A derivative of the orb web and its evolutionary significance

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1 New Zealand Journal of Zoology ISSN: (Print) (Online) Journal homepage: A derivative of the orb web and its evolutionary significance Lyn M. Forster & R. R. Forster To cite this article: Lyn M. Forster & R. R. Forster (1985) A derivative of the orb web and its evolutionary significance, New Zealand Journal of Zoology, 12:4, , DOI: / To link to this article: Published online: 27 Feb Submit your article to this journal Article views: 58 View related articles Citing articles: 12 View citing articles Full Terms & Conditions of access and use can be found at

2 New Zealand Journal of Zoology, 1985, Vol. 12: /85/ $2.50/0 Crown copyright A derivative of the orb web and its evolutionary significance LYN M. FORSTER R. R. FORSTER Otago Museum Great King Street Dunedin. New Zealand Abstract This paper describes a new kind of ladder-web structure in which there are two ladders. one above and one below a centrally positioned orb. It differs from previously described ladder-webs. not only because of the two ladders but also because of its 24 h (or more) duration. its vertical placement against the trunks oftrees, and the fact that it apparently offers the spider protection against parasitism. Both the spider taraneus atrihastulusi and its ladder-web are ideally adapted to the tree-trunk: the web with regard to its position. shape. and lack of visibility; and the spider in respect of its coloration. daytime posture. and proximity to the snare. It is concluded that the design of this web offers a number of advantages which evidently enhance the spider's survival and increase its capture potential over and above that ofthe simple orb. Keywords ladder-web; central orb; adaptation; protection; crypticity; prey-capture potential; spiders: Araneus INTRODUCTION The cosmopolitan araneid spiders build orb-webs which are regarded as highly efficient prey-catching devices (Denny 1976). A number of modifications of these webs, often entailing a reduction of the orb, are known (Robinson & Robinson 1975; Lubin 1980; Stowe 1978) but more recently several derivatives involving extensions to the orb have been discovered. These structures have apparently evolved to utilise particular features of the environment and to trap insects by taking advantage of certain aspects of their morphology and behaviour. Received 2 Februarv /985: accepted /9 April /985 One such web, the ladder-web snare, which was first described from Papua New Guinea by Robinson & Robinson (1972), consists of a typical symmetrical orb below which is an elongated structure appropriately called a ladder (Fig, la). The entire snare is usually strung between the upper lateral branches of small trees and their lower lateral branches, or occasionally the ground. The spider which builds this web has now been identified as Tylorida sp, (Araneidae) (see Robinson 1982). An almost identical web, but with an inverted ladder (Fig. Ib), was later discovered in South America by Eberhard (1975). This snare, produced by Scoloderus tuberculifer (sub-family Araneinae) represents an 'almost incredible convergence to the same web form', although it differs in several relatively minor details. In Florida, Stowe (1978) observed that a similar web was built by Seoloderus cordatus; he noted also that the method of construction, essentially the same as in S. tuberculifer, involved several modifications not seen in the typical orb-web. Eberhard (1975) recorded a third type of ladder-web constructed by Eustala sp. (Araneidae) in which a number of features, more particularly the shorter wedge-shaped ladder, suggests that it represents an intermediate stage between the advanced type of ladder-web and the typical orb. A further highly specialised web from Papua New Guinea, comparable in several respects to the complex ladder-webs described above, is built by Herennia ornatissima against tree trunks (Robinson & Lubin 1979). This structure consists of a densely matted hub-cup generally positioned in the upper portion of the rectangular snare but perhaps the most unusual feature is that this web curves part way round the trunk. Yet another kind of ladder-web has been discovered in New Zealand, but in this instance there is both an upper and lower ladder ofalmost identical design so that the orb occupies a more-or-less central position in the web structure. Like the Herennia web it is built against tree trunks but does not curve around them. Although the spider which builds this web (Araneus airihastulus, sub-family Araneinae) was mentioned earlier by us (Forster & Forster 1973), its ladder-like web was then unknown. There is, moreover, a closely related species, Araneus subcomptus, which builds a typical orb-web usually between two upright branches although webs are sometimes attached by one side Published online 27 Feb 2013

