiders, Frontinella pyramitela (Linyphiidae), males and females court and pseudocopulate with no transfer of sperm, then may proceed to true copulations during which sperm are transferred. A male assesses female virginity during courtship and pseudocopulation. Only if the female is a virgin will he build a sperm web, charge his pedipalps with sperm, and then begin insemination. The data also indicate that (1) the males assessment of the females virginity depends on a signal (in contrast to a structural change) given by the female, and (2) the female can signal ‘virgin’ but not ‘non-virgin’. Males that do not assess the females as virgin continue with pseudocopulations for highly variable periods of time (up to 4·5 h) and the decision to cease is apparently governed by a stochastic process, analogous to a random number generator, in the male. In constrast, the timings of other events (e.g. the virginity signal and the end of true copulation) during these reproductive behaviours are governed by quite different processes: the data are consistent with a model involving the steady accumulation or dissipation of some substance or drive.

Fitness consequences of prolonged copulation in the bowl and doily spider

SummaryBowl and doily spiders, Frontinella pyramitela (Araneae, Linyphiidae), copulate much longer than the 15 min required for insemination. This apparently maladaptive behavior has been satisfactorily explained in the literature by invoking hypotheses that involve sperm competition, resistance to predators and parasites, or foraging enhancement. In this study, the fertility, live progeny production, progeny size, female receptivity, and latency to oviposition of bowl and doily spiders were measured and related to copulation duration. Using these data, we were able to eliminate six hypotheses and support a seventh: in this species, longer copulations result in larger hatchlings in the first clutch of eggs produced by the female. Although the correlation between copulation duration and progeny size is positive and significant, much of the variation in progeny size remains unexplained, and the reason for the variability in copulation durations remains obscure.

Linyphid spider courtship: Releaser and attractant functions of a contact sex pheromone

Abstract Experiments with male bowl-and-doily spiders ( Frontinella communis ) on methanol-washed and unwashed female webs indicate that the courtship of males is chemically released. Parallel experiments on the webs of females of another linyphiid species and on immature F. communis webs indicate that the releaser pheromone is species specific and that it is produced only by mature females. The pheromone that releases male courtship also functions as an attractant, changing the males orientation from negatively to positively geotactic. The presence or absence of a female on the test web has only a slight effect on the males behaviour.

PREDATOR AVOIDANCE ON THE WATER SURFACE? KINEMATICS AND EFFICACY OF VERTICAL JUMPING BY DOLOMEDES (ARANEAE, PISAURIDAE)

Abstract Vertical jumps of fishing spiders (Dolomedes sp.) from the water surface have been presumed to be evasive behaviors directed against predatory fish. We used high-speed videography to analyze the jumps of fishing spiders and then constructed a numerical model to assess the effectiveness of these jumps in evading predatory strikes by trout. Jump height (mean = 3.7 cm) and duration (mean = 0.17 sec) were similar across spider masses (0.05–0.66 g) but latency to jump increased significantly with mass. To accomplish jumps of similar height, more massive spiders had to generate more force during the propulsive phase of the jump than did smaller spiders; and the contribution of fluid drag to the total force used in jumping was substantially greater for large spiders than for smaller ones. Our model juxtaposing the jumps of spiders and the attacks of trout revealed that jump heights and durations were inadequate: only the most lethargic strikes by trout could be successfully evaded by jumping vertically from the water surface.

WATER SURFACE LOCOMOTION BY SPIDERS: DISTINCT GAITS IN DIVERSE FAMILIES

Abstract Pisaurids such as Dolomedes triton (Walckenaer 1837) are well known as inhabitants of ponds and streams and are adept at locomotion on the water surface. In a broad survey of water surface locomotion in spiders, we have found that most taxa do not use specialized gaits under these circumstances. However, some tetragnathids, araneids, and salticids (three families that are outside of the superfamily Lycosoidea to which the pisaurids belong) resemble D. triton to the extent that they do use specialized gaits when on the water surface. Of these, the tetragnathids are particularly accomplished at water surface locomotion, achieving velocities that exceed those of D. triton when it rows, but not when it gallops.

Multiple web-borne pheromones in a spider Frontinella pyramitela (Araneae: Linyphiidae)

Abstract The bowl-and-doily spider, Frontinella pyramitela , is a common inhabitant of low vegetation throughout most of temperate North America. All instars build concave-upward, bowl-shaped, nonviscid webs supported above and below by meshworks of silk. Previous studies of this species have revealed that chemical(s) on the silk of adult females elicit both courtship behaviour and positive geotaxis from adult males that contact the silk. This study demonstrates (1) that two different contact pheromones are responsible for the dual action of the silk of adult females and (2) that the webs of different age and sex classes of bowl-and-doily spiders (including the webs of adult males) contain functionally different mixtures of the two pheromones.

