J. Zool., Lond. (1973) 171, 367-384

Stabilimenta on the webs of diversus (Araneae: ) and other

WILLIAMG. EBERHARD* Museum of Comparative Zoology, Harvard University, Cambridge, Massachusetts, U.S.A.

(Accepted 12 June 1973)

(With 4 plates and 3 figures in the text)

Uloborus diversus places extra silk (“stabilimenta”) near the hubs of its webs, preferentially on short radii ending near anchor threads. Spiders probably distinguish these radii from others by their relatively low extensibility. The stabilimentum probably functions as a camouflage device, and the orientation of the stabilimentum lines probably aids disturbed spiders in making quick exits from webs. Turning responses at radius-frame junctions during these exits are influenced by thread angles at the junctions (and possibly by other factors), and also enable disturbed spiders to reach hiding places quickly. The available data on other stabilimentum-building spiders suggests that they also use stabilimenta to provide defence against visually-hunting predators.

Contents Page Introduction...... 361 Methods ...... 369 Stabilimentum patterns ...... 369 Orientation of stabilimenta in the field ...... 312 Orientation of stabilimenta in the laboratory ...... 313 Movements preceding stabilimentum construction ...... 313 Stabilimentum construction ...... 314 Possible stimuli directing stabilimentum placement ...... 316 Spiders’ positions on webs in nature ...... 311 Escape behaviour ...... 318 Discussion ...... 380 References ...... 383

Introduction Many spiders which remain on their webs during the day rest near objects they have fastened to the web. These objects are usually called stabilimenta when found in orb webs. In these webs (made by spiders in the families Uloboridae and Araneidae), stabilimenta consist of bands or tufts of silk, egg sacs, and detritus (Table I). Although egg sac and detritus stabilimenta are generally thought to serve as camouflage (e.g. Gertsch, 1949), the function of silk stabilimenta has been controversial. Silk stabili- menta were first thought to stabilize orbs, supposedly by strengthening the connections between hub threads (McCook, 1889; Comstock, 1940). Other authors suggested they were reserve cables for use in swathing prey (Vinson, 1863 quoted in Wiehle, 1927), * Present address: Universidad del Valle, Departamento de Biologia, Cali, , South America. 361 368 W. G. EBERHARD TABLEI Orb weaving spiders which spin stabilirnenta, and their daytime resting sites

Spider at hub during the day? References SILK STABILIMENTA (apparently all species) Yes McCook, 1889; Wiehle, 1927, 1928; Hingston, 1927, 1932; Marson, 1947a; McKeown, 1952; Levi, 1968; Robinson & Robinson, 1970; Ewer, 1972 Uloborus (apparently all except U.gibbosa McCook, 1889; Wiehle, 1927, 1931; which often omits the stabilimentum) Hingston, 1927, 1932; B. J. Marples, 1955, 1962; Kullmann, 1972; this paper adiantus Wiehle, 1929 armatus Wiehle, 1931 redii Wiehle, 1927 Aculepeira ceropegia (= Aranea ceropegia) Wiehle, 1927 Cyclosa conica McCook, 1889; Marples & Marples. 1937 ginnage Yaginuma, 1966 inssulana Kullmann, 1972 sedecuolata Yaginuma, 1966 Cyrtophora moluccensis Yaginuma, 1966 Gasterucantha brevispina Marson, 19476 cancriyormis McCook, 1889; Comstock, 1940 pallida Hingston, 1932 versicolor Ernerit, 1968 Landana cygnea Eberhard, in prep. Micrathena armatus Hingston, 1932 gracilis McCook, 1889 sagittata Comstock, 1940 benjamina McCook, 1889 Singa haemata Wiehle. 1929 EGG SAC STABILIMENTA (GENERA) Cyrtophora Kullmann, 1961 Cyclosa McCook, 1889; Wiehle, 1927, 1928; Kaston, 1948

