Y. D. Lubin, W. G. Eberhard, and G. G. Montgomery.
Webs of Miagrammopes (Araneae: Uloboridae) in the Neotropics.
Psyche 85:1-23, 1978.

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PSYCHE
Vol. 85 March, 1978 No. 1
WEBS OF MIAGRAMMOPES (ARANEAE: ULOBORIDAE) IN THE NEOTROPICS
BY Y. D. LUBIN', W. G. EBERHARD~, AND G. G. MONTGOMERY' Uloboridspiders (Uloborus sens. lat.) typically construct orb webs composed of non-sticky threads (radii, frame threads, hub, and temporary spiral) which support a sticky spiral made of cribellar or hackled silk.
Specialization of the web in the uloborid genus Miagrammopes has involved the reduction of its structural com- plexity together with changes in its operation as an insect trap. The one described web of an unidentified species from Natal, South Africa is reduced to a single horizontal capture thread (Akerman 1932). In this paper we describe the webs of six more species of Miagrammopes and the prey capture behavior of the spiders, re- vealing a substantial range of variation in simple web design within the genus.
We studied M. simus on Barro Colorado Island, Panama Canal Zone during the wet season of 1976. At no time was this species common. In May and June, 1977, M, sp. 1 (ca. unipus) was studied in a bamboo (Guadua angustifolia) thicket in the Cauca valley near Cali, Colombia where it occurred in abundance. In August, 1977, M. intempus Chickering and M. sp. 2 were found in Vaile, Colom- bia. The former was common in some places on hanging moss on exposed roots and low branches near the Rio Anchicaya at 400 m elevation, while the latter was found in brush near the Rio Tulua at 1100 m elevation. A small tree in a clearing on Finca La Selva 'Smithsonian Tropical Research Institute, P.O. Box 2072, Balboa, Panama Canal Zone.
2Departamento de Biologia, Universidad del Valle, Cali, ~olombia, and Smith- sonian Tropical Research Institute.
Manuscript received by the editor April 16, 1978.



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2 Psyche [March
near Puerto Viejo, Heredia Province, Costa Rica, had substantial populations of M. sp 3. M. sp. 4 was found on low vegetation in January and February, 1978, in mid-elevation wet forest in Guatopo National Park, Miranda State, Venezuela. Individuals of the last
four species were observed in the field on only one or two days each, but in all cases more extensive observations had already been made on the other species, and it was thus possible to make critical ob- servations allowing comparisons among all six species. Miagram- mopes sp. 1-4 appear to be either undescribed species or females of species known only from males. Voucher specimens of these and of the two previously described species are deposited in the Museum of Comparative Zoology, Harvard University. M. simus
The web typically consisted of a single vertical capture thread about 1 m long, attached above to a short, horizontal resting thread strung under a leaf, and below to the ground or a leaf or twig (Fig. 1 a). The capture thread was covered with sticky, cribellar silk along the central 50 to 60 percent of its length, and one or more very fine, more or less horizontal threads often connected it to other supports. Both end portions of the capture thread were non-sticky. For an individual whose webs were measured periodically, the lengths of sticky and non-sticky sections in new webs were (in cm; lengths of sticky portions underlined): 20:2:30, 4:50:30, 6:52:34, 7:@:32, and 7:63:34.
One adult female which had been starved for seven days made a web with two vertical capture threads and several thin, non sticky lines between them.
One M. simus was seen laying sticky, cribellar silk on a non- sticky, vertical thread which was already in place. The spider moved slowly up the thread, combing out silk with legs IV until it was about 5 cm below the resting thread, then ran up and assumed the resting posture.
Individuals of M. simus rested under the horizontal thread and held onto the broken end of the capture thread with one leg I and one leg 11, while the other legs held the resting thread (Fig. I b). Ten- sion was exerted on the vertical capture thread both by pulling it up with leg I and by backing up and pulling in the resting thread with the fourth pair of legs. The spider which constructed a web with two



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19781 Lubin, Eberhard, & Montgomery - Miagrammopes 3 Figure 1. a) Typical web of Miagrammopes simus, showing the horizontal rest- ing thread under a leaf, the vertical capture thread with sticky segment and thin, non-sticky, horizontal threads; b) posture of M. simus as it holds its web and waits for prey.
capture threads rested in essentiallythe same position; the leg I holding the horizontal resting thread was in position to monitor vibrations from the second capture thread. When disturbed, or when hanging from a resting thread with no capture thread present, M. simus assumed a stick-like, cryptic pos- ture, orienting along the resting thread with the first and second pair of legs held straight forward and the fourth pair held straight behind. The small third pair of legs held the resting thread or the substrate, but were pressed close to the body and did not break the stick-like outline.




