2002. The Journal of Arachnology 30:494±502

ESTIMATING THE STICKINESS OF INDIVIDUAL ADHESIVE CAPTURE THREADS IN ORB WEBS

Brent D. Opell: Department of Biology, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061 USA. E-mail: [email protected]

ABSTRACT. Sticky threads improve an orb web's ability to retain the insects that strike it, allowing a spider more time to subdue these insects before they can escape from the web. The adhesive capture threads found in most orb webs feature small droplets of aqueous material. Inside each droplet, glycopro- tein granules coalesce to impart thread stickiness. An independent contrast analysis of threads produced by the adults of ®ve species (Leucauge venusta, Argiope trifasciata, Micrathena gracilis, Cyclosa conica, Araneus marmoreus) and ontogenetic studies of the threads of two of these species show that the volume of material in a thread's droplets is directly related to its stickiness. Models based on these analyses predict thread stickiness to within an average of 11% of the mean measured values using measurements of droplet diameter and distribution that are easily made with a compound microscope. This approach will facilitate the inclusion of thread stickiness in studies that examine the properties and performance of spider orb- webs. Keywords: Araneae, orb web, prey capture, spider thread, stickiness

The spirally-arrayed capture thread of a spi- et al. 1990). Within each droplet, a glycopro- der's orb web retains insects that strike the tein granule condenses and confers the web, giving the spider more time to subdue thread's stickiness (Peters 1995; Tillinghast et these prey before they escape from the web al. 1993; Vollrath & Tillinghast 1991). Rela- (ChacoÂn & Eberhard 1980; Eberhard 1986, tive to both spider size and capture thread vol- 1989, 1990). Orb weaving belong to ume, viscous thread is stickier than cribellar the , which is comprised of thread (Opell 1997, 1998). Consequently, ar- two subclades: the , whose mem- aneoids construct orb-webs with a greater bers produce primitive cribellar capture stickiness per capture area than do deinopoids threads, and the much larger , (Opell 1999b). whose members produce viscous capture The stickiness of adhesive capture threads threads (Bond & Opell 1998; Coddington differs greatly among species and is related to 1986, 1990a, 1990b; Coddington & Levi both spider mass and web architecture (Opell 1991; Griswold et al. 1998). Cribellar threads 1997, 1998, 1999b). Although few in number, are dry fuzzy threads in which the outer sur- studies that have examined the effect of thread faces are formed of thousands of ®ne, looped stickiness on orb web performance have found ®brils that are spun from the spigots of a spin- it to be signi®cant. Using arti®cial orb web ning plate termed the and, in the analogs, ChacoÂn & Eberhard (1980) showed Deinopoidea, are supported by a pair of inner that increasing the amount of adhesive on the axial ®bers (Eberhard & Pereira 1993; Opell lines of these ``webs'' increased the number 1994a, 1994b, 1995, 1996, 1999a; Opell & of prey that they retained. The observation Bond 2001). These cribellar ®brils snag on the that orb webs constructed by adult spiders re- setae and irregular surfaces of insects and ad- tained prey for longer periods than those of here to smooth surfaces by van der Waals and conspeci®c juveniles was attributed to the pu- hygroscopic forces (Opell 1994c; Hawthorn & tatively stickier threads produced by adults Opell pers. obs.). In contrast, the viscous cap- (Eberhard 1989), as was the greater size of ture threads of araneoids are formed of a pair prey captured by adults (Opell 1990). Using of supporting axial ®bers overlain by a com- measurements of capture thread stickiness, plex aqueous solution that coalesces into reg- capture thread length, and web capture area, ularly spaced droplets (Vollrath 1992; Vollrath Opell (1997, 1999b) computed and compared

