Viscous capture threads are highly evolved adhesive delivery systems found in orb-webs produced by over 4000 spider species(Fig. 1). They make crucial contributions to the operation of these webs by retaining insects, thereby giving spiders more time to locate, run to and subdue prey that their webs have intercepted (Chacón and Eberhard, 1980; Eberhard,1986; Eberhard,1989; Eberhard,1990). Viscous threads are spun from the spigots of two adjacent silk glands (Foelix, 1996). The flagelliform glands produce a pair of supporting axial fibers, and the aggregate glands coat these fibers with a viscous, aqueous solution that quickly forms into droplets (Peters,1986; Peters,1995; Vollrath et al.,1990). The glycoprotein granules that coalesce inside each droplet contribute to thread adhesion (Vollrath and Tillinghast, 1991; Tillinghast et al., 1993) and the hydrophilic compounds in the surrounding fluid attract atmospheric moisture to prevent droplets from drying(Townley, 1990; Vollrath et al., 1990; Townly et al., 1991).

Together with the cob- and sheet-web weaving species descended from them,these orb-weaving spiders comprise the Araneoidea clade, which includes 27% of the 40 024 living spider species(Platnick, 2008). The viscous threads produced by members of this clade replaced the cribellar prey capture threads spun by members of their sister clade, the Deinopoidea(Coddington, 1986; Coddington, 1989; Griswold et al., 1998; Garb et al., 2006). Cribellar capture threads are also supported by a pair of axial fibers. However, these fibers are covered by an outer sheath of fine, dry, looped protein fibrils(Peters, 1984; Peters, 1986; Peters, 1992; Eberhard and Pereira, 1993; Opell, 1999) that are drawn from the spigots of an oval spinning plate, termed the cribellum, by a spider's calamistrum, a setal comb on the metatarsus of each of its fourth legs (Eberhard, 1988; Opell, 2001). Rhythmic adductions of the median spinnerets press the fibril sheath around the supporting strands to produce a thread that has a complex, but often regular,surface configuration (Peters,1986).

The present study examines more precisely the contribution of axial fiber extensibility to viscous thread adhesion as it tests the hypothesis that reducing the extensibility of a thread's axial fibers reduces its expressed stickiness. It does so by examining the viscous threads of five araneoid species that have droplet profiles that range from small, closely spaced droplets to large widely spaced droplets(Fig. 1). The stickiness of these threads was first measured under their native tensions and again after they were stretched to two different lengths to reduce the extensibility of their axial fibers. Stretching threads also increases the distance between their droplets (Fig. 2). To maintain the number of droplets that contributed to the stickiness of a thread span, we measured the stickiness of stretched threads with contact plates whose widths were increased in proportion to the degree of thread elongation.

Fig. 1.

Capture threads of the five species included in the study, shown at the same magnification.

Fig. 1.

Capture threads of the five species included in the study, shown at the same magnification.

### Species studied and thread collection

We collected web samples from orb-webs constructed by adult females of five species of the family Araneidae [Araneus marmoreus Clerck, Argiope aurantia Lucas, Micrathena gracilis (Walckenaer), Verrucosa arenata (Walckenaer) and Cyclosa turbinata(Walckenaer)] from sites near Blacksburg, Montgomery Co., VA, USA. Orb-web sectors were collected in the morning, a few hours after webs were spun, using 18 cm-diameter aluminum rings with a 5 mm wide bar across their centers. Double-sided tape on the rim and center bar of each ring held the threads securely. We photographed and measured the stickiness of threads from each web sample in the laboratory under the same relative humidity and temperature within 6.5 h after web samples were collected.

### Altering axial fiber extensibility

Fig. 2.

Capture threads from the web of M. gracilis female number 632 at their native (1×) and stretched (2× and 3×) lengths.

Fig. 2.

Capture threads from the web of M. gracilis female number 632 at their native (1×) and stretched (2× and 3×) lengths.

### Measuring droplet size and spacing

Using techniques described more fully by Opell and Hendricks(Opell and Hendricks, 2007),we photographed the threads of each individual spider at each of the three elongations and measured these digital images with ImageJ (ImageJ, 2006; http://www.uhnresearch.ca/facilities/wcif/imagej/;Bethesda, MD, USA) to characterize the size and spacing of their primary droplets (Table 1). Threads spun by some individuals also have smaller secondary droplets between some of their primary droplets (Fig. 1). As these comprise only a small part of the thread's total volume per mm (A. aurantia 1.9%, A. marmoreus 3.4%, M. gracilis 4.0%, V. arenata 0.6%, C. turbinata 10.8%;B.D.O. and M.L.H., unpublished observations) and their presence and size were variable, we included only the primary droplets in this study. The profiles of viscous droplets best matched those of a parabola(Opell and Hendricks, 2007). Therefore, we determined droplet volume (DV) using the following formula generated from the formula of a parabola rotated around its x-axis(Opell and Hendricks, 2007):
$\mathrm{DV}=(2{\ }{\pi}{\ }\mathrm{droplet}{\ }\mathrm{width}^{2}{\times}\mathrm{droplet}{\ }\mathrm{length}){/}15.$
Table 1.