3 456 to the main trunk (Forster & Court unpublished data). The present paper describes the ladder-web snare of A. atrihastulus and advances some hypotheses about the function and evolutionary significance of this specialised type of web construction. MATERIALS AND METHODS The spiders, Araneus atrihastulus and A. subcomptus (Fig. 2a, b), are remarkably similar in appearance. Both have spiny legs and cryptically patterned abdomens which blend with the Iichen-covered tree trunks where they rest during the day. Adult females of both species are 6-8 mm in body length while males are about 5-6 mm. The web of A. atrihastulus, however, has been substantially modified from the basic orb by the addition ofan upper and a lower ladder. With two exceptions, the ladder-webs described here were found on the north-eastern slopes ofcoronation Scenic Reserve, Whangarei, Auckland. The area concerned fringes a substantial patch ofnative forest, often disturbed by grazing horses. A large web made by a mature female was found beside a bush track in the Monument Reserve in Whangarei, and another large web was located some distance from the road at the Mangangina Reserve, near Kaikohe. Since the original discovery, A. atrihastulus spiders and their ladder-webs have been found in Kitchener Park, near Feilding in the North Island, in Peel Forest, Canterbury, near Mt Hercules, Westland, as well as Trotter's Gorge and the Catlins Coastal Park, Otago, in the South Island. Araneus subcomptus has been found in the Auckland area (D. Court, pers. comm.) as well as Mt Hercules, Westland, but our observations relate to those spiders and their webs found in Peel Forest where they are sympatric with A. atrihastulus. Apparently both species are quite widely distributed throughout New Zealand although their prevalence has undoubtedly diminished as the native forests have receded. Although precise ecological data relating to ladder-web habitats are unavailable as yet, we surmise that a fairly specific and above-average humidity is important. Moreover, webs are generally found either along forest margins or where the forest is relatively sparse, thus enabling sunlight to glimmer periodically through the overhead canopy. These two conditions are not easily met since thinner patches of bush tend to dry out more quickly, but our failure to find webs in some otherwise likely areas can only be explained in the meantime by these suggestions. New Zealand Journal of Zoology, 1985, Vol. 12 Ladder-webs were photographed and measured in situ by first coating with cornstarch using a puffer designed by Dr J. Carico (1977). Measurements relating to the total height of snare, diameter of the orb and details of web design were taken from the first 10 complete webs found in Whangarei (Table I). These included webs made by juvenile spiders as well as those made by sub-adult males and mature females. In several instances, the spider was located and collected from its daytime resting site on the tree trunk. Webs found since range within the dimensions as originally measured and reported here. Captive A. atrihastulus were housed in glass cages (c. 30 X 70 X 80 ern) backed with nylon netting in the manner described by Forster & Forster (1976). Twigs and branches were strategically positioned around the cage to serve as attachment points for webs. Cages were kept on an outside porch so that they were sheltered from rain but subject to ambient temperatures and breezes. Periodically, water was sprayed into cages. Moths, house flies and Drosophila were introduced by peeling back a comer of the nylon netting away from the web; prey of one kind or another was always available. One female spider lived for 6 months in its cage and built a succession of ladder-webs during this time. In the Spring (August) it laid an eggsac and subsequently died. Data for the first 12 captive webs built by this spider over a period of 5 weeks were recorded (Table I) and analysed. RESULTS AND INTERPRETAnONS Web placement In contrast to the Tylorida and Scoloderus ladderwebs, the specialised A. atrihastulus structures found in New Zealand were almost always placed against (but not touching) tree trunks. In some localities the most preferred trunks were Dicksonia species (native 'ponga' ferns) (Fig. 3), while in other areas, tall straight trunks ofseveral different species were utilised. The longitudinal axis of the web always corresponded to the vertical axis ofthe tree; i.e., its ladder-like proportions appeared to be neatly adapted to the shape of the trunk. In this respect, they were remarkably similar to Herennia webs except that these latter webs exhibited a pronounced curvature around a portion of the trunk. In contrast, A. subcomptus webs were positioned away from the trunk and were sometimes inclined. Orbs were generally larger than those of A. atrihastulus but were also fine-meshed and equally hard to see. The ladder-webs surveyed here were all found in place during the daytime and appeared fresh and undamaged; spiders were almost always located nearby by tracing the silk thread from the hub to

4 Forster & Forster-A derivative of the orb web 457 mm (a) (b) -- \ j Fig. 1 (a) New Guinea ladder-web with ladder below the orb (from Robinson & Robinson 1972); (b) ladder-web ofscoloderus tuberculifer with ladder above the orb (from Eberhard 1975).