494 OPELLÐESTIMATING THREAD STICKINESS 495 the total stickiness and the stickiness per cap- ture area of orb webs constructed by several species. However, ecological studies of orb web performance have never incorporated thread stickiness. This is because the tech- niques for measuring stickiness have only re- cently been developed and are rather laborious (Opell 1997, 1998). Because glycoprotein granules condense from the material that forms a thread's drop- lets, I hypothesize that droplet volume is di- rectly related to thread stickiness. Phyloge- netic analysis of adhesive threads produced by ®ve species and ontogenetic analyses of the threads produced by two of these species sup- port this hypothesis. Formulas derived from these analyses make it possible to estimate ac- curately the stickiness of adhesive capture threads using a simple set of measurements made with a compound microscope. This pro- cedure thus makes estimates of the stickiness of individual threads accessible to ecological and comparative studies. METHODS Figure 1.ÐAn overview of araneoid phylogeny (summarized from ®g. 82 in Scharff & Coddington Species studied.ÐI selected for study ®ve 1997), showing the position of the ®ve genera in- species that represent major araneoid cluded in the current study. (Fig. 1), show considerable ecological diver- sity, and were abundant enough near Blacks- burg (Montgomery County), Virginia to per- (Walckenaer 1805) has a mean adult female mit the sample sizes required by my studies. body mass of 84.3 mg (n ϭ 21, SE ϭ 5.0). It These species were identi®ed using the sys- occupies moist forests, where it constructs orb tematic studies of Berman & Levi (1971) and webs that have very closely spaced capture Levi (1968, 1976, 1977, 1980, 1985). Voucher spirals and an orientation that ranges from specimens are deposited in the Museum of nearly horizontal to vertical. Cyclosa conica Comparative Zoology, Harvard University. (Pallas 1772) has a mean adult female body One value for droplet volume and one value mass of 8.9 mg (n ϭ 23, SE ϭ 0.8), constructs for thread stickiness were computed from the vertical orb webs, and can be found on ex- threads produced by each spider. Interspeci®c posed shrubbery and on vegetation along for- comparisons are based on threads produced by est edges. Araneus marmoreus Clerck 1757 adult females. has a mean adult female body mass of 677.1 Leucauge venusta (Walckenaer 1841) is mg (n ϭ 15, SE ϭ 75.0), constructs vertical member of the family Tetraghathidae. Adult orb webs on vegetation along forest edges, females have a mean body mass of 28.1 mg and uses a signal line to monitor its web from (n ϭ 26, SE ϭ 2.3), are typically found in a retreat made of a curled leaf. shaded forest edges, and construct horizontal Thread collection and storage.ÐI collect- orb webs. The remaining species are members ed some capture threads directly from orb of the family Araneidae. Argiope trifasciata webs on samplers made from microscope (ForskaÊl 1775) is a large orb-weaver with a slides with raised, parallel, rectangular sup- mean adult female body mass of 474.0 mg (n ports glued at 4.8 mm intervals to their upper ϭ 25, SE ϭ 51.6). It is found on weedy veg- surfaces. Doubled-sided tape atop these sup- etation, typically in exposed areas, where it ports maintained the thread's native tension. constructs vertical orb webs with widely In some cases, a sector of an orb web was spaced capture spirals. Micrathena gracilis ®rst captured on an 18 cm diameter polished 496 THE JOURNAL OF ARACHNOLOGY aluminum ring with double-sided tape on its ing formulas to compute the volume (␮m3)of upper rim and thread samples were collected viscous material in the droplets of a 1-mm from this ring in the laboratory. length of adhesive thread. Threads were collected in the morning to Mean Droplet Radius (MDR) ensure that they were fresh. However, it was ϭ (Grand Mean of L and W)/2. (1) not possible to determine precisely how old these