Micrathena (1Acrosoma) McCook, 1889 Uloborus Emerton, 1883; McCook, 1889; Scheffer, 1905 DETRITUS STABILIMENTA (GENERA) Gasteracantha Hingston, 1927 Cyclosa McCook, 1889; Hingston, 1927; Wiehle, 1927 Landana Eberhard, in prep.

deposits of superfluous silk left over after web construction (Wiehle, 192Q or “love paths” to guide the male to the female (Wolfram, 1924 quoted in Wiehle, 1927). Hingston (1927, 1932), and later Marson (1947a, b) showed the great variety of patterns of silk stabilimenta, and, noting a general correlation between the shape of the stabilimentum and the posture of the as it rested at the hub, concluded that stabilimenta function as defence against visually orienting predators by obscuring the outline of the spider or STABILIMENTA ON WEBS 3 69 by distracting the attention of such predators from the spider. Several recent authors (e.g. Gertsch, 1949; Robinson & Robinson, 1970) are not convinced of the defensive function of stabilimenta, at least in certain cases, and propose other possible functions such as “tuning” or strengthening the web. The data of this study, which is the first detailed examination of stabilimenta on the webs of Uloborus diversus, suggest that the stabilimenta on these webs function as camou- flage devices, and that they are oriented so the resting spider is in position to follow a short escape route off the web when disturbed.

Methods Webs were studied both in the field and in the laboratory. Approximately 325 webs, each spun by a different spider in a different location, were observed in high Sonoran desert near Cave Creek, Arizona (elevation c. lo00 m) between 20 July and 5 August 1966. Webs of spiders of all ages except the first instars were studied. More than 600 webs and more than 30 stabilimentum constructions made by a stock of about 50 spiders were observed between September 1965 and March 1966 in a laboratory in Cambridge, Mass. Light levels were measured with a “Gossen Luna Pro” meter. Strong light did not inhibit stabilimentum construction, and movies (48 f/sec) of this behaviour were obtained and analysed. All significance tests reported below are 2 x 2 two-tailed x2 tests. Stabilimentum patterns Uloborus diversus constructed two types of stabilimentum,* both composed entirely of silk: the “linear” type (Plate I(a)) consisted of one or more mats of white silk laid in lines (“arms”) along radii; the “circular” type (Plate I(b)) contained, in addition to an arm or arms, silk laid in roughly circular loops at the hub (sometimes there was no arm visible in a circular stabilimentum). Occasionally a spider laid stabilimentum silk on lines at the edge of the web or below it (Plate 11). These lines were usually attached to frames or radii in the web, but were sometimes completely free of the web. Linear stabilimenta were most common both in the field and in the laboratory: 65 % of 268 webs in the field had linear stabilimenta, 30 % had circular stabilimenta, and 5 % had none; 42 % of 590 webs constructed in the laboratory had linear stabilimenta, 25 % had circular stabilimenta, and 33 % had none. Pairs of arms were most common in linear stabilimenta, and although the arms tended to be on opposite sides of the hub like those in Plate I(a), there was substantial variation in patterns (Table 11). Ninety-six per cent of

TABLEI1 Stabilirnentumpatterns of U. diversus

Type of Number of arms in stabilimentum stabilimentum N 0 1 2 3 4 5 Laboratory line 455 - 16% 76% 6% 1% 1% circle 135 1 48 31 14 5 1 Field line 182 - 5 93 2 0 0 circle 86 8 53 32 6 1 0

* The hubs of webs of first instar spiders were usually dotted with white specks which were often so numerous that there was a white “mat” at the hub. These specks were probably by-products of the construction of supple- mentary radii rather than a distinct structure (see Eberhard, in prep.). 3 70 W. G. EBERHARD

PLATEI. Linear (a) and circular (b) stabilimenta at the hubs of U. diversus webs. The spider resting at the hub normally just fills the gap between the arms of the linear stabilimenturn. STABILIMENTA ON ULOBORUS DIVERSUS WEBS 371

PLATE11. Web of U. diversus with stabilimentum lines below the web, and the spider at the hub.