Features of threads at their native and extended lengths

A. aurantia (N=4)A. marmoreus (N=8)M. gracilis (N=9)V. arenata (N=10)C. turbinata (N=8)
Droplet length (μm)
Unstretched 63.24±4.76 57.67±2.18 29.43±2.20 23.55±2.37 10.73±0.70
2× 62.01±3.40 55.36±3.16 27.28±2.08 25.01±3.12 10.09±0.68
3× 64.46±4.65 60.26±2.70 29.06±2.03 25.98±2.55 10.89±0.53
Droplet width (μm)
Unstretched 45.39±4.09 43.31±1.67 22.37±1.83 19.18±2.05 8.73±0.63
2× 47.31±3.00 45.42±2.63 21.75±1.61 21.21±2.61 8.59±0.66
3× 49.00±4.13 50.19±2.38 23.52±1.79 21.60±2.10 8.94±0.51
Range of intraindividual droplet volume (%) 31.0±12.0 47.4±5.7 58.5±10.8 57.5±14.3 61.3±12.5
Droplet volume (μm3
Unstretched 58,745±16,383 47,277±5,330 7,132±1,674 4,969±1,902 397±77
2× 60,203±11,575 52,431±9,304 6,236±1,393 6,894±2,545 349±77
3× 67,971±16,516 66,941±9,574 7,716±1,920 6,455±1,836 390±54
Droplets per mm
Unstretched 3.83±1.36 3.46±0.26 10.98±1.06 10.12±1.24 23.39±7.10
2× 1.69±0.33 1.80±0.13 5.98±0.34 5.38±0.62 11.25±2.16
3× 1.28±0.10 1.24±0.12 4.22±0.31 3.83±0.35 8.83±1.66
Breaking factor 6.33±0.49 6.53±0.62 8.69±0.60 3.53±0.33 5.63±0.38
Threads per spider 6.5±0.9 11.8±1.8 6.5±0.6 6.0±1.1 7.0±1.0
Relative Young's modulus
2× 0.35±0.06 0.31±0.02 0.24±0.02 0.58±0.05 0.39±0.03
3× 0.47±0.13 0.46±0.03 0.32±0.02 0.78±0.03 0.50±0.05
A. aurantia (N=4)A. marmoreus (N=8)M. gracilis (N=9)V. arenata (N=10)C. turbinata (N=8)
Droplet length (μm)
Unstretched 63.24±4.76 57.67±2.18 29.43±2.20 23.55±2.37 10.73±0.70
2× 62.01±3.40 55.36±3.16 27.28±2.08 25.01±3.12 10.09±0.68
3× 64.46±4.65 60.26±2.70 29.06±2.03 25.98±2.55 10.89±0.53
Droplet width (μm)
Unstretched 45.39±4.09 43.31±1.67 22.37±1.83 19.18±2.05 8.73±0.63
2× 47.31±3.00 45.42±2.63 21.75±1.61 21.21±2.61 8.59±0.66
3× 49.00±4.13 50.19±2.38 23.52±1.79 21.60±2.10 8.94±0.51
Range of intraindividual droplet volume (%) 31.0±12.0 47.4±5.7 58.5±10.8 57.5±14.3 61.3±12.5
Droplet volume (μm3
Unstretched 58,745±16,383 47,277±5,330 7,132±1,674 4,969±1,902 397±77
2× 60,203±11,575 52,431±9,304 6,236±1,393 6,894±2,545 349±77
3× 67,971±16,516 66,941±9,574 7,716±1,920 6,455±1,836 390±54
Droplets per mm
Unstretched 3.83±1.36 3.46±0.26 10.98±1.06 10.12±1.24 23.39±7.10
2× 1.69±0.33 1.80±0.13 5.98±0.34 5.38±0.62 11.25±2.16
3× 1.28±0.10 1.24±0.12 4.22±0.31 3.83±0.35 8.83±1.66
Breaking factor 6.33±0.49 6.53±0.62 8.69±0.60 3.53±0.33 5.63±0.38
Threads per spider 6.5±0.9 11.8±1.8 6.5±0.6 6.0±1.1 7.0±1.0
Relative Young's modulus
2× 0.35±0.06 0.31±0.02 0.24±0.02 0.58±0.05 0.39±0.03
3× 0.47±0.13 0.46±0.03 0.32±0.02 0.78±0.03 0.50±0.05

Values are means ±1 s.e.m. Unstretched threads had RYM values of 0.083843 (Fig. 5)

We assessed the range of droplet volumes for an individual spider's threads by first subtracting the mean droplet volume of the thread strand (1×,2× or 3×) with the smallest mean droplet volume from the thread strand with the greatest mean droplet volume. Next, we divided this difference by the mean droplet volume of the individual's three thread strands at the three elongations and multiplied this value by 100 to obtain an index that we term the range of intraindividual droplet volume(Table 1).