threads were. Thread droplets were mea- Droplet Volume (DV) sured 2±4 h thereafter, and their stickiness was ϭ 4 ϫ pi ϫ MDR3 /3. (2) measured no more than 3 h later. Threads were maintained and measured at 23Ð25 oC and Droplets per mm (DPMM) 60Ð62% RH (Table 1). Under these condi- ϭ ((N ϩ N )/(D ϩ D))ϫ 1000. (3) tions, the hygroscopic properties of viscous 1212 threads maintain droplet volume (Townley et Droplet Volume per mm (DVPMM) al. 1991). However, I did not attempt to mea- sure the stability of droplet dimensions. ϭ DV ϫ DPMM. (4) The thread properties of each species prob- ably perform optimally under the environmen- Interdroplet volume was not included in tal conditions typical for that species. How- these calculations, as it is small and does not ever, Townley et al. (1991) show that the size appear to contribute to glycoprotein granule of viscous thread droplets can change as they formation (Opell 1997). Values of droplet vol- take up the atmospheric water and that this is ume include the volume of the supporting ax- in¯uenced by ambient humidity. Thus, tem- ial ®bers that run through the droplets. I did perature and humidity differences among hab- not attempt to factor out this volume for two itats probably in¯uence droplet volume and reasons. First, for the species studied axial ®- this feature may change over the course of a ber diameters are small, ranging from 1.03± day. By maintaining threads under stable and 5.37 ␮m (Opell & Bond 2001); and ®brils fairly uniform temperature and humidity con- comprise only a small part (0.07±0.75%, mean ditions, I believe that this ¯uctuation was min- 0.35%) of a droplet's volume. Second, the ob- imized and that I established conditions that jective of this study was to devise a simple were acceptable for the broad comparisons method for estimating thread stickiness. Axial made in this study. ®ber diameter is dif®cult and time-consuming Droplet volume.ÐI ®rst examined a thread to measure (Opell & Bond 2001) and its in- under a dissecting microscope to ensure that clusion is inconsistent with this objective. it was not damaged. Its droplets were then Thread stickiness.ÐThread stickiness is measured at 500 X under a compound micro- reported as the force (␮N) required to over- scope. Both the repeatability and resolution of come the stickiness of a 1 mm length of measurement of droplet diameters were about thread. As described in more detail by Opell 0.4 ␮m (Opell 1997). I determined thread vol- (1997), this was determined by ®rst pressing ume from measurements (Fig. 2) of the dis- a 2 mm wide piece of 320 grit, 3M waterproof tance (D) spanned by a series of droplets (N) silicon carbide sandpaper against a thread and the lengths (L, dimension parallel to with a standard force. The force required to thread length) and widths (W) of two droplets. pull the contact plate from the thread was then Droplet length in¯uenced the number of drop- measured with a strain gauge made from a lets included in a series as shown in Table 1. stainless steel needle. The particles on the sur- Two thread sectors were measured for each face of these sandpaper plates were of uniform spider's web (subscripts 1 and 2). When drop- size and distribution (Opell 1993) and these let size was not uniform, I measured represen- plates registered the same stickiness for ad- tatives of the larger and smaller droplets. Ta- hesive threads as did contact plates made from ble 1 gives the mean intra-sample range of ¯esh¯y wings (Opell 1997). Thus, the sticki- droplet length and width for each species. In- ness values obtained by this method were sim- creasing the number of droplets and thread ilar to those registered by a representative in- sectors measured increases the accuracy with sect surface. which thread volume can be computed. All Phylogenetic analyses.ÐFeatures of spe- measurements were in ␮m. I used the follow- cies that are evolutionarily related are not, in OPELLÐESTIMATING THREAD STICKINESS 497