167 webs in the field and 96 % of 379 webs in the laboratory which had linear stabilimenta had at least one pair of stabilimentum arms which were on opposite sides of the hub from each other. These trends were reduced to 74 % of 35 webs in the field and 52 % of 61 webs in the laboratory which had circular stabilimenta. As noted by B. J. Maples (1962) in other species of Uloborus, given individuals were capable of making both types of stabilimentum, but most spiders usually spun only one type under constant conditions in the laboratory. Laboratory experiments showed that the light level during the night (when the spiders spun their webs) was correlated with changes in stabilimentum pattern. The amount of night light was changed for 21 consecutive nights, and 18 spiders (instar No. 3 and older) were fed one fruit fly per web, and their webs destroyed each day. The spiders consistently constructed both more circular stabilimenta and more lines under their webs following nights of bright (4.5 Lamberts) illumination; and they constructed fewer, predominantly linear stabilimenta with fewer lines under their webs (both P<0-05)after low light level (less than 1-0 Lamberts) nights (Table 111). Thus spiders resting in spots more exposed to the night sky may construct more elaborate stabilimenta. Webs at such sites would probably be most exposed to large, visually orienting predators, and these trends in stabilimentum size may function to provide maximum camouflage for those spiders most susceptible to attack, and preserve silk in the others. 372 W. G. EBERHARD TABLEI11 Correlation between diffuse Iight during the night and the occurrence of different stabilimentum types. Numbers in parentheses are the per cent of webs with lines under the web; * indicates Pt0.01 comparing bright and dark nights

% linear % circular % without stabilimenta stabilimenta stabilimenta Totals Bright nights 36 (13) 44 (26)* 20 (3) 100 (42)* (N=60) Dark nights 58 (1) 13 (0) 30 (0) 101 (1) (N=91)

Recent feeding failed to influence the types of stabilimenta constructed in the laboratory (Table IV). Spiders fed one fruit fly (Drosophilu melunoguster) on day 0 and then not fed on subsequent days although their webs were destroyed daily, constructed the same types of stabilimenta with the same numbers of loops and arms on days 1 and 2 (Pt0.05). Nor was there correlation between the age of spiders and the types of stabilimenta con- structed in the field except that mature females tended to construct fewer stabilimenta than immatures (P<0.05,N= 46).

TABLEIV Lack of influence of recent feeding on occurrence of different stabiIimentum types of U. diversus

One day Two days after feeding after feeding (N=76) (N=61)

% linear type 33 34 % circular type 28 31 % without stabilimentum 39 34 Average number stabilimentum arms 2.8 2-4 Average number stabilimentum loops 2.8 2.9

Orientation of stabilirnenta in the field All the webs studied in the field were at the bases or on the sides of roughly dome-shaped piles of debris (sticks, pieces of cactus, cow dung, etc.) made by pack rats (Neotornu lepida). The site of the hub, the slant of the orb, and the orientation of stabilimentum lines all generally bore consistent relationships to the orientation of the web on the pack rat nest. Most of the stabilimenta on these webs had a pair of arms (Table 11) which almost always formed a continuous line across the hub (Plate I(a)). These composite lines usually ran from the side of the web farthest from the centre of the pack rat nest toward the nest centre: 80 % of the composite lines on 167 webs were directed to within 25" of the centre of the nest (P

Orientation of stabilimenta in the laboratory The hubs of webs built in the laboratory, like those in the field, were usually not at the geometric centres of the webs. The stabilimenta on these webs were also oriented toward relatively short radii. In 110 webs containing a total of 212 stabilimentum arms, 94 had at least one stabilimentum arm on a shorter than average radius (P