Fig. 3.

Achieved thread extension for the five species compared to the intended 2.215 and 3.346× extensions. Light shaded bars represent 2×extensions, and dark shaded bars represent 3× extensions.

Fig. 3.

Achieved thread extension for the five species compared to the intended 2.215 and 3.346× extensions. Light shaded bars represent 2×extensions, and dark shaded bars represent 3× extensions.

### Computing adjusted stickiness per droplet

Fig. 4.

Relationship between the number of C. turbinata thread droplets contributing to thread stickiness, as measured with contact plates of four widths, and the mean stickiness per thread droplet. This example shows how we compensated for a decrease in the per droplet stickiness due to inadequate thread elongation, which resulted in five additional droplets contributing to a thread's stickiness. Multiplying these five droplets by the slope of the regression line and adding this product to the measured stickiness per droplet corrects for the effects of thread elongation, which was less than intended.

Fig. 4.

Relationship between the number of C. turbinata thread droplets contributing to thread stickiness, as measured with contact plates of four widths, and the mean stickiness per thread droplet. This example shows how we compensated for a decrease in the per droplet stickiness due to inadequate thread elongation, which resulted in five additional droplets contributing to a thread's stickiness. Multiplying these five droplets by the slope of the regression line and adding this product to the measured stickiness per droplet corrects for the effects of thread elongation, which was less than intended.

### Measuring the breaking length of threads

As the native extensibility of viscous threads differs among species(Opell and Bond, 2001), we judged that our thread elongation procedure probably did not affect the extensibility of each species' axial fibers in the same way. To evaluate this,we measured the breaking lengths of capture threads relative to their native lengths, using the same caliper apparatus described above, the same methods for affixing threads to the bars on this caliper's jaws, and the same rate of elongation to measure the breaking lengths of capture threads. We collected a series of 3 mm-long thread spans from each web, extended these threads and recorded the breaking length of each strand. We then computed breaking factor by dividing a thread's initial length by its length at rupture.

### Evaluating breaking factors of threads

Fig. 5.

Plot of relative thread elongation (elongation/elongation at rupture) and relative Young's modulus (Young's modulus/Young's modulus at rupture) derived from the stress–strain curve of Araneus diadematus(Köhler and Vollrath,1995).

Fig. 5.

Plot of relative thread elongation (elongation/elongation at rupture) and relative Young's modulus (Young's modulus/Young's modulus at rupture) derived from the stress–strain curve of Araneus diadematus(Köhler and Vollrath,1995).

### Testing the effects of droplet volume and thread elongation on stickiness

For each individual's threads at each elongation we determined DV, ASPD and RYM. Within a species, droplet volume is directly related to the stickiness of viscous threads (Opell, 2002; Opell and Schwend, 2007),although this relationship may not be as strong among species(Opell and Schwend, 2008). Moreover, our hypothesis predicted that RYM should contribute negatively to stickiness, as larger values of RYM indicate stiffer threads. We used the SAS statistical package (SAS Inc., Cary, NC, USA) to test the normality of droplet volumes, to compare the droplet volumes of threads stretched to different lengths and to generate regression models that tested the hypothesized contribution of DV and RYM to ASPD in each of the five species. Data were considered normally distributed if P>0.05 for a Shapiro–Wilk W-statistic test. We examined normally distributed data with one-way analyses of variance (ANOVA) and t-tests (T). Data that were not normally distributed were compared with Kruskal–Wallis χ2 tests(KW). We considered regression models with 0.10⩾P>0.05 to provide weak support for the hypothesis and P⩽0.05 to provide strong support for the hypothesis.

Thread elongation clearly altered the RYM of an individual's thread samples(Fig. 6). Moreover, the range of intraindividual droplet volume of the 1×, 2× and 3×threads was considerable, from 31 to 61%, and averaged 51% of mean individual droplet volume (Table 1). Given these differences, the separate measurements of the RYM, DV and ASPD that we obtained for each individual's 1×, 2× and 3× threads were largely independent of one another. However, there is still the possibility of a spider-specific effect among the three factors (RYM, DV, individual) that contributed to ASPD. Therefore, we report two sets of P values for each species' regression model: P (EDF1), whose F value was computed using an error degree of freedom (EDF) based on the number of individuals sampled, and P (EDF2), whose EDF was based on a sample size reduced by one-third to account for any effect of measuring the threads of individuals at three elongations. Thus, EDF2 provides a more conservative test of the hypothesis by diminishing the individual component through reduced F values and increased P values.

