William G. Eberhard.
The Natural History and Behavior of the Bolas Spider, Mastophora dizzydeani sp. n. (Araneae).
Psyche 87:143-170, 1980.

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PSYCHE
Vol. 87 1980 No. 3 4
THE NATURAL HISTORY AND BEHAVIOR
OF THE BOLAS SPIDER
MASTOPHORA DIZZYDEANI SP. N. (ARANEIDAE) Smithsonian Tropical Research Institute and Departamento de Biologia,
Universidad del Valle, Cali, Colombia
The unusual hunting techniques of the bolas spiders of the tribe Mastophoreae ( Mastophora, Dichrostichus, and Cladomelea) were described long ago (Hutchinson 1903, Longman 1922, Akerman 1923). These spiders make a large sticky ball on the end of a short thread, and swing the ball at passing insects while hanging on another short, horizontal "trapeze" line. Probably because the spiders are difficult to find, however, little has been done since to solve the problems which these first observations raised. Gertsch (1955) gives a complete and clear resume of what had been discovered of the biology of -the entire group to that date. A recent study of an undescribed species of Mastophora which included observations of the behavior of both the spider and its prey plus extensive series of prey has finally confirmed the suspicion of the early naturalists that Mastophora lures its prey with a volatile substance which mimics the sex attractant pheromone of virgin female moths (Eberhard 1977). The present paper gives a taxonomic description of the species on which this work was done and presents Present address; S.T.R.I. and Escuela de Biologia, Universidad de Costa Rica, Ciudad Universitaria, Costa Rica.
Manuscript received by the editor December 22, 1980.



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144 Psyche [vo~. 87
further observations on other aspects of its natural history and behavior, including web forms and their construction, the structure and function of the sticky ball, variations in prey species, egg sac construction behavior, and rates of egg sac production. These observations seem to link Mastophora to both araneids such as Cyrtarachne, Poecilopachys, and Pasilobus which spin more nearly typical orb webs, and to the completely webless Celaenia and Taczano wskia.
The main study site and the observation methods are described in Eberhard, 1977. The spiders were sedentary, seldom moving more than 10-20 cm in a night and usually returning day after day to the same daytime resting site where a pad of silk gradually accumulated. These sites were extremely exposed; one was on the barb of a barbed wire, and others were on fence posts, the upper surfaces of leaves, etc. in an open field. During the day in Cali the spiders sometimes experienced temperatures of up to at least 42OC combined with brisk (>lo kmph) winds.
Additional observations were made in open areas near the edge of Lago Calima (el. 1400 m) in grassland with scattered small bushes and trees, where spiders were relatively common in September, 1977, but absent in January, 1979.
Unless otherwise noted, all descriptions of behavior refer to observations of mature females.
Mastophora dizzydeani new species
Figure 1-9
Dr. Willis Gertsch kindly studied male and female specimens, and stated (in litt.) that "I am confident that it represents an undescribed species." Like Dr. Gertsch, I have been unable to match this species to any published description, and keeping in mind the overworked state of most spider taxonomists plus the parasitic relationship which behaviorists and ecologists generally enjoy with them, will undertake the description of this new species.
Etymology. Since this spider's livelihood depends on throwing a ball fast and accurately, it seems appropriate to name it in honor of one of the greatest baseball pitchers of all time, Jerome "Dizzy" Dean. All measurements in mm; colors in parenthesis from living spiders.



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19801 Eberhard - Bolas Spider 145
Female: The measurements of the holotype female from Cali, Colombia, are as follows: total length 15.32, carapace 5.44 long, 6.29 wide; abdomen 10.78 long, 15.99 wide (Figs. 1-3). Carapace mahogany brown, darkest on sides and the posterior declivity, without noticeable hairs. Sternum yellow-brown in central areas and brown at margins; labium and endites brown with white on margins nearest mouth; coxae and legs brown, the first leg weakly annulated with two bands of light color on femur, three on tibia, and two on metatarsus, similar markings on other legs. Abdomen Figs. 1-9. Mastophora dizzydeani new species. 1. Female cephalothorax, anterior view without appendages. 3. Female, dorsal view without appendages. 2. Female, lateral view without appendages. 4. Epigynum, subcaudal view. 5. Male, dorsal view without appendages. 6. Male, right leg I dorsal view. 7-9. Male palpus. 7. Ventral view. 8. Lateral view. 9. Dorsal view