5 458 the trunk (Fig. 4). Spiders were seldom found in association with badly tom or distorted webs, nor were the remains of prey evident in any web that we examined. For all observations, the maximum width of the web was less than the diameter ofthe trunk so that the web was, in effect, protected by the trunk. However, we found two large webs (Fig. 5 illustrates one of these) which had been placed between the tree trunk and a twig growing from it (described below). Web characteristics The most striking feature of the present structure was the presence oftwo ladders, one above and one below the orb. Neither ladder was as long as those of the adult Scoloderus or Tylorida webs yet the total length ofthe structure shown in Fig. 5 was 75 em, which comes within the range reported for S. cordatus (Stowe 1978). The shape ofthe ladder-web in Fig. 5 was because its unusual placement dictated a structure based on a triangular framework; accordingly, both ladders tapered at their extremities, the upper more so than the lower. A second ladder-web of comparable dimensions and shape was found later in an almost identical situation. In this latter instance, the spider was retrieved and subsequently built many ladder-webs in captivity, enabling comparisons to be made with its 'wild' structure which was indistinguishable from the one iilustrated in Fig. 5. Mean values (±SO) of the 10 webs surveyed in the field are set out in Table I; these were calculated for adult females, subadult males and juvenile spiders. Webs were almost invariably rectangular as the height to width ratio shows, this basic configuration being established by two vertical threads which formed the lateral boundaries of the entire structure (see Fig. 3). Based on present data, ladder area shows a consistent ratio to the orb (c. 3: I) throughout the life of the spider, offering a considerably greater opportunity for the interception and trapping of prey than the orb itself. In their natural setting, webs were almost invisible, being constructed of very fine, closely meshed silk. Field measurements, verified from photographs, indicated that the distance between the sticky threads of the spiral and cross threads ofthe ladder varied from 0.5 mm in central regions of the web to 2 mm in outer areas. The spacing of similar threads in captive webs could not be measured with the same accuracy without damaging them (largely because of the confines of the cage) but estimates by eye suggested that mesh size ranged within the above limits. New Zealand Journal of Zoology, 1985, Vol. 12 Web construction Although several spiders were caged, only one built a succession ofladder-webs. The following account of this spider's web construction and its associated behaviour have been pieced together from many observations at various stages of web-building although on one occasion most phases of the sequence were witnessed. In this instance we viewed events under red light, having already found that these spiders were easily distracted by light, a sensitivity noted in Araneus diadematus (Reed 1969). Unfortunately, the red light rendered the web completely invisible and made the spider's position, relative to various sections of it, less easily determined. Webs were always built at night and remained in place during the day while the spider rested nearby, connected to the hub by a silk thread; confirming our field observations. The spider emerged at dusk and took up a position head downwards at the hub on the outside of the web (relative to the trunk). Where there was neither web nor existing framework, the spider spent several hours exploring the area beforecommencingconstruction. Various scaffolding threads, including two long vertical lines, were fastened to twigs and other suitable attachment points but the position of the hub itself seemed to be largely determined by its relationship to the cranny that the spider had previously selected as its daytime resting site. Evidently a short lead from the trunk to the hub is desirable for reasons discussed later. This meant that the first bridging thread had to be placed with due regard to the eventual position of the hub, which always remained linked to the tree trunk. With the framework in place, radial threads were then laid with several very long lines running above and below the hub, and relatively short ones to the sides. After the temporary spiral had been deposited, it appeared that a few non-sticky rungs ofthe upper ladder were put in place. Then, starting at the highest level ofthe structure, the spider crossed back and forth as it laid down sticky ladder threads. At the spiral itself, the spider climbed down a vertical framework thread exhibiting a somewhat perambulating motion which suggested that it was attaching silk to the lateral radials. Upon reaching the lower part of the spiral it crossed back and forth over the elongated vertical radials apparently laying down non-sticky threads until it reached the base ofthe structure. It then commenced an upward progression, looping from side to side as before. As the spider neared the non-sticky outer spiral these cross threads became increasingly crescent-shaped, after which it began the sticky spiral and eventually