Fig. 6.

Relative Young's modulus values for unstretched and stretched threads derived from the curve shown in Fig. 5.

Fig. 6.

Relative Young's modulus values for unstretched and stretched threads derived from the curve shown in Fig. 5.

### Testing hypotheses

Tables 1 and 2 report the thread features and stickiness values for native and elongated threads of the five species. If thread elongation resulted in more viscous material being withdrawn from droplets and distributed along interdroplet regions, then a species' droplet volume should decrease as threads are elongated. In fact, it appears as though there might be a tendency for droplet volume to increase as threads are elongated (Table 1). However,this is not supported by comparisons of either the droplet volumes of threads at their three elongations or of the droplet volumes of the unstretched and 3× threads. For A. aurantia, A. marmoreus, M. gracilis, V. arenata and C. turbinata, the results of these tests were: KW P=0.3897, KW P=0.2482; ANOVA P=0.2441, T P=0.0944; KW P=0.8208, KW P=0.6272; KW P=0.6977, KW P=0.3258; ANOVA P=0.8749, T P=0.9397, respectively.

Table 2.

Thread stickiness and the environmental conditions under which measurements were taken

A. aurantia (N=4)A. marmoreus (N=8)M. gracilis (N=9)V. arenata (N=10)C. turbinata (N=8)
Conditions
Temperature (°C) 23.8±0.3 24.5±0.2 23.5±0.2 23.9±0.2 24.3±0.03
Relative humidity (%) 42.6±4 44.9±2 46.7±1 46.9±1 44.0±3.3
Barometric pressure (kPa) 134.922±0.400 134.789±0.133 135.189±0.133 135.456±0.133 135.722±0.133
Stickiness (μN)
Unstretched 101.50±31.74 48.40±4.08 57.76±4.05 149.42±14.22 40.04±2.28
2× 96.44±27.07 65.52±8.70 59.14±4.77 142.42±11.64 42.18±1.98
3× 80.88±4.81 67.85±7.69 45.92±5.96 146.38±15.85 38.48±1.66
Stickiness per droplet (SPD) (μN)
Unstretched 28.42±5.13 14.80±2.08 6.06±1.16 17.31±2.66 1.86±0.23
2× 25.10±3.36 17.17±2.98 4.51±0.35 14.39±2.75 1.76±0.17
3× 19.18±1.60 18.57±4.32 3.55±0.70 12.73±2.13 1.33±0.08
SPD adjusted for achieved stretch (ASPD) (μN)
Unstretched 28.42±5.13 14.80±2.08 6.06±1.16 17.31±2.66 1.86±0.23
2× 24.85±3.36 17.56±2.98 4.89±0.44 16.20±2.60 1.80±0.17
3× 20.36±3.84 19.11±4.21 4.05±0.71 15.43±2.08 1.51±0.12
A. aurantia (N=4)A. marmoreus (N=8)M. gracilis (N=9)V. arenata (N=10)C. turbinata (N=8)
Conditions
Temperature (°C) 23.8±0.3 24.5±0.2 23.5±0.2 23.9±0.2 24.3±0.03
Relative humidity (%) 42.6±4 44.9±2 46.7±1 46.9±1 44.0±3.3
Barometric pressure (kPa) 134.922±0.400 134.789±0.133 135.189±0.133 135.456±0.133 135.722±0.133
Stickiness (μN)
Unstretched 101.50±31.74 48.40±4.08 57.76±4.05 149.42±14.22 40.04±2.28
2× 96.44±27.07 65.52±8.70 59.14±4.77 142.42±11.64 42.18±1.98
3× 80.88±4.81 67.85±7.69 45.92±5.96 146.38±15.85 38.48±1.66
Stickiness per droplet (SPD) (μN)
Unstretched 28.42±5.13 14.80±2.08 6.06±1.16 17.31±2.66 1.86±0.23
2× 25.10±3.36 17.17±2.98 4.51±0.35 14.39±2.75 1.76±0.17
3× 19.18±1.60 18.57±4.32 3.55±0.70 12.73±2.13 1.33±0.08
SPD adjusted for achieved stretch (ASPD) (μN)
Unstretched 28.42±5.13 14.80±2.08 6.06±1.16 17.31±2.66 1.86±0.23
2× 24.85±3.36 17.56±2.98 4.89±0.44 16.20±2.60 1.80±0.17
3× 20.36±3.84 19.11±4.21 4.05±0.71 15.43±2.08 1.51±0.12

Values are means ±1 s.e.m.