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M. sp. 1 (ca. unipus)
The web of this species differed in that there was usually more than one capture thread attached to a single horizontal resting thread (Fig. 2). The average was 2.4 capture threads and some webs had up to five (Table 1). There was no apparent relationship between the number of capture threads in the web and the size of the spider that constructed it. The capture threads were usually not perfectly vertical and were often in different planes with angles of less than 90' between them. They were shorter and thinner than the capture threads of M. simus and it was necessary to powder them with corn- starch in order to count them. The horizontal resting thread was always under a thin twig rather than a leaf, as in webs of M. simus. In some webs of M. sp. 1 there were one or more very slack, non- sticky, horizontal threads connecting the multiple capture threads. Because of their looseness and their variable location and orienta- tion, these lines were at first thought to be incidental (perhaps float- ing threads made by other spiders), but their presence in many webs of both this species and M. simus argues otherwise. Web construction appeared to be similar to that of M. simus. One spider was seen laying cribellar silk while moving up along a vertical thread which was already in place. The spider advanced
slowly, combing out silk continuously with legs IV and attaching it to the thread periodically with brisk dabs of the abdomen. Total construction time for one capture thread was about 3 minutes. At night, M. sp. I assumed a capture position similar to that of M. simus, resting under the horizontal thread and holding a cap- ture thread with legs I and I1 (Fig. lb). During the day it either held the capture thread in the same way, or, more often, assumed a more cryptic resting posture. The spider positioned itself near one end of the resting thread which it broke and spanned with its body. It held one end with one or both pairs of front legs, and then pulled in the line behind it with the hind legs (and, occasionally, the Table 1.
Numbers of sticky capture threads in 66 webs of Miagrammopes sp. 1 (ca. unipus) and 22 webs of M. sp. 3.
Number of Capture Threads
Number of Webs
M. sp. 1
M. sp. 3
6 or more
0
1




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19781 Lubin, Eberhard, & Montgomery - Miagrammopes 5 Figure 2.
Typical web of Miagrammopes sp. 1 (ca. unipus) showing horizontal resting thread under twig and three capture threads. line in front of it with legs I). The result was to draw the spider close to the twig.
When adopting the cryptic posture, the spider reached out briefly with legs I1 and I11 to pull itself closer to the twig, then positioned legs I1 against legs I, holding the broken end of the resting thread, and legs I11 against the sides of the abdomen. In this position it was nearly invisible (see Fig. 3). M. sp. 2
The web of one adult female was found in the morning (the spider was without a web at 2100 the night before), and was similar to some of the webs of M. sp. 1. The spider rested pressed to the undersurface of a branch, at the end of a horizontal thread about 3 cm long that was strung under the branch (Fig. 3). She held the broken end of the horizontal thread with one leg I1 and kept it tense by pulling in the thread with her hind legs, as described for M. simus and M. sp. 1. A single, vertical, capture thread (invisible until powdered) was attached near the other end of the horizontal thread. The lengths of the non-sticky and sticky portions of the capture thread were 7:53:40.




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Figure 3. Miagrammopes sp. 2 in cryptic posture as it feeds and holds the non- sticky resting thread.
M. intempus
Webs of this species were variable and most were different from those of other Miagrammopes species. One mature female held both a horizontal and a vertical sticky thread with her front legs, and a single, short, non-sticky line with her rear legs (Fig. 4). A second female also held two capture threads, but both were at an angle rather than being either horizontal or vertical. The first spider was induced to move forward along the horizontal thread several times and her return to the waiting position was observed carefully (Fig. 4). Each time she tensed the sticky threads by pulling them in with her front legs; she did not move her hind legs. Another individual, on a web which was similar except that the horizontal thread did not appear to be sticky, held the sticky vertical thread in the same way that M. simus held the capture thread and tensed it by pulling thread with both front and hind legs. Still other in- dividuals with single, horizontal, sticky threads (Fig. 5) failed to pull in silk as they assumed the waiting position. One vertical thread had several very fine, loose, horizontal lines attached to it, similar to those shown in Fig. la for M. simus.