6 Forster & Forster-A derivative of the orb web 459 I, Iv (l"\~' 'l-"*" I.t ~.. a Fig. 2 (a) Araneusatrihastulus; (b) Araneussubcomptus. Both species are cryptically marked so that they are almost impossible to see against the lichen-encrusted trees where they rest during the day. Table 1 Ladder-web data (in em) for 10 webs surveyed in the field. Within the limits of the present sample, the ratio of the area of the ladder to the area of the orb is consistent for the three subdivisions set out here. Ratio of height to width varied, however, being greatest for sub-adult males. Adult <f (n=4) Mean±SD Sub-adult cl (n = 2) Mean±SD Juvenile (n=4) Mean±SD Height of web Upper ladder (height) Lower ladder (height) Diameter of orb Height: Width Ladder area: Orb area : ±1.0 5: I 3: I :1.5 6: I 3: I : 1.1 4: I 3: I finished up at the hub. Unlike Scoloderus, it is presumed that non-sticky silk was removed during the final construction since it was not present in the completed web. This account supposes that the spider interrupted the sticky thread deposition in the upper ladder to lay down more non-sticky silk in the lower ladder, a switch in silk-deposition procedures not known in any other araneid (Eberhard, pers. comm.). For A. atrihastulus to conform to the more typical practice it would need to put down the nonsticky cross threads in both the upper and lower ladders first. then lay the sticky threads in these extended regions from bottom to top (or vice versa), thus bypassing the spiral orb section twice. This we never saw. The spider always moved upwards after laying the non-sticky spiral and later bypassed this web element on its way downwards; the sticky spiral was always laid immediately following the lower

7 460 New Zealand Journal of Zoology, 1985, Vol. 12 ladder construction. One possible explanation for this apparent anomaly in the order of silk deposition is that we were mistaken in our interpretation ofthe spider's movements and that no nonsticky threads were laid down in the ladders. In this event the spider should have gone directly to the upper extremity and commenced laying sticky threads from side to side as it moved downwards towards the orb; then after bypassing the orb, it should have moved immediately to the lowest extremity and begun to lay sticky threads as it crisscrossed upwards. If this explanation is found to represent the actual sequence of silk-laying, no unorthodox switch in type of silk deposition would be involved. Structural variability Table 2 sets out values for the wild web as well as for the 12 captive webs made by one spider. Cage dimensions limited web size and in particular, height, and no web was built to match that of the wild structure. The orb itself was apparently the most stable aspect of the structure because mean orb diameter more closely matched that of the wild web than any other section. For the first eight captive webs the upper ladder was either reduced or omitted from the structure; we attribute this primarily to the spider resting in a topmost comer of the cage during the day. This meant that, because ofprobable restrictions on the length of the hub-trunk connection, the orb filled the upper zone leaving little or no space for the construction of the upper ladder. Moreover, we noticed that the first web built at a new site was invariably larger than subsequent replacements (after damage during prey capture) within the same silk boundaries. After the spider had taken up a resting place some 13 cm below the ceiling, more typical ladder-webs (nos. 9-12) having both upper and lower extensions were built. Their overall size was greater than that of previous webs and the number of prey captured per web-unit day increased (unpublished data). Ladder design, in particular, varied from site to site; in one location ladders were wide and squat, in another, where the lower threads formed a triangle at the base ofthe cage, ladders were long and attenuated. Where webs were built within an existing framework, specific characteristics tended to be repeated. Field observations reinforced this notion - in one instance we retrieved a spider whose web was damaged in the process but upon returning to this site on a later occasion found another occupant with an almost identical web, shaped to meet with the natural attachment points. Fig. 3 Web of young Araneus atrihastulus, Web is attached to leaf stubs of ponga fern. The upper ladder has been damaged. Site selection In between building new webs, spiders spent several nights suspended from a single thread looped between twigs. In the wild this strategem would keep the spider safe from predation, but in captivity (and probably in the wild) it also resulted in an