Table 1.ÐDroplet measurements and thread stickiness values and the conditions under which threads were stored and measured (mean Ϯ 1 standard error). The mean intra-sample range of droplet length and width is given in parentheses.

Family Percent Mean Droplet Droplet Droplets Droplet Thread Species Temp. relative droplets length width per volume stickiness (individuals) (ЊC) humidity per series (␮m) (␮m) mm (␮m3/mm ϫ 103) (␮N/mm)

Tetragnathidae 25 Ϯ 0.1 61 Ϯ 0.5 Leucauge venusta 8 12.5 Ϯ 0.51 10.1 Ϯ 0.42 41.4 Ϯ 2.1 31.3 Ϯ 2.6 19.3 Ϯ 1.3 (25) (2.2) (2.1) Araneidae 23 Ϯ 0.3 60 Ϯ 0.5 Argiope trifasciata 13 43.0 Ϯ 2.1 29.3 Ϯ 1.3 9.4 Ϯ 0.56 257.7 Ϯ 31.4 28.2 Ϯ 1.9 (33) (21.0) (18.8) Micrathena gracilis 23. Ϯ 0.2 62 Ϯ 0.4 4 24.9 Ϯ 1.0 19.8 Ϯ 0.72 20.2 Ϯ 0.92 117.2 Ϯ 9.3 26.4 Ϯ 1.6 (17) (5.3) (6.2) Cyclosa conica 24 Ϯ 0.1 61 Ϯ 0.5 6 11.8 Ϯ 0.64 9.6 Ϯ 0.56 34.0 Ϯ 2.4 21.3 Ϯ 2.2 9.6 Ϯ 0.76 (18) (3.9) (3.9) Araneus marmoreus 23 Ϯ 0.2 61 Ϯ 0.2 9 59.6 Ϯ 3.6 46.9 Ϯ 3.1 6.9 Ϯ 0.80 480.2 Ϯ 55.7 32.0 Ϯ 2.7 (21) (23.2) (21.5) a strict sense, independent and, therefore, vi- I collected threads from the ®eld rather than olate the assumptions of parametric statistics from spiderlings reared in the laboratory to (Felsenstein 1985; Harvey & Pagel 1991). reduce the possibility that spider diet would Therefore, I employed the independent con- unnaturally affect the results of this study. I trast (IC) method of Felsenstein (1985) to de- collected threads from A. trifasciata from ear- termine if the droplet volume and stickiness ly June to late October at a site where the of threads produced by adult female spiders previous year I observed a large number of were related. This method accounts for the in- adult females of this species and only one in- ¯uence of phylogeny on continuous characters dividual of the sympatric species, A. argentata by analyzing differences in the values ex- Lucas. I collected threads of M. gracilis from pressed by sister taxa (both extant taxa and early June to late August at a site where, for their inferred ancestors). These differences are two years, this was the only species of this then normalized and relationships among the genus that I observed. As spiderlings do not resulting independent contrast values are an- emerge synchronously from egg sacs and as alyzed with regression statistics. I used the it was not possible to permanently mark spi- Comparative Analysis of Independent Con- derlings, I was unable to determine precisely the age of individuals included in each devel- trasts program of Purvis & Rambaut (1995) opmental series. My approach was, therefore, to compute normally distributed independent to sample these populations regularly to ob- contrast values and the S.A.S statistical pack- tain threads produced by immatures of in- age (S.A.S. Institute Inc., Cary, North Caro- creasing size and, eventually, by adult spiders, lina) to perform this and the other statistical thereby obtaining capture threads from each tests reported in this study. species that had droplets of increasing size. Ontogenetic analyses.ÐI examined devel- opmental changes in the droplet volume per RESULTS mm and the thread stickiness per mm of Phylogenetic analysis.ÐDroplet volume threads produced by A. trifasciata and M. ranged from 21±480 x 103 ␮m3 per mm length gracilis. I selected these species because threads produced by adult females have sim- ilar stickiness values but differ greatly in the size and distribution of their droplets (Table 1). Argiope trifasciata produces threads with much greater droplet volume but only half as many droplets per mm as threads of M. grac- Figure 2.ÐMeasurements used to compute drop- ilis. let volume. 498 THE JOURNAL OF ARACHNOLOGY