Movements preceding stabilimentum construction The stabilimentum was always constructed after the sticky spiral was finished and the web was otherwise complete. Spiders usually paused 5-lominutes at the hub after finishing the sticky spiral before making the stabilimentum. Sometimes they turned slowly at the hub, stopping repeatedly to grip one or two radii with legs I and jerk them or bounce the web (see descriptions of these movements below) before coming to rest. The spiders almost always performed another series of these test-like movements after the pause, just before they began to lay the stabilimentum. These movements, which probably served to orient the stabilimentum, were analysed using motion pictures. They included one type of bounce and two types of jerk-a “slow flex” and a “sharp jerk”. The bounces occurred in series. A bounce began when the spider drew its body away from the horizontal web, extended its legs upward (ventrally), and pulled them together. Then it released the tension and swung its abdomen upward causing its body and the whole hub to swing upwards. Spiders often made similar movements during subsequent down and up swings so that a series of bounces often lasted up to two or three seconds. The frequency of the bounces, about 5/sec for a mature female, was probably the resonant frequency of the web loaded with the spider’s weight. Both types of jerk were produced when the legs (especially legs I) were flexed as they held radial and/or hub threads. Neither type of jerk was as vigorous as the otherwise 314 W. G. EBERHARD similar jerks associated with attack behaviour. Typically a spider performed a series of jerks and bounces, turned and performed another series, and so on. There was no set order of jerks and bounces within a series, although sharp jerks were often preceded by series of slow flexes. Occasionally a spider performed only slow flexes and omitted the other movements. The turns made between early bursts of jerking and bouncing were often 90” or more, but became smaller and smaller until finally all jerks were directed to the same small sector of the web. There was some variation in this pattern, and sometimes spiders turned very little. The relative positions of legs I also changed during a series of turns. Legs I usually gripped adjacent or nearly adjacent radii during early jerks and bounces, but both usually gripped the same radius during the later ones. Thus the spiders “zeroed in” on a particular radius by both leg and body position before starting stabilimentum construction on that radius.

Stabilimentum construction Stabilimentum construction, which always immediately followed the last jerks and bounces, began with several dabs by the spinnerets against threads near the edge of the hub. Then the spider walked slowly from the hub along the radius its legs I were holding. Legs I, 11, and I11 held this radius while the abdomen shuttled from side to side (about 3 times/sec in one adult female) and legs IV pulled silk from the spinnerets at each swing and packed it into a white band which stuck against the radius. The spider periodically dabbed its spinnerets against the radius as it moved (about every 3-5 mm). It turned back part way out the radius (usually less than half way) and retraced its steps to the hub, laying another band of silk against the underside of the first. Another series of small turns, sometimes accompanied by jerks, usually preceded the laying of each additional stabilimentum arm, but sometimes the spider continued across the hub without pausing and laid a stabilimentum arm under a radius opposite the first. Spiders which crossed the hub in this way usually laid much less stabilimentum silk at the hub itself than on the radii. Often spiders “recoated” radii already having stabilimentum arms, usually covering only the inner portion of the arm. Finished stabilimentum arms thus often began abruptly near the edge of the hub, and seldom intersected (Plate I(a)). Loops of circular type stabilimenta were laid outermost loop first. The spider made few probing movements as it laid loops, only occasionally tapping outward with outside leg I until it touched the last loop of stabilimentum just laid. The other legs did not hold the line(s) onto which the silk was laid, but the movements of legs IV and the abdomen were identical to those during the construction of linear stabilimenta. The spacing between the loops was often irregular, but although a given loop often merged with the preceding loop, it seldom crossed over it. As noted by McCook (1889) and Wiehle (1927), the movements of the abdomen and legs IV during stabilimentum construction resemble those made during prey wrapping. Stabilimentum silk differs from wrapping silk however in being composed of smooth and curly strands rather than uneven and kinky ones (Plate 111). The stabilimentum silk of U. diversus is loose and unstressed (Plate III), and since it often does not connect threads but is just laid as a cottony mat on a single thread, it is extremely unlikely that it contributes to the structural stability of the web. STABILIMENTA ON ULOBORUS DIVERSVS WEBS 375

~~ ~ PLATE111. Light micrographs of (a) stabilimentum silk laid on two radial lines, and (b) wrapping silk. 376 W. G. EBERHARD Possible stimuli directing stabilimentum placement There were almost certainly no temperature, humidity, light, or wind gradients in the laboratory situation, so none of these provided cues to orient the spider as it constructed stabilimentum lines. Most of the webs were close to horizontal so gravity may also be excluded. Properties of the web itself thus probably provided the cues used to orient the stabilimentum.