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146 Psyche ". [vo~. 87
yellowish (white) with a greyish-brown (dark green with brown tinge) band across the anterior margin with grey (dark green) on the anterior margins of the shoulders and a dark band extending to base; irregularly shaped intercalations of white along the anterior margin - - - - -- - -.
of the band. Three pairs of sclerotized depressions (in addition to many scattered minor ones) of which the first pair between the humps is dark brown and more conspicuous. Venter of abdomen whitish (yellow) between epigastric furrow and the brownish spinnerets, with a darker central area.
The carapace about as long as wide, widest posteriorly, rounded on the sides, the pars cephalica at the posterior eye row more than half as wide as carapace (3.421 6.34). Pars cephalica subtriangular as seen from side, the occipital horn bifurcate apically, separated by deep rounded depression, and greatest width of horns more than half of width of carapace. Pars cephalica with well developed warts and cones which are mostly restricted to this area of carapace. Lateral eyes of each side on a single projecting cone and the four median eyes on a single, less pointed elevation. Clypeus vertical, equal in height to about 3-4 diameters of anterior median eye. Anterior eye row slightly procurved, posterior row slightly more so. Anterior medians sepa- rated by 2.3 diameters, posterior medians by 3.3 diameters; anterior medians from anterior laterals by 8 diameters, posterior medians from posterior laterals by 8.7 diameters. Median ocular quadrangle broader than long (ratio 1.49: 1) and front eyes larger. Chelicerae bluntly conical, the upper margin with three sharp teeth, the middle one longest, lower margin with one tooth. Sternum subtriangular slightly longer than wide (ratio 1.13: l), broadly emarginate in front with rounded point opposite the posterior margin of each coxa. Posterior coxae separated by 1 /4 width of fourth coxa. Labium broader than long (ratio 2.12: 1) with central point projecting somewhat at tip and extending to about 113 of distance to tips of parallel endites.
Legs clothed with fine hairs.
-
I I I Ill IV
Femur
5.33
4.27
2.83 4.07
Patella 2.92
2.53
1.49 1.94
Tibia 4.88 3.42
1.91 2.95
Metatarsus 5.05 3.09
1.71 2.67
Tarsus 1.40 1.01
.79 .73
Total 19.58 14.32
8.73 12.36




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19801 Eberhard - Bolas Spider 147
/
First leg with metatarsus much thinner than tibia and slightly curved, median claws large and strongly curved. Abdomen broad and subtriangular as seen from above, shallowly emarginate anteriorly and broadly rounded on sides, presenting a pair of clearly defined but not large tubercles slightly posterior and median to the shoulder humps. Abdomen overlapping carapace about to the horns, highest just anterior to the tubercles and declining steeply behind, with the caudal portion rounded. Abdomen hairless except for ventral surface anterior to spinnerets. Deepness of wrinkles along sides related to how recently spider oviposited. Spinnerets with apical segments short and subconical. The epi- gynum is illustrated in Fig. 4.
Mde: Total length of holotype from Lago Calima, Valle, Colombia is 1.8 1; carapace .87 long; abdomen .91 long and 1.35 wide. Coloration of carapace brown (reddish orange) with longitudinal white stripe forming dorsal triangle, eyes lighter. Abdomen white with scattered small grey blotches at anterior dorsal margin. Three pairs of large sclerotized depressions present, and tubercles posterior to first pair and separated slightly more than the depressions. The palpus is illustrated in Figs. 7-9.
Type Localities: Female holotype from field on Melendez campus of Universidad del Valle at southern edge of Cali, Colombia. Male holotype from field at eastern edge of Lago Calima, near Darien, Valle, Colombia.
DISTRIBUTION: Western Colombia in Departamento Valle del Cauca. 1000-1400m el. in western range of Andres. Other Records: mature female, other mature males, several juveniles from type locality of male holotype.
Web construction and prey capture
The form of the bolas and its construction were basically the same in M. dizzydeani as those of M. cornigera as described by Hutchinson (1903) and Gertsch (1947). A number of details were different, however, perhaps due to species differences, and are described below. As the spider moved back and forth on the horizontal (trapeze) line prior to starting a bolas (Fig. lOa), she did not reinforce the line, but rather repeatedly broke it, and reeled up the old line as she payed out a new one behind, her body thus forming a bridge between the two lines. She often descended somewhat as she broke the line, and waved