8 Forster & Forster-A derivative of the orb web 461 Fig. 4 Thread linking resting spider (hidden in fibrous bark of ponga fern) with the hub of its web. occasional meal. We saw spiders eating flies and moths and presumed that these insects had been captured when they ventured onto these threads. Following this sedentary phase, the spider spent one or more nights in exploratory activity. The first sign that web-building was imminent occurred when the spider dropped several times from a dragline between various upper and lower projections. Perhaps this behaviour provides the spider with information about the distance and attachment points below it, as well as potential obstacles. We saw no behaviour, however, which suggested that the spider made similar assessments about the area above its projected orb. Nevertheless, cage dimensions indicated that the spider could have built a wildsized web with two ladders had it put the orb in the centre ofthe available space and foreshortened some ofthe lower supporting guylines. Since it did not do so, we must assume that such judgements are beyond it and that assessment ofsite suitability is limited to an ordered routine in which certain primary requirements have to be met, the lower ladder apparently having priority over the upper, despite the upper one being the first to be constructed. Perhaps the sedentary phase also provides the spider with information about the suitability of a site with respect to prey, predators, air movement, moisture and sunlight. The exploratory phase allows it to determine the physical parameters of its habitat and hence the potential for web-building. Having thus surveyed the general area, it then establishes a resting place, subsequently connecting it to the frame thread and ultimately the hub; at some stage it measures the distance below it probably laying the vertical framework at this time. Bearing in mind that the spider normally builds against a tree trunk, perhaps there is no inherent need to survey the area above it since tree trunks almost always meet their architectural requirements. In none of the captive webs, for instance, was the orb placed in the lower zone with an upper ladder attached and the lower ladder omitted, an equally feasible solution for a spider confronted with limited space. The capture of prey Except when A. atrihastulus was required to rebuild at an entirely new site, webs served as 24 h traps or for prey storage. House flies, which became entangled during the day and enticed the spider from its lair, were wrapped at the capture site and left attached to the nearest radial. Indeed, all prey

9 462 items including the first, whether snared by day or by night, were left at capture sites while the spider rested at the hub for a time, or if it was daytime, returned to its resting spot until nightfall. This routine is not always exhibited by A. pustulosa, for instance; after prey is wrapped it may be hauled back to the hub immediately and consumed (pers. obs.) or alternatively, left in situ until later (D. Court, pers. comm.). After emerging at dusk, A. atrihastulus would sit at the hub for some time before retrieving stored prey. This was accomplished by first cutting the wrapped prey free from the web and repairing the radial line. The spider would return to the hub with its abdomen angled away from the web so that the wrapped prey bundle dangling by a thread from the spinnerets was kept clear. On one occasion, the spider was seen at night eating a fly at the hub while two more flies awaited retrieval from other parts of the web. Moreover, when a moth subsequently became caught in the web, the spider left its partially eaten meal, ran down the web and seized the moth. This was bitten, then wrapped and left in situ while the spider returned to the hub and finished the first fly. As each prey item was consumed, the remains were cut away and discarded. Surplus prey, however, was left until the following night. The way in which moths were caught in the web and seized by the spider in captivity was observed on many occasions. The fineness of the mesh did not allow even the smallest moth to fly through; large ones, however, always managed to bounce off and as the web aged moths of all sizes bounced off. About halfof the moths which struck captive webs escaped. Some of them slid downwards, sticking and fluttering intermittently as they fell, until they were eithercaught by the spideror reached the bottom of the web and escaped (also see Stowe 1975). As Eisner et al. (1964) pointed out, moth scales detach with great ease when a moth strikes a web, leaving behind scale-laden patches or 'moth scars'. These moth scars, ranging from em in length, were a prominent feature of captive webs, almost always running down the vertical dimension of the web. We seldom saw moth scars in wild webs during the day but this may be because spiders replaced their webs immediately after consuming the night's catch. Incaptivity, we left the entanglement of