FIG.1. Balances of stresses in an orb web. The threads which are stressed when the spider flexes legs I are drawn as solid lines. The jerking and bouncing movements preceding stabilimentum construction resemble testing movements and are probably used to accomplish the orientation. Resonant vibrations of radii are not likely to yield accurate information on radius length (Eberhard, 1969, 1972; R. M. Langer, pers. comm.), and neither bouncing nor slow flex movements STABILIMENTA ON ULOBORUS DIVERSUS WEBS 377 appear to be “designed” to sense vibrations of the radii, so thread vibrations can be tentatively ruled out as cues. Although inter-radial angles could yield information about both radius length and anchor sites (Eberhard, 1972), the jerking and bouncing movements do not appear suited to make this measurement either. The extensibility of the radii is another possible cue; the extensibility of a thread of known diameter and tension is proportional to its length, so the extensibility of a radius could possible give information about both its length and also its distance from an anchor thread since short radii ending near anchors would be least extensible. All the “testing” movements, particularly the slow flex movement, seem suited to sense the extensibility of the radii gripped by legs I, and it seems likely that this is the cue used by the spiders to orient their stabilimenta. One argument against this hypothesis is that since the spider is in the midst of a network of elastic threads and has no contact with immobile objects, a movement like a slow flex would not test the extensibility of just the threads gripped by legs I, but that of a whole system of threads in front of and behind the spider (Fig. 1). The extensibility of many threads behind the spider, however, is balanced against the extensibility of just the two (or one) held by legs I (the ratio in a web of 40 radii would be about 10 : l), so most of the extension perceived by a spider doing a slow jerk would be the result of the extension of the two radii gripped by legs I. Another difficulty with the extensibility hypothesis is that the tension on the radii also affects their extensibility, and there are reasons to believe that the tensions on all radii are not equal (Eberhard, 1969, 1972). However, as a result of tension exerted by sticky spiral attachments, long radii would be less tense and thus more extensible at the hub (Eberhard, 1969), emphasizing rather than obscuring the difference between their extensibility and that of shorter radii. Since the weight of a spider in a non-horizontal web would tense (and thus reduce the extensibility of) radii above it and loosen those below it, one would predict that the direction in which a web slants would influence the orientation of stabilimentum lines if extensibility is the cue used to orient them. This was not tested in the nearly horizontal laboratory webs, but may have been the case in webs in the field (above). On the other hand, the lack of strong evidence that radius length affected stabilimentum placement in webs in nature argues that additional cues may be involved in the natural situation.

Spiders’ positions on webs in nature Uloborus diversus’ positions on webs in nature supported the hypothesis that their stabilimenta function to obscure the spiders’ outlines. The spiders usually remained at the centres of their webs both day and night, and usually rested facing along a stabilimentum arm; 90 % of those with line type stabilimenta (N= 125), and 56 % of those with circle stabilimenta (N= 57) faced within 5” of a stabilimentum arm (both Pt0.01). In most cases the spider’s outline just filled the gap between two stabilimentum arms so that the arms and the spider formed one continuous line. Most spiders (71 %) faced toward the centre of the pack rat nest (and thus in most cases toward the nearest edge of the web). The reduced alignment on webs with circle stabilimenta may indicate that tactile cues from stabilimentum silk influence the direction the spider faces, but the tendency to face toward instead of away from the nearest edge of the web indicates that additional factors, probably the same as those influencing stabilimentum placement, also influence the spider’s position at the hub. 378 W. G. EBERHARD The spiders’ positions at the hub during the night were different from those during the day. During the day spiders usually assumed “aligned” postures (Plate IV(a)) which tended to make their outlines merge with the stabilimentum lines; both legs I held a single radius at the edge of the hub, both legs I1 were pressed against the 1’s and did not touch any threads, and legs 111 and IV held hub threads close to the abdomen. This posture differs only slightly from that reported for U. walckenaerius (B. J. Marples, 1962; Szlep, 1964).