In M. gracilis, V. arenata and C. turbinata, DV and RYM jointly explained ASPD in models judged on both full and reduced EDF values(Table 3). In these three species, DV was a significant and positive contributor to ASPD under both full and reduced EDF values. In all species but A. marmoreus, RYM was a significant and negative contributor to ASPD under full EDF values. That is,as a thread's extensibility was reduced, its per droplet stickiness also decreased. When EDF2 values were considered, strong support for a negative contribution of RYM remained in M. gracilis and V. arenatabut there was only weak support for a negative contribution of RYM in A. aurantia and C. turbinata.

Table 3.

Results of regression analyses of the relationship between droplet volume(DV) and relative Young's Modulus (RYM) and adjusted stickiness per droplet(ASPD) that accounts for realized thread elongations

A. aurantiaA. marmoreusM. gracilisV. arenataC. turbinata
Model
R2 0.457 0.107 0.637 0.735 0.469
P (EDF1) 0.0640 (9) 0.3064 (21) 0.0001 (24) 0.0001 (27) 0.0013 (21)
P (EDF2) 0.151 (6) 0.455 (14) 0.0001 (16) 0.0001 (18) 0.012 (14)
ASPD=DV × 8.676×10–5 1.676×10–5 3.530×10–4 9.877×10–4 1.550×10–3
+ RYM × –22.750 16.654 –10.008 –6.697 –0.874
+ (intercept) 25.857 11.481 4.642 13.507 1.423
DV
P (EDF1) 0.2601 (9) 0.5527 (21) 0.0001 (24) 0.0001 (27) 0.0013 (21)
P (EDF2) 0.352 (6) 0.650 (14) 0.0001 (16) 0.0001 (18) 0.009 (14)
RYM
P (EDF1) 0.0352 (9) 0.1583 (21) 0.0016 (24) 0.0094 (27) 0.0385 (21)
P (EDF2) 0.090 (6) 0.250 (14) 0.011(16) 0.035 (18) 0.092 (14)
A. aurantiaA. marmoreusM. gracilisV. arenataC. turbinata
Model
R2 0.457 0.107 0.637 0.735 0.469
P (EDF1) 0.0640 (9) 0.3064 (21) 0.0001 (24) 0.0001 (27) 0.0013 (21)
P (EDF2) 0.151 (6) 0.455 (14) 0.0001 (16) 0.0001 (18) 0.012 (14)
ASPD=DV × 8.676×10–5 1.676×10–5 3.530×10–4 9.877×10–4 1.550×10–3
+ RYM × –22.750 16.654 –10.008 –6.697 –0.874
+ (intercept) 25.857 11.481 4.642 13.507 1.423
DV
P (EDF1) 0.2601 (9) 0.5527 (21) 0.0001 (24) 0.0001 (27) 0.0013 (21)
P (EDF2) 0.352 (6) 0.650 (14) 0.0001 (16) 0.0001 (18) 0.009 (14)
RYM
P (EDF1) 0.0352 (9) 0.1583 (21) 0.0016 (24) 0.0094 (27) 0.0385 (21)
P (EDF2) 0.090 (6) 0.250 (14) 0.011(16) 0.035 (18) 0.092 (14)

Two P values are given, P (EDF1), whose error degree of freedom was based on the number of individuals sampled, and P (EDF2),whose error degree of freedom was two-thirds of this value. The numbers in parentheses following P values are the sample size on which these P values are based

Our failure to find a strong support for models of ASPD based on DV and RYM in A. aurantia and A. marmoreus may result from high DV variance in these species or to a low correlation between the DV and ASPD in the individuals that we studied. The relationships between DV and ASPD for the unstretched threads of A. aurantia and A. marmoreus were not significant (P=0.1327 and 0.1859, respectively) whereas this relationship was significant for M. gracilis, V. arenata and C. turbinata (P=0.0038, P=0.0061 and 0.0303,respectively).

We used the significant regression models for M. gracilis, V. arenata and C. turbinata to illustrate graphically the contributions of RYM and DV to ASPD (Fig. 7). These models showed that, as threads are elongated, the adhesion attributed to droplet volume alone increasingly exceeded measured ASPD whereas increasing amounts of potential adhesion are lost to reduced thread extensibility.

Axial fibers contribute indirectly to thread stickiness by recruiting the adhesion of multiple droplets. To assess this contribution we used the regression models of M. gracilis, V. arenata and C. turbinata to estimate the percentage of an unstretchced thread's stickiness that can be attributed to the extensibility of its axial fibers. We express this contribution as percent elastic component (PEC), the reduced stickiness of a 3× elongated thread attributed to its lost extensibility divided by the stickiness of an unstretched thread. The PECs for M. gracilis, V. arenata and C. turbinata threads were 50.0%, 29.2%and 22.5%, respectively, yielding a mean PEC of 33.9%.