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19781 Lubin, Eberhard, & Montgomery - Miagrammopes 7 Figure 4.
Movements of a female M. intempus, illustrating how thread attach- ments are manipulated. Letters mark spots on horizontal thread. The spider rested (a) holding both sticky lines with its front legs, and a third, short, non-sticky line with its hind legs at point x (the ends of the capture threads were not drawn as they could not be seen). When lured out onto the horizontal sticky line, the spider car- ried the vertical thread for some distance (b), then attached it to the horizontal line and continued on (c). When she returned, she shifted the point of attachment of the vertical thread again (d), then turned around and pulled in the line with her front legs and resumed her original position (e). The shifts in attachment were extremely rapid; the actual motions involved could not be followed, and the shifts were noticed only by comparing thread positions before and after the spider passed by.



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M. sp. 3
The webs of this species were similar to those of M. sp. 1 in hav- ing variable numbers of capture threads (Table 1). The sticky lines were not all attached to a non-sticky line at one end, however, but rather radiated in several directions from a more or less centrally placed thread (Fig. 6). The spider rested on this thread, often break- ing one of the capture threads and holding it as described for M. simus (Figs. lb and 6). This position was also similar to that of M. intempus in that the spider held a non-sticky line behind it and a sticky line in front of it. In other cases the spider rested holding only the non-sticky thread with both front legs. The sticky threads differed from those of other species of Miagrammopes in being relatively short (all less than 25 cm) and sticky all the way to the lower end. The webs were found at night and were gone the next morning.
M. sp. 4
Webs of M. sp. 4 had one or two capture threads (invisible until powdered or sprayed with water), 20 to 40 cm long each. The cap- tured threads were vertical or nearly vertical, but not necessarily parallel or in the same plane. Of 9 spiders found during the day, three had two capture threads each, three had a single capture thread, and three had no capture thread. As in M. simus, the rest- ing thread was generally under a leaf and often placed at an angle. Spiders with capture threads rested with one leg I holding a vertical thread (see Fig. lb) and adjusted the tension both by pulling in the resting thread with legs IV and the capture thread with leg I. Spiders without capture threads rested in a cryptic position sim- ilar to that of M. simus. Often after going into the cryptic posture (and particularly when disturbed), the spider bounced up and down on the resting thread in a rhythmic motion reminiscent of rocking motions of stick insects (Phasmidae). The significance of these movements is not known.
We observed in detail prey captures made by four M. simus, two M. sp. 1, and one M. intempus. Insects that we gave to the spiders as prey included fruitflies (2-3 mm long), moths (3-7 mm long), and ants (3-5 mm long). In general, the sequences of prey capture behavior were similar, but the spiders moved so rapidly that stop-



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19781 Lubin, Eberhard, & Montgomery - Miagrammopes 9 Figure 5. Miagrammopes intempus female holding a single thread web. Note
the loose line just anterior to the tip of leg IV(a), and leg I1 holding the end of the capture thread (b).
action analysis of video-recording was needed to permit adequate analysis. Only M. simus was video-taped, using a SONY AV-3400 videorecorder and a macro lens. The descriptions below are based mainly on analyses of these video-recordings. Stage I: Prey detection -jerking the capture thread When an insect was placed on the capture thread, the spider re- sponded by jerking the thread, The spider quickly flexed her lower leg I, which held the capture thread, and immediately extended it again. The maximum distance travelled by the tip of the leg on an upward jerk was 0.3 leg length (about 2.8 mm), and the quickest jerks were accomplished in less than 1/60 second (the time span of a single "frame" of the video-recording). It is tempting to think that jerking functions in gauging the weight or size of the prey, as seems to be the case in other uloborids (Eberhard 1969). Spiders with multiple capture threads (both M. sp. 1 and M. simus) jerked only the thread on which prey had been placed.



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Figure 6. Miagrammopes sp. 3 on its web, as seen from below and slightly to the side. The brighter threads are sticky (the web was not powdered). Note that the spider has broken the end of the capture thread and holds it with the front legs bent to the side in a manner similar to that shown for M. simus (Fig. 1 b). Stage 11: Entanglement of the prey - sagging the line The spider sagged the capture thread by dropping the loose silk it had pulled in with its hind legs, and perhaps also letting out additional dragline. At almost the same time it manipulated the capture thread with a series of complex movements of leg I (Fig. 7a) which resulted in the prey being jerked rapidly back up and down again (Fig. 7b). Whereas the jerks in stage I displaced a fruitfly only 5-6 mm, sagging the capture thread caused the prey to drop 26-33 mm in less than 1/30 second. As the prey dropped, it was often displaced sideways as much as 6 mm (due to air currents?). Rapid and repeated sagging of the capture thread resulted in the formation of one or more loops of silk that enveloped the prey. Such loops were seen in the capture threads of both M. simus and M. sp, 1.
The mechanism responsible for the formation of these loops is not clear.
One possible explanation is that, due to the relatively higher air resistance and lower weight of the silk, the prey drops more rapidly than the silk during a sag, and therefore falls into the silk below it (Fig. 8a). An alternative explanation (Fig. 8b) is



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19781 Lubin, Eberhard, & Montgomery - Miagrammopes 11 that, at the end of a sag, when the spider jerks the line up again, the prey is "snapped" back upward and accelerated more than the silk just above it so that it "runs into" the thread above it. The second of these hypotheses is more appealing since 1) it would work with non-vertical capture threads whereas the first would not, and 2) we saw two instances in which a loop clearly formed in the thread just above the prey. In any event, the spider is somehow able to entangle the prey from a distance by manipulating the capture thread.
Stage 111: Immobilization of prey-wrapping After manipulating the capture thread to cause one or more sags, Figure 7. a) Movements of the tip of leg I of a female Miagrammopes simus as she sagged the capture thread. Points are locations of the tip of leg I holding the capture thread, taken from a video-taped sequence with "frames" 1/60 sec apart. In frames 3-5 the tip of the leg remained in the same spot. In frames 9 and 11 the tip of the leg was not visible; these points are not shown in the figure. b) Movements of a prey on the capture thread while the thread is being sagged and jerked back up and down, taken from a video-taped sequence (as above). Numbers refer to segments of the path of movement of the prey on the line during consecutive 1/60 sec intervals. Scale marker represents 10 mm.




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the spider attached a dragline to the resting thread and moved rap- idly down the capture thread, pulling in the capture thread and wadding it up loosely with legs I1 as it moved. It touched the prey one or more times with legs I, probably receiving tactile and chem- ical clues as to the identity of the prey, and then turned 180' and began wrapping. The wadded up capture thread was transferred to legs 111 and wrapped onto the prey, probably thereby increasing the effectiveness of the initial wraps.
While wrapping, the spider faced away from the prey, holding the capture thread just above the prey with one leg I and the prey itself with legs I1 and 111. After 20-30 seconds of wrapping, the spider cut the capture thread just above and below the prey. It then rotated the prey package rapidly with legs I1 (and the palps?) while continuing to wrap by pulling silk out from the spinnerets and throwing it onto the prey with legs IV (rotation-wrapping in the nomenclature of Robinson and Olazarri 1971). While wrapping the prey, the spider spanned the gap between the two ends of the capture thread, holding each end with one leg I as do other ulo- borids (Marples 1962).
Stage IV: Transport of prey to the feeding site The wrapped prey was transferred to the palps, and the spider attached a dragline to the thread she had laid on her way down and then to the broken end of the capture thread. After thus re- pairing the web, she ran up to the resting thread, holding the prey In the palps. Once on the resting thread, the spider transferred the prey to the third pair of legs and again wrapped it. She wrapped as described above, rotating the prey package with legs I1 while hanging from the resting thread with legs I. After wrapping as long as 5 minutes, the spider transferred the prey back to the palps, turned facing away from the capture thread, and pulled the resting thread with legs I as though testing the tension. She then turned 180å and resumed a resting posture with one leg I monitoring the capture thread. As in other uloborids, the prey package was held "overhead" in the palps and chelicerae while the spider fed (Fig 3) and re-wrapped several times during the process of feeding. Feeding often lasted an hour or more.
Variations in the prey capture sequence
We saw several modifications of the basic prey capture sequence in M. simus. Small dolichoderine ants were rejected by a spider