PLATE1V. Positions of U. diversus on its web during the day (a) and at night (b).

Often spiders assumed “partially aligned” postures, spreading the 1’s slightly or gripping threads with the 11’s. The aligned posture did not occur at night, when the spiders spread legs I and I1 and gripped the web with all eight legs (Plate IV(b)).

Escape behaviour The behaviour of U. diversus suggests that the orientation of both stabilimentum and spider toward the nearest edge of the web function as defense against predators. The spiders usually reacted to disturbances by moving to the edge of the web, often after bouncing and jerking for a short time at the hub. The orientation of stabilimentum and STABILIMENTA ON ULOBORUS DIYERSUS WEBS 379 spider toward the shortest (and in most webs in nature, the most protected, at least from any large predator) side of the web minimized the time it took to reach the edge and brought the spider to the least exposed portion of the web. When a spider reached the end of a radius (a radius-frame junction) it usually turned and moved along the frame thread toward the nearest anchor thread, thus continuing along the path constituting the shortest exit from the web. Observations of U. diversus and U. geniculatus in the laboratory indicate that this orientation towards the nearest anchor may have been accomplished by using the angle between the radius and the frame thread as a cue. Six post-first instar U. geniculatus which left the hub in response to relatively undirectional stimuli (a bump on the side of the cage for example) chose the shortest path to an anchor 19 of 19 times (100 %) when the radius-frame angle on the shortest side (6 in Fig. 2) was >90°, 2 of 4 times (50%) when 8 was =90°, and 7 of 13 times when 8 was <90°. Similarly, U. diversus (9 post-first instar individuals) which were prodded onto radii

FIG.2. Spider encounters a frame thread (F) at angle 8 after leaving the hub along a radius (R). which ended on long frame threads (the spiders usually chose radii leading to the shorter sides of the webs unless they were prodded) chose the shorter path 27 of 34 times (79 %) when 8 was >90°, 5 of 13 times (38 %) when 8 was =90°, and 5 of 9 times (55 %) when 8 was <90°. Response to the same radius-frame angle has been demonstrated in orb weavers once before in a completely different situation-during the construction of radii and secondary frame threads by the araneids and Zygiella x-notata (= Zilla x-notata) (Mayer, 1952). Spiders apparently sensed the radius-frame angle with legs I as they approached a junction. Often, especially when they encountered nearly 90” angles or when the frame thread was especially large, they paused and bounced with one I holding the frame thread on each side of the junction (Fig. 2). The relatively high frequency of “correct” choices when 8 was <90° may have resulted from signals other than angle sizes (perhaps the extensibility of both segments of frame thread) sensed during these pauses. Uloborus diversus responded to more violent (and occasionally to sudden mild) dis- turbances by dropping from their webs at the end of a line. Usually in nature they dropped 380 W. G. EBERHARD far enough to reach an object below the web. If the object was not large (a twig for example) the spider immediately crawled under it and rested motionless. Spiders which landed in the middle of large objects usually remained immobile with all their legs drawn up tight against their bodies (Fig. 3). If a spider did not encounter any objects after dropping a foot or so, it hung motionless at the end of the line, usually with legs I, 11, and I11 pressed together and extended in front of its body (Fig. 3), but occasionally with its legs drawn up tight. These defensive responses of U. diversus appear designed to obscure the spider’s eight-legged outline and probably serve to help it escape visually orienting predators.