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[Vol. 87
Fig. 10.
Construction of a sticky ball. A. Spider moves horizontally, waving her front legs actively in the space where she will produce and swing her bolas. Probably this behavior serves to sense the presence of objects which would interfere with hunting behavior. B. Spider hangs from the horizontal trapeze line and begins to pull sticky silk from her spinnerets onto the bolas line which is attached above to the trapeze line. C. Spider continues to produce sticky silk and the sticky ball grows. D. The last of the sticky silk has been produced and it is slowly absorbed into the ball as the ball hangs free. The spider then walks across the trapeze line, grasps the bolas line, and assumes the hunting posture.




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19801 Eberhard - Bolas Spider 149
her front legs actively as she moved (Fig. lOa), perhaps in order to sense the presence of objects beneath her future hunting site. As she produced the ball of sticky material at the end of the bolas line (Fig. lob-d), it did not seem that she swept viscid material along a central line, but rather that a line as well as viscid material was pulled out with her IV legs. This impression was confirmed by microscopic examination of completed balls which showed large accumulations of thread (below). About 80-100 pulls with alternating legs IV were performed before the ball was complete, rather than 16-20 as with M. cornigera. Typically the spider began with shorter and quicker strokes, and then slowed as the ball began to form. When the ball was finished, it appeared that the silk simply ran out rather than that the spider somehow cut the line with one leg IV. The production of a ball took only 1-2 minutes from start to finish. On several occasions spiders assumed predatory positions without balls (Fig. 11) some of these after a ball being constructed was experimentally removed or became stuck to a leg and was ingested; I assumed that these spiders were hunting since some moths ap- proached them, but I never saw a prey capture. Hunting without first making a ball seemed to be associated with particularly windy evenings, but no careful measurements were made. Smaller, immature spiders made balls less often. One female which was an estimated 3-4 moults from maturity was seen hunting both Fig. 11.
A mature female M. dizzydeani in hunting position without a ball, with both legs I extended anteriorly (seen from the side and slightly below-drawn from a photograph).




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with and without a ball; another slightly larger one hunted without; and numerous newly emerged spiderlings appeared to hunt without. Perhaps this tendency is related to the fact that drying out of the sticky ball (see Fig. 14 and below) would be a more serious problem for smaller spiders due to surface-volume relations. As noted in Eberhard (1977), hunting spiders generally positioned themselves on the trapeze line with their ventral surfaces downwind. Since the direction of the wind at the study site varied erratically, the spiders assumed angles varying from 0' to 90' with respect to the trapeze lines. On two occasions when the wind changed 180å¡ the spider responded by releasing the ball and changing her position to hold it with the other leg I, changing her orientation 180å as she did so.
Both those spiders with balls and ones in hunting position without balls responded to my humming (but not to the high-pitched hum of an electronic flash), suggesting that sound was not sensed through the bolas line as suggested by Gertsch (1947). The responses were different however. Spiders with balls extended one or both legs I, especially the lower one which was generally either extended laterally to point straight down, or else "cocked" ready to swing the ball (see Fig. Id in Eberhard 1977). Occasionally they actually swung the ball with a quick ventral flick of the leg. Those without balls sometimes flexed their legs I quickly, and other times extended them, especially the lower of the two. Even in the absence of sudden noises spiders holding balls twitched and jerked their legs I almost continuously, especially the upper one, giving the impression that they were extremely alert. Although the upper leg I seemed to be in position to grab prey, this was not its function since photos of spiders in the act of swinging the ball (Fig. 12) show this leg held back and away from the Prey -
Despite the spiders' extremely quick reaction time and tne relatively slow flight of approaching moths (Eberhard 1980), it was not easy for the spiders to hit them, and 12 of 21 strikes I observed were misses. Most of the moths left after a miss and apparently did not return. When a spider succeeded in hittinga moth, the ball always stuck tight despite the moth's struggles, and the spider descended the line, embraced the moth with her legs, and bit it. After several seconds she released her hold and wrapped the prey with slow alternate strokes of legs IV. The prey was rotated s@wfy or (usually) not at all during the wrapping. The spider then ascended the line as she held the



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Fig. 12.
Mastophora dizzydeani missing with a swing of its bolas at a moth (out of picture). Note the extension of the ball into a line, and the retracted position of the upper leg I.