Figure 3.ÐRelationship between change in drop- let volume per mm and thread stickiness, as deter- mined by an independent contrast analysis. Num- Figure 4.ÐDevelopmental changes in droplet bers refer to independent contrast values between volume and thread stickiness of Argiope trifasciata the sister taxa identi®ed in the pruned phylogeny. and Micrathena gracilis. A common regression line is shown, as the slopes and intercepts of the two species' regression lines do not differ. of capture thread and stickiness ranged from 9.6±32.0 ␮N per mm of thread length (Table 1). IC analysis supports the hypothesized as- droplet volume and thread stickiness (Fig. 3). sociation between droplet volume and thread However, as IC values are derived values, they stickiness by showing that the IC values for do not depict the actual relationship between droplet volume and thread stickiness are di- these features. Therefore, I employed tradi- rectly related (Fig. 3; n ϭ 4, F ϭ 356.64, P tional regression analysis to describe the re- ϭ 0.0028, R2 ϭ 0.99). lationship between droplet volume per mm of Ontogenetic analysis.ÐFor threads of both thread length and thread stickiness. In one A. trifasciata and M. gracilis with droplets of analysis I used the pooled values of adult fe- increasing size, droplet volume per mm was males from the ®ve species (Fig. 5; n ϭ 113, directly related to thread stickiness per mm F ϭ 58.26, P ϭ 0.0001, R2 ϭ 0.34) and in (Fig. 4; n ϭ 94, F ϭ 33.25, P ϭ 0.0001, R2 another the mean values of each of the ®ve ϭ 0.27 and n ϭ 67, F ϭ 69.25, P ϭ 0.0001, 2 R ϭ 0.52, respectively). When logn droplet volume per mm was used, a better ®t for A. trifasciata was obtained and the ®t for M. gracilis did not change appreciably (F ϭ 83.44, P ϭ 0.0001, R2 ϭ 0.48 and F ϭ 67.90, P ϭ 0.0001, R2 ϭ 0.51, respectively). An anal- ysis of covariance test showed that slopes of the regression lines for logn droplet volume per mm and thread stickiness did not differ (F ϭ 0.20, P ϭ 0.65) and a comparison of the intercepts of the two species regression lines showed that they also did not differ (F ϭ 1.41, P ϭ 0.24). Thus, a single regression line sat- isfactorily describes developmental changes in logn droplet volume per mm and the stickiness of these two species' capture threads (Fig. 4). Characteristics of threads produced by Figure 5.ÐRelationship between droplet volume adult females.ÐThe IC analysis documented per mm of thread and thread stickiness for adults that there is a direct relationship between of ®ve araneoid species. OPELLÐESTIMATING THREAD STICKINESS 499

Figure 7.ÐComparison of measured stickiness Figure 6.ÐRelationship between the mean adult (S) and the values predicted by three models. Model values of droplet volume per mm of thread and 1 is based on the regression of mean adult values thread stickiness for ®ve araneoid species. and has the formula: S ϭ Logn droplet volume per mm X 6.262±48.95. Model 2 is based on the on- togenetic regression and has the formula: S Log species (Fig. 6; n ϭ 5, F ϭ 25.03, P ϭ 0.015, ϭ n droplet volume per mm X 4.063±23.183. Model 3 R2 ϭ 0.89). In both analyses droplet volume is an empirical model and has the formula: S ϭ was directly related to thread stickiness and in Logn (droplet width X droplet length X droplets per both this relationship was curvilinear. That is, mm) X 13.184±96.288. stickiness increased at a slower rate than did droplet volume. Consequently this relation- ship was best explained by formulas that used model on a regression of the mean adult val- log values of droplet volume (Figs. 5, 6). The ues of these variables (Fig. 6). The second plot of all values (Fig. 5) shows that as a model uses the common ontogenetic regres- thread's droplet volume increases, so too does sion formula shown in Fig. 4 to predict stick- the variance of its measured stickiness. iness. The third model is based on the obser- Only for the threads produced by adult fe- vation that the product of droplet length (DL), male C. conica was there a direct relationship droplet width (DW), and the number of drop- between droplet volume per mm thread length lets per mm (DPMM) successfully predicted and thread stickiness (n ϭ 18, F ϭ 11.71, P thread stickiness (F ϭ 40.24, P ϭ 0.008, R2 ϭ 0.0041, R2 ϭ 0.41, stickiness (␮N/mm) ϭ ϭ 0.93) according to the following formula: 0.00022 thread volume (␮m3/mm) ϩ 4.860. In the other four species this relationship was not Thread Stickiness signi®cant (0.13 Ͻ P Ͻ 0.94). When log n ϭ log (DL ϫ DW ϫ DPMM) ϫ 13.184 droplet volume per mm thread length was n used, the relationship for C. conica remained Ϫ 96.288. (5) signi®cant (n ϭ 18, F ϭ 12.29, P ϭ 0.0029, 2 R ϭ 0.43, stickiness (␮N/mm) ϭ 4.370 logn Figure 7 compares the performance of these thread volume (␮m3/mm)Ϫ33.53) and for the three models. For model 1 the mean absolute other four species it was insigni®cant (0.08 Ͻ difference between each of the ®ve species' P Ͻ 0.84). measured and predicted stickiness values was Modeling thread stickiness.ÐI developed 13.23%, for model 2 it was 20.52%, and for and evaluated three models to predict the model 3 it was 10.76%. stickiness of adhesive threads (Fig. 7): one based on adult thread features, one based on DISCUSSION ontogenetic data, and one based on a trial-and- Phylogenetic and ontogenetic analyses sup- error empirical examination of droplet values. port the hypothesis that the volume of material Because phylogenetic analyses demonstrate a in an adhesive thread's droplets is directly re- direct relationship between droplet volume per lated to its stickiness. Models based on these mm and thread stickiness, I based the ®rst comparisons provide good estimates of the 500 THE JOURNAL OF ARACHNOLOGY

increases relative to droplet volume, granule area and, presumably, thread stickiness in- creases. However, this model also shows that when granule volume is a constant percent of a droplet, granule surface area increases more slowly than does droplet volume. Thus, it ap- pears that the bene®ts (in terms increased thread stickiness) of producing capture threads with larger adhesive droplets diminish unless there is also an increase in granule volume relative to droplet volume. Similarities in the shapes of the curves in Figs. 5 & 8 suggest that no such compensatory increase in granule volume has occurred in the species that were studied. However, the similarity of these curves may be coincidental and the shape of the curve in Fig. 5 may instead re¯ect inter- speci®c differences in the composition and Figure 8.ÐRelationship between the volume of concentration of glycoproteins and hydroscop- a viscous droplet and the surface areas of its gly- ic compounds in the droplets of these species. coprotein granule, with granule volume being com- The ability of an orb web to intercept prey puted as 5%, 10%, and 20% of droplet volume. is related to its capture area (the area between the inner- and outer-most capture spiral of the stickiness of adhesive threads that have a wide web), whereas the web's ability to retain prey range of droplet size and distribution patterns is related to its stickiness per capture area and that are produced by spiders of greatly (Opell 1999b). The origin of modern adhesive different masses. The simplicity and accessi- orb webs like those treated in this study was bility of this approach will facilitate the inclu- associated with an increase in the stickiness sion of thread stickiness in studies that ex- per capture area (Opell 1999b). However, amine the properties and performance of there were differences in the stickiness per spider orb webs. However, it is important to capture area among the ®ve adhesive orb webs remember that these models are based on the examined by Opell (1999b). The methods de- threads of only ®ve species from the temper- scribed in this study make it easier to examine ate region of North America that belong to the consequences of these differences in web two families and that they represent only a design. For example, do larger spiders tend to small part of araneoid diversity. Additionally, construct webs characterized by a greater cap- variance in the stickiness predicted by the ture area and a smaller stickiness per capture droplet volume of threads (Fig. 5), particularly area because they are better equipped to sub- the threads of large araneoids, such as A. tri- due larger prey than are smaller spiders? Are fasciata and A. marmoreus, requires that the the running and response speeds of a spider capture threads of many individuals be mea- associated with the capture area and stickiness sured to usefully estimate the stickiness of a per area of its orb web? Are horizontal orb species' threads. webs characterized by a greater capture areas The curvilinear relationship between drop- and smaller stickiness per capture area be- let volume per mm of thread length and thread cause they tend to capture smaller, more er- stickiness (Figs. 4, 5) is most easily explained ratically ¯ying insects (Craig 1987)? Does the by the relationship between the volume of a stickiness per capture area of an orb web in- viscous droplet and the surface area of the ¯uence the guild of insects that it captures? These and other questions can be addressed glycoprotein granule that lies within it. If con- by estimating and considering capture thread tact between a thread's granules and an object stickiness. is responsible for thread stickiness, then the surface areas of granules should be directly ACKNOWLEDGMENTS related to thread stickiness. The model pre- Jason E. Bond assisted with ontogenetic sented in Fig. 8 shows that as granule size studies and Sarah C. Crews helped with data OPELLÐESTIMATING THREAD STICKINESS 501 entry and analysis. Jonathan Coddington and Griswold, C.E., J.A. Coddington, G. Hormiga & N. Robert Jackson provided useful comments on Scharff. 1998. Phylogeny of the orb-web build- the manuscript. 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