### Assessing the effects of stress relaxation

The time required to screen and photograph threads before measuring their stickiness allowed all of the threads used in this study to reach their stress relaxation equilibriums. This means that the residual extensibility of all of the stretched threads was probably greater than our indices of relative Young's modulus indicate and was probably proportionately greater for threads that were stretched to a higher percentage of their breaking elongations. The mean realized 3× extensions of the threads of M. gracilis, A. marmoreus, A. trifasciata, C. turbinata and V. arenata,expressed as a percentage of their mean breaking extensions, were 30%, 43%,47%, 47% and 75%, respectively. Thus, stress relaxation should have had the most pronounced effect on stretched threads of V. arenata.

Fig. 7.

Adhesive components of the threads of M. gracilis, V. arenata and C. turbinata based on regression models(Table 3) of the positive contributions of droplet volume and the negative contribution of relative Young's modulus to mean stickiness per thread droplet.

Fig. 7.

Adhesive components of the threads of M. gracilis, V. arenata and C. turbinata based on regression models(Table 3) of the positive contributions of droplet volume and the negative contribution of relative Young's modulus to mean stickiness per thread droplet.

To assess this effect, we performed an additional regression for V. arenata threads using reduced RYM values computed from the data on A. sericatus. Relative to breaking elongation, the 3× elongation of V. arenata threads was 86% that of A. sericatus. As the stress-relaxed RYM of A. sericatus threads was 37% less than their elongated RYM, this translates to a 32% reduction in the RYM values of 3× V. arenata threads. The mean realized 2× extension of V. arenata threads was 53% of their mean breaking extensions. If the RYM of a stress-relaxed thread decreases in proportion to its elongation, then the values of a 2× thread should be 71% that of a 3× thread or 23%that of thread that has not undergone stress relaxation. The regression model based on these modified RYM values is significant (Model PEDF1=0.0001, DV P EDF1=0.0001, RYM DV P EDF1=0.0125) and shows that ASPD is directly related to DV and inversely related to RYM(ASPD=0.001DV–8.923RYM+13.529).

Our results support the hypothesized positive contribution of a viscous thread's axial fiber extensibility to its stickiness. When threads are stretched and their extensibility is reduced, their per droplet stickiness decreases. We observed this response both in threads of M. gracilis,which had the greatest native extensibility, and in threads of V. arenata, which had the least. We also observed it in threads whose droplet volumes differed by a factor as great as 152 and whose droplet spacing differed by a factor as great as 6. These findings are consistent with the operation of a SBM that enhances the stickiness of viscous capture threads by recruiting the adhesion of droplets interior to the edges of a thread's contact with a surface (Opell and Hendricks, 2007).

Our attempt to evaluate the effect of stress relaxation on stretched threads used the conservative assumption that the slope of the stress–strain curve of stress-relaxed threads is identical to that of threads that have not undergone stress relaxation. Nonetheless, it confirmed that thread extensibility contributes positively to the stickiness of V. arenata threads, which were elongated to a much greater percentage of their breaking lengths than were threads of the other species. Consequently,we believe that stress relaxation did not confound the broader conclusions of our study. A complete explanation of viscous thread performance must incorporate this phenomenon, although the preliminary calculations that we present suggest that this will be challenging.

The molecular structure of silk affects its mechanical properties(Hayashi et al., 1999; Hayashi et al., 2001; Hayashi et al.,2004; Hayashi and Lewis,2001; Craig, 2003; Ayoub et al., 2007) and appears to explain intraspecific (Blackledge and Hayashi, 2006) and interspecific(Swanson et al., 2006a; Swanson et al., 2006b)differences in thread properties. However, the observed 2.5-fold difference among the breaking factors of the five species' viscous threads(Table 1) cannot be attributed solely to differences in the molecular composition of their axial fibers. As we measured threads at their native, in-web tensions and did not standardize their tensions prior to measuring their breaking factors, we were unable to factor out the contribution that differences in web construction behavior may have made to the observed differences in thread breaking factors. Members of some species may stretch their capture threads more than others before they attach them to the web's radial lines. If they do, then a greater portion of the potential extensibility of these threads would have been expended, leaving them with less usable extensibility.

The mean 33.9% lost adhesion that can be attributed to reduced extensibility in the 3× stretched threads documents the important contribution that thread extensibility makes to thread stickiness. Threads of all species could be elongated more than the realized 3× extensions on which this estimate was based. However, this 33.9% is probably a reasonable estimate of the typical contribution of axial fiber extensibility to thread adhesion because features of orb-web architecture constrain the elongation that viscous threads undergo when intercepting and retaining prey. Insects usually strike multiple spiral turns, thereby distributing impact forces and struggling stresses over several thread spans. Moreover, aerodynamic dampening helps vertical orb-webs absorb the forces of prey impact as webs flex through the air (Lin et al., 1995). Even this web flexibility is constrained by the combined extensibility of the web's radial and capture lines (Craig,1987; Craig,2003).