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152 Psyche [vo~. 87
prey with one leg IV, climbed onto the trapeze line, and turned the moth so its anterior end was at her mouth and fed. Several variations on this general scheme were observed. On some occasions a spider which had captured a moth did not feed immediately, but fastened it to the trapeze line, spun a new ball, and resumed hunting; in one case a spider had three prey on the trapeze line when she caught a fourth. Longman (1922) observed a Dichro- stichus magnificus which also temporarily stored prey on the trapeze line. In no case did I see an extended coating along the bolas line as has been observed with M. bisaccata (Gertsch 1955) and D. magnificus (Longman 1922), and only one of an estimated 60 balls observed was double. On one occasion however I saw an even stranger trap. One particularly warm and windless evening a spider had an unusually long trapeze line which had three different balls hanging from it. The spider was at one end of the trapeze in a predatory stance, and did not hold a ball. The spider soon ate the balls, and later the same night made a single ball and held it with one leg in the usual fashion.
Balls were ingested if after a period of waiting no prey were captured; such lapses averaged 24 min (range 14-41, N=8). Usually the spider did not make another ball immediately after ingesting an earlier one, but rested immobile for up to an hour or more. Moths are difficult prey for web spiders to capture since their abundant and easily detached scales more or less insulate them from sticky traps (Eisner et al. 1964), and some araneids have evolved special attack behavior towards them to offset this defense (Robinson 1969, Robinson, Robinson and Graney 1972). It is thus surprising that M. dizzydeani was able to catch moths regularly with a sticky trap, and the structure and function of the sticky ball assume particular interest.
The balls of M. dizzydeani were largely liquid. When touched to a piece of filter paper, a ball immediately wet an area two to three times its diameter and, to the naked eye, disappeared. When touched to a nonabsorbant object like a glass slide, the ball remained visible as a mass of jelly-like material surrounded by a pool of liquid (Fig. 13). That the ball's wetness isessential to its stickiness was shown by taking a newly made ball away from a spider and letting it hang free at



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Eberhard - Bolas Spider
Fig. 13. The internal structure of a sticky ball (diagrammatic) which was lowered onto a glass slide, showing the apparent layers of material of different viscosities near the edges, and the mass of folded thread in the center (regularity of folding is overemphasized).




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154 Psyche [Vol. 8'
the hunting site for 90 minutes. At the end of this time its size was substantially reduced (Fig. 14-an estimated 40% loss of volume) anc it was no longer sticky. The spiders' ingestion of balls after about 3( minute intervals thus probably served to insure that the ball i; sufficiently sticky. This agrees with Hutchinson's observations of tht balls of M. cornigera. The reduction in size contrasts with the almos complete lack of shrinkage of the balls of adhesive on the stick! spirals of the orbs of araneids such as Metazygia sp., Leucauge sp. Fig. 14.
Shrinkage of a sticky ball due to evaporation: A. a freshly made ball; B. the same ball 90 minutes later.