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19781 Lubin, Eberhard, & Montgomery - Miagrammopes 13 1
Figure 8. Two possible mechanisms which could result in prey becoming en- tangled as a result of sagging behavior. A) The prey drops faster than the line below it, and thus becomes entangled. B) The prey's momentum, acquired when the spider jerks the line up after a sag, causes it to become entangled in the line just above it. This hypothesis depends on the thread below the prey being extensible. on four occasions. Each time the spider jerked and sagged the cap- ture thread several times, ran a short distance down the capture thread, wadding it up as it went, and then cut the line above the ant and ran back up to the resting thread. These ants were thus recog- nized from a distance, perhaps by their strong alarm odor. After an ant was rejected, the wadded-up section of the capture thread was manipulated in the mouthparts for several minutes (feeding?), then dropped. Rejection of prey thus resulted in destruction of the capture thread. A new thread was often built within a few hours. Three other ants, two Camponotus sp. and one Ectatuma sp., all about the same size as the spider (6-7 rnm long), were attacked successfully, but modifications of the capture sequence occurred in all three trials. In two, the spider dropped the lower portion of the



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14 Psyche [March
capture thread after wrapping the prey instead of re-attaching it to the dragline. In these trials, the spider did not rotate-wrap the prey, but cut it out after the initial wrap and carried it directly back to the resting thread. In all three trials, the ants were carried up to the resting thread dangling from the spinnerets on a 1.5 to 2 cm thread which was held with one or both legs IV. After reaching the resting thread, the spider pulled the prey in with legs IV and rotate- wrapped it.
Live moths of about the same length as the spider escaped readily from the capture thread by fluttering down it, leaving behind a con- spicuous trail of scales stuck to the cribellar silk. We observed four complete prey capture sequences with moths and saw no major modifications in prey capture behavior, such as those seen with some araneids (Robinson 1969, Robinson et al. 1971). In three of the trials, the spider discarded the remaining capture thread after wrapping; as with the ants as prey, the rotate-wrap stage was omitted from these captures.
These observations suggest that the decision to retain or discard the remaining capture thread is made early in the attack sequence, and is perhaps related to the size of the prey. If the capture thread is to be abandoned, it may be advantageous for the spider to delay rotation-wrapping until it reaches the resting thread, where it is less exposed to visual predators. This explanation is not entirely satis- factory, however, since if rotation-wrapping is not necessary at the capture site (it would seem most necessary for just those large prey for which it is omitted), it would seem advantageous to perform all rotation-wrapping at the more protected resting thread. Capture sequences with multiple prey
Capture of small prey such as fruitflies caused little damage to the capture thread, because the repair of the thread left the remain- ing sticky portion intact. When presented with a second or third prey, the spider rushed down the capture thread holding the first prey in its palps, and attacked the new prey in the usual manner. Second prey were wrapped together with the first prey and carried up to the resting thread in the palps in one large package, or wrapped separately and carried up hanging from the spinnerets, then wrapped with the first prey.
After only a few prey items were captured, the spider destroyed the remaining capture thread by dropping the lower end of the



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19781 Lubin, Eberhard, & Montgomery - Miagrammopes 15 thread after wrapping the prey instead of attaching it to the drag- line. The capture thread was destroyed even when a substantial portion remained undamaged, suggesting that the catching capa- city of the thread does not limit the number of prey items the spider will attack. Since Miagrammopes does not attach prey at the feed- ing site (this is also true of Uloborus diversus - Eberhard 1967), it is likely that the size of the prey package the spider can hold in its palps limits it to capturing only a few insects in succession.