LIST OF ABBREVIATIONS

• ASPD

•
• DPMM

•
• DV

droplet volume

•
• EDF

error degree of freedom

•
• PEC

percent elastic component

•
• RYM

relative Young's modulus

•
• SBM

suspension bridge mechanism

•
• SPD

stickiness per droplet

Caitlin Flora and Genine Lipkey assisted with fieldwork. Harry Schwend helped compute RYM values and provided suggestions for improving figures. National Science Foundation grant IOB-0445137 supported this research.

Ayoub, N. A., Garb, J. E., Tinghitella, R. M., Collin, M. A. and Hayashi, C. Y. (
2007
). Blueprint for a high-performance biomaterial: full-length spider silk genes.
PLoS ONE
2
e514,
1
-13.
Blackledge, T. A. and Hayashi, C. Y. (
2006
). Unraveling the mechanical properties of composite silk threads spun by cribellate orbweaving spiders.
J. Exp. Biol.
209
,
3131
-3140.
Chacón, P. and Eberhard, W. G. (
1980
). Factors affecting numbers and kinds of prey caught in artificial spider webs with considerations of how orb-webs trap prey.
Bull. Br. Arachnol. Soc.
5
,
29
-38.
Coddington, J. A. (
1986
). The monophyletic origin of the orb-web. In
Spiders: Webs, Behavior and Evolution
(ed W. A. Shear), pp.
319
-363. Stanford: Stanford University Press.
Coddington, J. A. (
1989
). Spinneret silk spigot morphology: evidence for the monophyly of orb-weaving spiders, Cyrtophorinae(Araneidae), and the group Theridiidae plus Nesticidae.
J. Arachnol.
17
,
71
-96.
Craig, C. L. (
1987
). The ecological and evolutionary interdependence between web architecture and web silk spun by orb web weaving spiders.
Biol. J. Linn. Soc.
30
,
135
-162.
Craig, C. L. (
2003
).
Spider Webs and Silk: Tracing Evolution from Molecules to Genes to Phenotypes
. New York: Oxford University Press.
Denny, M. (
1976
). Physical properties of spider silks and their role in design of orb-webs.
J. Exp. Biol.
65
,
483
-506.
Eberhard, W. G. (
1986
). Effect of orb-web geometry on prey interception and retention. In
Spiders, Webs,Behavior, and Evolution
(ed. W. A. Shear), pp.
70
-100. Stanford: Stanford University Press.
Eberhard, W. G. (
1988
). Combing and sticky silk attachment behaviour by cribellate spiders and its taxonomic implications.
Bull. Br. Arachnol. Soc.
7
,
247
-251.
Eberhard, W. G. (
1989
). Effects of orb-web orientation and spider size on prey retention.
Bull. Br. Arachnol. Soc.
8
,
45
-48.
Eberhard, W. G. (
1990
). Function and phylogeny of spider webs.
Annu. Rev. Ecol. Syst.
21
,
341
-372.
Eberhard, W. G. and Pereira, F. (
1993
). Ultrastructure of cribellate silk of nine species in eight families and possible taxonomic implications. (Araneae: Amaurobiidae, Deinopidae, Desidae,Dictynidae, Filistatidae, Hypochilidae, Stiphidiidae, Tengellidae).
J. Arachnol.
21
,
161
-174.
Foelix, R. F. (
1996
).
Biology of Spiders, 2nd edn.
New York: Oxford University Press.
Garb, J. E., DiMauro, T., Vo, V. and Hayashi, C. Y.(
2006
). Silk genes support the single origin of orb-webs.
Science
312
,
1762
.
Griswold, C. E., Coddington, J. A., Hormiga, G. and Scharff,N. (
1998
). Phylogeny of the orb-web building spiders(Araneae, Orbiculariae: Deinopoidea, Araneoidea).
Zool. J. Linn. Soc.
123
,
1
-99.
Hawthorn, A. C. and Opell, B. D. (
2003
). van der Waals and hygroscopic forces of adhesion generated by spider capture threads.
J. Exp. Biol.
206
,
3905
-3911.
Hayashi, C. Y. and Lewis, R. V. (
2001
). Spider flagelliform silk: lessons in protein design, gene structure, and molecular evolution.
BioEssays
23
,
750
-756.
Hayashi, C. Y., Shipley, N. H. and Lewis, R. V.(
1999
). Hypotheses that correlate the sequence, structure, and mechanical properties of spider silks.
Int. J. Biol. Macromol.
24
,
271
-275.
Hayashi, C. Y., Blackledge, T. A. and Lewis, R. V.(
2004
). Molecular and mechanical characterization of aciniform silk: uniformity of iterated sequence modules in a novel member of the spider silk fibroin gene family.