flies and moths to chance, i.e., we did not place them in webs. The likelihood of their being snared was greatly increased when webs were fresh or when they were damp (after spraying). House flies trapped in the daytime were probably detected only because of the short hub-trunk lead but although Drosophila were readily entangled by the silk the spider never emerged as a result oftheir struggles; New Zealand Journal of Zoology, 1985, Vol. 12 Fig.5 Ladder web of adultfemale Araneusatrihastulus. This web stood out from trunk in an atypical fashion. Only by dusting with cornstarch could the web be seen and this treatment caused the damage. presumably because such weak signals could not be transmitted over that distance to the spider. Hence the short hub-trunk lead is vital if the spider is to seize sizable insects during the day. New webs were built after all prey had been consumed although the

10 Forster & Forster-A derivative of the orb web 463 o ",,"'NV" -~N~-OO "": coming ofdawn would induce the spider to postpone construction until the next night. If no prey was caught, webs were left in place, sometimes for several days. In some instances, unsuccessful webs were simply abandoned. By contrast, A. subcomptus retained contact with the web via an extended radial attached to the trunk beside the spider's resting site. Webs were much more exposed and, although they probably intercepted a wider range of prey items, they were also more vulnerable to damage. Like A. atrihastulus, however, A. subcomptus rested inconspicuously during the day on a lichen-encrusted tree trunk. Surprisingly, an analysis ofthe araneids parasitised by Pison species shows that A. subcomptus was captured in large numbers while A. atrihastulus was never a victim (Forster & Court unpublished data). Could it be that the transition to the ladder-like structure so appropriately positioned against a treetrunk also provided A. atrihastulus with a protective barrier behind which it can no longer be so easily seized by some visually acute, parasitic wasp? -N I v... _ N- 1 0 " -~.:.: ONVN" ("fj- -lr')tr) NM DISCUSSION The main features which distinguish A. atrihastu Ius structures from Tylorida and Scoloderus ladderwebs are the central placement ofthe orb (between two vertical ladder-like extensions) and the modified construction techniques thus required; the 24 h (or longer) duration of the web, and its direct association with tree trunks. In this latter respect, however, it has some similarity to the Herennia web. Since ladder-webs are clearly derived from orb-webs, it is of interest to speculate about the circumstances under which such changes might have been brought about. Many orb-weaver species, such as Zygiella, build asymmetrical orbs (see Witt & Reed 1965) in which the lower sector is extended and more closely woven than the upper sector. This is achieved by 'looping' actions during which the spider doubles back on the spiral in the lower region of the orb a number of times in the early stages of sticky spiral formation. It is not too difficult to envisage a series of steps in which some ancestral species began to enlarge on this looping behaviour so that eventually a ladder-web type of structure, such as that seen in Eustala sp., is developed, as Eberhard (1975) suggests. To postulate such a gradual transition to a ladder-like web in Scoloderus and Tylorida. however, we must also assume that at some stage in this evolutionary sequence the spider began its looping behaviour with non-sticky silk instead of sticky thread. Subsequently, sticky thread deposition commenced; not in the outer fringes of the lower orb but in the most distant region of either

11 464 a lower ladder, e.g., Tylorida, or an upper ladder, e.g., Scoloderus, after suitably extended radials had developed. Where A. atrihastulus is concerned, moreover, two such evolutionary transitions must have taken place with the result that the spider not only constructs first, an upper and then, a lower ladder but also works towards the centre from the two extremities of the web. This means that the sequence of events in this, and all other presently known ladder-webs, allows the final stage of construction to coincide with the spider's return to the hub, as happens in the typical orb-web. However, our observations for both wild and captive webs of A. atrihastulus show that, where space is limited or the surroundings unsuitable, the lower ladder takes priority over the upper, suggesting that in the course of its evolutionary history, the lower ladder was developed first and is therefore more primitive. That the upper ladder is constructed before the lower (space and appropriate stimuli permitting) does not contradict this notion, since it is clear that all modifications leading to ladder-web construction have been inserted into the original orb-web programme. We can be reasonably confident of this because construction begins with the temporary spiral and concludes with the deposition