FIG.3. Positions of U. diversus when it drops from its web.

Discussion There are several reasons for believing that stabilimenta constructed by U. diversus function as protection against visually orienting predators: the spiders rest at the hub during the day ; greater intensity of night-time illumination (and thus, perhaps, greater likelihood of exposure to predators) leads to more elaborate stabilimenta ; and the spiders assume positions making their outlines merge with those of their stabilimenta during the day but not at night. The precise effects of the stabilimenta are not known. As outlined by Robinson & Robinson (1970)’ stabilimenta could serve to actually hide the spider, to change its apparent shape, or to divert predators’ attention away from the spider. Linear stabilimenta almost certainly function to change the spiders’ apparent shape, but circular stabilimenta and lines below the plane of the web could function to hide the spider or to divert predators’ attention. Similar considerations suggest that the stabilimenta of other orb weavers function in the same ways. Although many (probably most) orb weavers rest on their orbs only during the night, tearing the web down or resting at its edge during the day (e.g. Kaston, 1948), many and perhaps all (information is lacking for some) of those species which construct stabilimenta rest at the hub during the day (Table I). The amount of silk in the STABILIMENTA ON ULOBORVS DIVERSVS WEBS 381 stabilimenta of Argiope pulchella may be correlated positively with the degree of exposure of the web to the sky (Marson, 1947a), although J. Edmunds has found a negative cor- relation between these factors in A.Jlavipalpis (pers. comm.). Argiope argentata, Uloborus arizonicus, U. geniculatus, and U. oweni (which all construct silk stabilimenta), several species of Cyclosa (which construct silk, detritus, and egg-sac stabilimenta), and Cyrtophora citricola (which makes egg-sac stabilimenta) all sit at the hub with their legs pressed together or drawn tight against their bodies during the day, but spread their legs at dusk (Peters, 1953; Wiehle, 1928; Robinson & Robinson, 1970; Eberhard, unpubl. obs.). In addition, the structure of the stabilimenta of many species precludes their use as web-strengthening devices. The silk is usually fastened to only a single thread (even sometimes in Argiope-Wiehle, 1928: 117), and in some other species it is laid so loosely that it could never absorb any stress in an intact web (e.g. Hingston, 1927). The detritus and egg sac stabilimenta of various genera (Table I) probably lower rather than raise web stability by loading the web and thus making it less resistant to additional strains. These types of stabilimenta are generally conceded to function to protect the spider from visual predators (e.g. Gertsch, 1949); that they can be appropriately included in this discussion of silk stabilimenta is indicated by the fact that at least some species (e.g. Cyclosu conica) which generally make detritus stabilimenta sometimes substitute bands of pure silk (Eberhard, unpubl. obs.). It has also been found that individuals of the species Argiope bruennichi (M. J. Marples, 1935; Tilquin, 1942), A. argentata (Robinson & Robinson, 1970), Cyclosa conica (Tilquin, 1942), Gusteracantha versicolor (Emerit, 1968), and G. cancriformis (B. J. Marples, 1969) which were without orbs placed stabilimenta on networks of non-sticky thread where the spider rested. In addition 1 have observed that individuals of Uloborus geniculatus, Uloborus spp., and Gusteracantha sp. without orbs often lay stabilimentum silk on single lines under or near which the spider rests. The presence of normal or even larger than normal stabili- menta (Robinson & Robinson, 1970) in situations such as these in which the spider has a reduced or nearly non-existent web (e.g. a single line) which need not function in prey capture and which would thus require less “stabilizing”, is a strong argument against a mechanical function for the stabilimentum. In fact, the basic function of these “resting” webs is probably defensive; they isolate the spider from substrate-bound predators, and, by means of the stabilimenta, defend it from larger or flying predators. A common objection to the idea that stabilimenta serve as defence against visual predators is that they often make otherwise relatively inconspicuous spiders and their webs more visible, at least to humans. This is certainly the case for U. diversus-I always look for stabilimenta rather than spiders or their webs when I search for U. diversus