Mol. Biol. Evol.
21
,
1950
-1959.
Köhler, T. and Vollrath, F. (
1995
). Thread biomechanics in the two orb-weaving spiders Araneus diadematus(Araneae, Araneidae) and Uloborus walckenaerius (Araneae,Uloboridae).
J. Exp. Zool.
271
,
1
-17.
Lin, L. H., Edmonds, D. T. and Vollrath, F.(
1995
). Structural engineering of an orb-spider's web.
Nature
373
,
146
-148.
Opell, B. D. (
1998
). Economics of spider orb-webs: the benefits of producing adhesive capture thread and of recycling silk.
Funct. Ecol.
12
,
613
-624.
Opell, B. D. (
1999
). Changes in spinning anatomy and thread stickiness associated with the origin of orb-weaving spiders.
Biol. J. Linn. Soc.
68
,
593
-612.
Opell, B. D. (
2001
). Cribellum and calamistrum ontogeny in the spider family Uloboridae: linking functionally related but separate silk spinning features.
J. Arachnol.
29
,
220
-226.
Opell, B. D. (
2002
.). Estimating the stickiness of individual adhesive capture threads in spider orb-webs.
J. Arachnol.
30
,
494
-502.
Opell, B. D. and Bond, J. E. (
2001
). Changes in the mechanical properties of capture threads and the evolution of modern orb-weaving spiders.
Evol. Ecol. Res.
3
,
567
-581.
Opell, B. D. and Hendricks, M. L. (
2007
). Adhesive recruitment by the viscous capture threads of araneoid orb-weaving spiders of araneoid orb-weaving spiders.
J. Exp. Biol.
210
,
553
-560.
Opell, B. D. and Schwend, H. S. (
2007
). The effect of insect surface features on the adhesion of viscous capture threads spun by orb-weaving spiders.
J. Exp. Biol.
210
,
2352
-2360.
Opell, B. D. and Schwend, H. S. (
2008
). Persistent stickiness of viscous capture threads produced by araneoid orb-weaving spiders.
J. Exp. Zool.
309A
,
11
-16.
Opell, B. D. and Schwend, H. S. (in press). Adhesive efficiency of spider prey capture threads.
Zoology
.
Peters, H. M. (
1984
). The spinning apparatus of Uloboridae in relation to the structure and construction of capture threads(Arachnida, Araneida).
Zoomorphology
104
,
96
-104.
Peters, H. M. (
1986
). Fine structure and function of capture threads. In
Ecophysiology of Spiders
(ed W. Nentwig), pp.
187
-202. New York: Springer Verlag.
Peters, H. M. (
1992
). On the spinning apparatus and structure of the capture threads of Deinopis subrufus (Araneae,Deinopidae).
Zoomorphology
112
,
27
-37.
Peters, H. M. (
1995
). Ultrastructure of orb spiders' gluey capture threads.
Naturwissenschaften
82
,
380
-382.
Platnick, N. I. (
2008
).
The World Spider Catalog
, v. 8.5. http://research.amnh.org/entomology/spiders/catalog/INTRO1.html.
Swanson, B. O., Blackledge, T. A., Beltrán, J. and Hayashi, C. Y. (
2006a
). Variation in the material properties of spider dragline silk across species.
Appl. Physics A
82
,
213
-218.
Swanson, B. O., Blackledge, T. A., Summers, A. P. and Hayashi,C. Y. (
2006b
). Spider dragline silk: correlated and mosaic evolution in high-performance biological materials.
Evolution
60
,
2539
-2551.
Tillinghast, E. K., Townley, M. A., Wight, T. N., Uhlenbruck, G. and Janssen, E. (
1993
). The adhesive glycoprotein of the orb-web of Argiope aurantia (Araneae, Araneidae).
Mat. Res. Soc. Symp. Proc.
292
,
9
-23.
Townley, M. A. (
1990
). Compounds in the droplets of the orb spider's viscid spiral.
Nature
345
,
526
-528.
Townley, M. A., Bernstein, D. T., Gallanger, K. S. and Tillinghast, E. K. (
1991
). Comparative study of orb-web hydroscopicity and adhesive spiral composition in three areneid spiders.
J. Exp. Zool.
259
,
154
-165.
Vollrath, F., Fairbrother, W. J., Williams, R. J. P.,Tillinghast, E. K., Bernstein, D. T., Gallagher, K. S. and Townley, M. A.(
1990
). Compounds in the droplets of the orb spider's viscid spiral.
Nature
345
,
526
-528.
Vollrath, F. and Tillinghast, E. K. (
1991
). Glycoprotein glue beneath a spider web's aqueous coat.
Naturwissenschaften
78
,
557
-559.