Spitting Performance Parameters and Their Biomechanical Implications in the Spitting Spider, Scytodes thoracica

Abstract Spitting spiders Scytodes spp. subdue prey by entangling them at a distance with a mixture of silk, glue, and venom. Using high-speed videography and differential interference contrast microscopy, the performance parameters involved in spit ejection by Scytodes thoracica (Araneae, Scytodidae) were measured. These will ultimately need to be explained in biomechanical and fluid dynamic terms. It was found that the ejection of “spit” from the opening of the venom duct (near the proximal end of the fang) was orderly. It resulted in a pattern that scanned along a lateral-medial axis (due to fang oscillations) while traversing from ventral to dorsal (due to cheliceral elevation). Each lateral-to-medial sweep of a fang produced silk-borne beads of glue that were not present during each subsequent medial-to-lateral sweep. The ejection of “spit” was very rapid. A full scan (5–57 fang cycles, one upsweep of a chelicera) typically occupied less than 30 ms and involved fang oscillations at 278–1781 Hz. Ejection velocities were measured as high as 28.8 m/s. The “spit” was contractile. During the 0.2 s following ejection, silk shortened by 40–60% and the product of a full scan by both of the chelicerae could exert an aggregate contractile force of 0.1 – 0.3 mN. Based on these parameters, hypotheses are described concerning the biomechanical and fluid dynamic processes that could enable this kind of material ejection.

SCYTODES VS. SCHIZOCOSA: PREDATORY TECHNIQUES AND THEIR MORPHOLOGICAL CORRELATES

Abstract Wolf spiders (Lycosidae) typically subdue prey using their legs for capture and their fangs for the injection of venom. Spitting spiders (Scytodidae), in contrast, subdue prey by entangling them, at a distance, in a spitted mixture of silk, glue, and venom that immobilizes and may also kill them. We selected individuals of Schizocosa duplex (Lycosidae) and Scytodes sp. (Scytodidae) of approximately the same mass and carapace width to provide a quantitative assessment of their relative allocations of biomass to morphological features that might be expected to vary with prey-capture technique. As expected, the wolf spiders allocated significantly more to legs, chelicerae, and fangs, and significantly less to the venom glands, than did the spitting spiders. Further comparisons of the legs and chelicerae of the two species provided surprises. First, the legs of Scytodes were 42% longer than those of Schizocosa despite smaller overall allocation to the legs in Scytodes. And second, although the relative sizes of the chelicerae differ greatly, the shapes of the chelicerae of Schizocosa and Scytodes were not significantly different despite the radically different tasks those structures must fulfill.

SPIDER SIZE AND LOCOMOTION ON THE WATER SURFACE (ARANEAE, PISAURIDAE)

Abstract Newly emerged fishing spiders, Dolomedes triton (Walckenaer 1837), can achieve rowing velocities as high as those of adults despite an approximately 600-fold difference in mass (1.7 mg vs. 1.1 g). In contrast, when velocity is measured in relative terms (body lengths/sec), small spiders move much more rapidly than adults, with Vrel ∝ mass−0.31. This surprising performance of very small spiders can be attributed both to their very high stride frequency (fs ∝ mass−0.43) and to the high angular velocity of their propulsive legs (ω ∝ mass−0.33). Calculations of leg tip velocities, based on measurements of both angular velocities and leg lengths, reveal that maximum leg tip velocities are achieved by spiders of about 33 mg, nineteen times more massive than the smallest spiders we tested. Some very small spiders perform conspicuously and consistently less well than do others of the same size. A detailed dissection of the motion of these underachievers reveals that a disproportionate amount of their rowing effort goes into vertical as opposed to horizontal work: the ratio of vertical to horizontal work during rowing is 1.03 ± 0.89 : 1 in normal fishing spiders and 5.18 ± 1.73 : 1 in the underachievers.

Behavioral thermoregulation: Solar orientation in Frontinella communis (Linyphiidae), a 6-mg spider

Summary1.The Linyphiid spider Frontinella communis changes its orientation from random with respect to the sun (Fig. 1A) to aligned with the rays of the sun (Fig. 1B) on days when both insolation and ambient temperature are high (Fig. 2). The orientation is not mediated by visual photoreception. Because the orientation behavior is not elicited when insolation is high and ambient temperature is low, an antipredator or antiprey behavioral role is rejected and a thermoregulatory function proposed.2.Body temperature measurements of a spider at different orientations to the sun reveal that alignment of the spiders oral-anal axis with the suns rays results in at least a 0.5°C decrease in TB relative to TB when the spiders axis is perpendicular to the sun.3.F. communis has a LT50 of 41.8°C, a temperature that the spider probably never approaches over much of its geographic range. Therefore, it is unlikely that the orientation behavior functions to keep the spider within a nonlethal temperature range. Instead, the behavior probably functions in depressing this poikilotherms metabolic rate over a broad temperature range, and thus permits reallocation of nutrient resources from maintenance to reproduction.