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19801 Eberhard - Bolas Spider 155
and Gasteracantha cancriformis (Eberhard in prep.), and again serves to emphasize the unusually fluid nature of Mastophora's adhesive.
Spiders never discarded balls, always carefully ingesting them even when they accidentally stuck to their own bodies (observed three times) or to nearby objects (observed once-to a cardboard wind- screen held near the spider). In addition to the obvious conservation of nutrients which this represents, it may also be important in the conservation of water, since the spiders sometimes go for long periods without drinking and also endure high temperatures at exposed sites during the day (above).
Cursory examination of balls on glass slides showed that their internal structure was complex, and consisted of a mass of curled or folded fibers which was embedded in a viscousmatrix which was in turn surrounded by a less viscous layer (Fig. 13). More refined observations by R. W. Work (pers. comm.) showed that the folded "spring" fibers are noodle-like ribbons which are nearly rectangular in cross section (Fig. 15), with only slightly rounded corners and slight bulges on their flat sides. He reports as follows: "The dimensions of two samples of these fibers from Mastophora dizzy- deani were 10.7 X 3.4pm and 10.0 X 3.2pm. A sample from a closely related species from Costa Rica gave 11.4 X 4.3 pm. They are possessed of axial birefringence, being of the same order of magni- tude (0.005-0.007) in planes perpendicular to both thick and thin axes of their cross sections. In this way they differ markedly from the -- -- - - -
elastomeric baselinesof the stickyspirals of araneid orb webs,&hich are birefringent in neither the non-extended nor extended states. It follows that the bolas throwing and recovery phenomena are not based on elastic properties. Rather, the combination of rectangular cross section (typical of a steel spring), folded or sinusoidal con- figuration while at rest, and axial molecular orientation (as evidenced by bifrigence) is the basis of the springiness. The spring fibers are also different from the fibers making up the line from which the ball was suspended since these latter, which are also birefrigent (values approximated .02-.03, typical for lines from the major ampullate glands of other arachneids), were round (4-5 ,urn in diameter) as is typical of other spider fibers. The details of the junction between the bolas and the suspension line could not be resolved, although it appears that spring fibers may be present in the 'bundle' near the end of the line."




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156 Psyche [Vol. 87
The ball lost most of its adhesiveness after a single long extension, but shorter extensions appeared to be reversible. In one case a ball taken from a spider was touched lightly against a nearby object and then pulled away gently. A small mass of "liquid" stayed on the object, and was connected to the ball by a thin, dry thread which was not sticky when touched. When I moved the ball several mm farther away more of the dry line was drawn out, and when I moved it back this line drew back into the ball. This was repeated several times with identical results. Probably the dry line was one of the folded threads, and its immediate withdrawal into the ball when tension was lowered was due to its tendency to resume a folded configuration. In other cases a greater quantity of material was stuck to an object and when the object was moved away the entire ball rather than a single thread stretched with it. Hutchinson (1903) saw similar extraction and withdrawal of line in M. cornigera (in this case the line was the bolas line from which the ball was suspended). The extensibility of the ball had an unsuspected consequence which was revealed in photographs taken as the spider swung at moths. As seen in Fig. 12, the ball stretched into a line during the Pig. 15. Micrograph of fibers inside a sttcky bal! of M. sp. near dizzydeanifrom San Jose Costa Rica, showing regular folding, and ribbon-like shape of the fibers (unmodified illumination of ball ona glasssiide; lO#m = 6.6cm, original magnification 250X). Photograph by R. W. Work.




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19801 Eberhard - Bolas Spider 157
swing. The greatest extension observed amounted to about 3.5-4 cm. This change was completely reversible and the recovery was evidently very rapid. I never noticed any elongation after swings by spiders, nor was I able to see any elongation when I myself swung a newly-made ball and looked quickly after each swing. The significance of this property of the ball is probably that it increases the spider's striking distance while maintaining between swings the compact ball form which is essential for quick and accurate strikes. Such use of spring- like action is unusual if not unique in animal structures. In summary, it appears that the ball may function in the following way. The low viscosity liquid is sufficiently wet and abundant to flow past the moth's scales and reach a relatively large area of the cuticle below. The more viscous liquid forms the actual bond to the thread which sustains the moth's weight, and the thread folded inside the ball functions to permit quick, reversible elongations which extend the spider's striking range and perhaps also serves to hold prey once it is hit.
The prey caught by a given individual of M. dizzydeani are relatively constant, but, as shown in Table 1, there is variation between individuals even at the same site. Some but not all the differences might be due to differences in prey abundances at different times of the year, although casual observations suggest that this was not the case. Small spiders seem to differ radically from larger ones in prey identity, which is expected given the relatively large size of the larger spiders' prey.
Rates of prey capture were estimated from numbers of moths