in the field. But this does not rule out a protective function for stabilimenta. As noted above, stabilimenta may tend to obscure a spider’s outline or offer a strong visual stimulus distracting the eye from the spider. Thus “generalized” predators which take many kinds of spiders, presumably by searching for “spider-like” forms, might be less likely to recognize prominent, immobile “spider plus stabilimentum” objects as prospective prey. Such predators would have to learn that stabilimenta signal the presence of spiders, and stabilimenta may fool predators often enough to make learning experiences so infrequent that learning seldom occurs. The demonstration that a predator is capable of learning to associate stabilimenta with food (Robinson & Robinson, 1970) does not rule out the possibility that stabilimenta normally function as defence against these predators ; the 382 W. G. EBERHARD critical question is whether such learning normally occurs in nature. The Robinsons’ observations are the only data available on this point, and they suggest that such learning had not occurred in nature in their small sample of eight birds (of the genera Myiodynastes, Thamnophilus, and Tyrannus), since the birds avoided stabilimentum patterns at the start of training. The fact that spiders of a given species do not always build “complete” stabilimenta (e.g. B. J. Marples, 1969; Robinson & Robinson, 1970, and above) does not necessarily imply that stabilimenta do not function against visually orienting predators as Robinson & Robinson (1970: 652) suggest. On the contrary, as also noted by Ewer (1972), when the shape and pattern of stabilimenta vary, it becomes even less likely that a predator will succeed in learning to associate them with food. Of course, visual predators specializing on at least some stabilimentum-constructing spiders may either learn or respond innately to stabilimentum patterns and thus have less trouble locating prey as a result of the stabilimenta. Other adaptations by stabilimentum- constructing species such as web shaking (Argiope, Cyclosa), barrier meshes at one side of the orb (Argiope), orientation of the stabilimentum (Uloborus), flight to the edge of the web (Uloborus and probably others), dropping from the web (Argiope, Cyclosa, Micra- thena, Uloborus), assumption of cryptic postures when they drop (Uloborus), shuttling to the other side of the web (Argiope), and spiny bodies (Gasteracantha, Micrathena) probably serve as additional defences against predators which have located the spider, making it difficult for the predator to actually capture it. These adaptations also lower the chance that “generalized” predators would learn to associate stabilimenta with food. Although the data for many stabilimentum-building species are not yet in, there is a striking lack of evidence contradicting the hypothesis that stabilimenta provide defence against visual predators. Most importantly, not a single stabilimentum builder is known to rest off its web during the day (Table I). In addition, none of those which hold their legs in constrained positions making them blend with the stabilimentum during the day are known not to spread their legs at night. These facts, in conjunction with those presented above, suggest that all stabilimenta function to hide the spider, in one way or another, from visually orienting predators. Stabilimenta would thus be included among other defensive devices such as the suspension of curled leaves and other detritus covering the spider’s retreat in the midst of the web (several theridiids-eg. Comstock, 1940; Kaston, 1948), and the construction of orbs with branches running through the centre and under which the spider rests ( Uloborus-Kullmann, 1970; Tetragnatha-Eberhard, unpubl. obs.). The combination of the obvious advantage of staying on the web and attacking prey caught during the day, and the array of visually orienting predators, which includes birds (Warburton, 1912; B. J. Marples, 1969; Robinson & Robinson, 1970), lizards (Thakur & Tembe, 1956), wasps (Warburton, 1912; Williams, 1919; Duncan, 1949; MacNulty, 1961 ; Tsuneki, 1968; Eberhard, 1970), and asilid flies (Bristowe, 1958), which attack spiders in their webs during the day must produce strong selection for devices to obscure the spider. This is part of a thesis done under the direction of Dr H. W. Levi and submitted to the Harvard Department of Biology in partial fulfillment of the requirements of a Ph.D. I thank Dr W. Kerfoot for help with light measurements, Dr P. B. 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