of the sticky spiral. Since A. atrihastulus leaves its web in place until it is ready to replace it with another (except when seeking a new site), it maintains its connection with the web at all times. Because of its procryptic characteristics, A. atrihastulus disappears against the tree-trunk during the daytime, its near invisible web usually just a short distance in front of it. Apparently, too, the web protects the spider from one of its greatest enemies, the parasitic wasp. The treetrunk, in tum, protects the web from birds in flight and large air-borne insects, as well as from climatic extremes. Both spider and web are thus ideally adapted to the tree-trunk; the web with regard to its position, shape and lack of visibility, and the spider in respect of its colouration, daytime posture and proximity to the snare. The morphological characters that A. atrihastu Ius and A. subcomptus have in common lead to the conclusion that they are sibling species (Forster & Court unpublished data) but it seems probable that, like A. subcomptus, their common ancestor also built a simple orb-web. The association between this plesiomorphic stage and lichen-encrusted trees probably developed first with crypticity being a strong factor influencing selection. In the process of speciation, the web of one form became modified to fit the architecture of the tree trunk while the other retained its more primitive characteristics. However, no intermediate ladder-web structures are known in New Zealand although A. New Zealand Journal of Zoology, 1985, Vol. 12 atrihastulus sometimes builds out from the tree trunk (see Fig. 5) in the normal A. subcomptus position. Our observations also suggest that these elongated webs are particularly effective in intercepting and trapping moths (although we do not claim that moths are this spider's only wild prey). However, many moths flyintermittently during the night and probably seek to rest on tree trunks from time to time. The opportunity to exploit this prey resource might be yet another factor influencing the evolution of the ladder-web described here (also see Stowe 1978). Having the orb in the centre of the structure also reduces the effect of one disadvantage ascribed to the New Guinea ladder-web - that of the extended distance the spider has to travel to catch its prey (Robinson & Robinson 1972). For instance, we observed that when a moth struck the upper part of the web, the spider frequently seized it as it slid past, only a few centimetres from the hub. It seems that, by extending its web in this fashion, A. atrihastulus has increased its capture potential beyond that of the original orb. ACKNOWLEDGMENTS We are grateful to the Scientific Distribution Committee of the Golden Kiwi Lottery Board for financial support during the preparation of this paper. We thank Dr W. G. Eberhard for reading an earlier draft of this manuscript and for valuable comments and suggestions. We appreciate the comments of Mr David Court on the final draft. REFERENCES Carico, J. E. 1977: A simple device for coating orbwebs for field photography. Bulletin British Arachnological Society 4(2): 100. Denny, M. 1976: The physical properties of spider's silk and their role in the design of orbwebs. Journal experimental biology 65(2): Eberhard, W. G. 1975: The 'inverted ladder' orb web of Scoloderus sp. and the intermediate orb of Eustala (?) sp. Araneae: Araneidae. Journal ofnatural history 9: Eisner, T.; Alsop, R.; Ettershank, G. 1964: Adhesiveness of spider silk. Science J46: I. Forster, R. R.; Forster, L. M. 1973: New Zealand spiders, an introduction. Auckland, London, Collins. 254 p : The small world: the community life of small land animals. Wellington, School Publications Branch, Department of Education. 99 p. Lubin, Y. D. 1980: The predatory behaviour of Cyrtophora (Araneae: Araneidae). Journal ofarachnology8:

12 Forster & Forster-A derivative of the orb web Reed, C. F. 1969: Cues in the web-building process. American zoologist 9: Robinson, M. H. 1982: The ecology and biogeography of spiders in Papua New Guinea. Monographiae Biologicae. Ed. J. L. Gressitt. Vol. 42: Dr W. Junk Publishers, The Hague. Robinson, M. H.; Lubin, Y. D. 1979: Specialists and generalists: The ecology and behaviour of some web-building spiders from Papua New Guinea. I. Herennia ornatissima, Argiope ocyaloides and Arachnura melanura (Araneae: Araneidae). Pacific insects 2/(2-3): Robinson, M. H.; Robinson, B. 1972: The structure, possible function and origin ofthe remarkable ladderweb built by a New Guinea orb-web spider. Journal ofnatural history 6: : Evolution beyond the orb-web, the web of Pasilobus sp: its structure, construction and function. Zoological journal ofthe Linnean Society. London 56: Stowe, M. K. 1978: Observations of two nocturnal orbweavers that build specialised webs: Scoloderus cordatus and Wixia ectypa (Araneae: Araneidae). Journal arachnology 6: Witt, P. N.; Reed, C. F. 1965: Spider-web building. Science /49: