Smith, DRR 1985. Habitat Use by Colonies of Philoponella Republicana

Smith, D. R. R. 1985. Habitat use by colonies of Philoponella republicana (Araneae, Uloboridae). J. Arachnol., 13:363-373. HABITAT USE BY COLONIES OF PHILOPONELLA REPUBLICANA (ARANEAE, ULOBORIDAE) Deborah R. R. Smith Department of Entomology Cornell University Ithaca, New York 148531 ABSTRACT Philoponella republicana (Araneae, Uloboridae) is a communal orb-weaving spider. Colonies of this spider were found more frequently in interface forest than in high forest or mountain savannah forest. This does not appear to be due to differences in insect abundance among forest types, but is correlated with greater complexity of the understory in the interface forest. This may be due to the need for supports for colony attachment lines. Within the interface forest, the location of colonies is correlated with local insect abundance. When flying insects are excluded from colonies, individual spiders can respond by increasing the distance between orbs in the colony, and colonies can respond by abandoning the site and moving to a new location. INTRODUCTION Philoponella republicana (Simon) is a communal orb-weaving uloborid spider, found in Panama, Trinidad, and northern South America (Opell 1979). It occurs in the rainforest understory, frequently in small tree-fall gaps and other openings in the forest. It is a seasonal species, with as many as three discrete generations per year in Panama (Lubin 1980). The colonies consist of attachment lines, individual prey capture orbs, and a central retreat area (Figure 1). The retreat is an irregular tangle of non-sticky threads; individuals leave their orbs and move to the retreat in the evenings and when disturbed. Females with egg-cases and adult males may also spend much of their time in the retreat (see also illustration in Simon 1891). Prey capture generally takes place in the orbs. The orbs are placed above and around the retreat, sometimes several layers deep (rarely directly below the retreat); orbs are occupied by one individual at a time. The body of the colony is suspended a short distance above the ground by the attachment lines. These are large conspicuous bundles of non-sticky threads running from the colony to objects in the environment used as supports (e.g., shrubs, herbs). ?Currentaddress: Museum of Zoology, Insect Division, University of Michigan, Ann Arbor, Michigan 48109. 364 THE JOURNAL OF ARACHNOLOGY The communal societies of P republicana are simple compared with those of cooperative spider species such as Agelena consociata (Agelenidae; Kraft 1970), Anelosimus eximius (Theridiidae; Brach 1975, Christenson 1984, Vollrath 1982), or Stegodyphus sarasinorum (Eresidae, Jambunathan 1905). There is no maternal care of the young other than guarding the egg-case, and no cooperation in orb construction. Nor do females cooperate in prey capture: although several females may be attracted to a large struggling insect and help to wrap it, a short aggressive interactidn ensues and one female claims the prey packet. There may, however, be more integration of colony members than this description implies, since colony mates share the support lines and the retreat, and there is some evidence (presented below) that the colony may respond as a group to unfavorable conditions. Fig. 1.?Sketch of a Philoponella republicana colony; a = support lines; b = individual prey capture orbs; c = central retreat area; d = objects used as supports (herbs, lianas, palms). Study of the facultatively communal species Philoponella oweni in Arizona, U.S.A. (Smith 1982, 1983) showed that this species forms communal groups in response to several environmental factors. Philoponella oweni builds its long- lasting webs in protected sites, such as hollow trees or clefts among rocks. These sites may be scarce in some habitats, and the same sites are often used year after year by succeeding generations. Females are Solitary if such sites are abundant, or if food is scarce. Communal groups form in areas where suitable sites for web construction are in short supply and insects are locally abundant, allowing several females to share a protected site and still obtain enough prey. Lubin (1980) reports that new colonies of P. republicana are often founded by groups of immatures dispersing en masse. It is possible that P republicana, with its larger and more complex groups, has evolved from an ancestor in which groups of immatures responded to patchily distributed resources in a way similar SMITH?HABITAT USE BY COLONIES OF PHILOPONELLA 365 to that of P. oweni. For instance, if food were abundant groups of siblings might remain together, whereas if food were scarce they would disperse. Later they might evolve the habit of remaining in groups even when local food supplies were low, moving as a group to a better location. Here I examine the location of P. republicana colonies with respect to those environmental factors already known to be important to P. oweni?insect abundance and substrates for web attachment?and with respect to forest type. I also present natural history information on colony size and development. METHODS Forest type.?I carried out observations of P. republicana in the Voltzberg- Raleighvallen reserve, Saramacca Province, Suriname (04? 32' N, 56? 32' W) during February-April 1980 and February 1982. The Voltzberg reserve is located in primary lowland rainforest. The vegetation of Suriname is relatively well known and several forest types have been described from the Voltzberg region. The names used here for forest types, and the brief descriptions below follow Schultz(1960). High forest is characterized by having two or three stories, the lower stories appearing very open. The main canopy is ca. 30 m tail, with emergent trees reaching about 40 m. Palms, particularly "boegroe makka" (Astrocaryon sciophilum), are abundant in the understory and form a fairly continuous layer at ca. 8 m. The understory is sparse. Mountain savannah forest is a semi-deciduous forest which occurs on shallow stony soils, as on the edges of granite plates and bergs. It resembles true savannah somewhat in appearance (hence the name) but differs floristically. Trees are thin-stemmed and there is little stratification. There may be a dense herb layer. I also included a third type: interface forest. Interface forest occurs where two or more forest types meet. This forest is characterized by a very dense understory of palms, lianas, shrubs, woody plants and herbs. The forest in the Voltzberg reserve was essentially undisturbed except for trails, which passed through tracts of each of the three forest types mentioned here. I located colonies by searching along trails; because the trails did not pass through equal distances of each forest type, the amount of each forest type sampled was not equal. The understory in mountain savannah forest was much less dense than in either high or interface forest; although colonies a few meters off the trails in the latter two forest types might not be visible, one could easily see objects which were reasonable distances from the trail in mountain savannah forest. Insect Abundance.?I measured insect abundance using sticky traps; my traps were fresh-cut leaves of Heliconia sp. (Because all equipment and food for two weeks at a time had to be backpacked into the study area, it was necessary to rely on natural materials as much as possible. I selected Heliconia leaves because they were large, abundant, and relatively uniform in size, and provided a smooth tough surface to spread the trap substance on.) I traced a 15 X 30 or 10 X 20 cm rectangle on the underside of the leaf, and coated an area larger than the rectangle with Stick'em Special. Insects which crawled onto the leaves would 366 THE JOURNAL OF ARACHNOLOGY presumably be caught before they reached the rectangle; insects captured inside the rectangle were assumed to be flying insects, By coating the underside of the leaf I ensured that the trapping surface would not be obscured if the leaf began to wilt. The leaf traps were suspended from trees and saplings. I measured insect abundance at colony sites and at non-colony sites using a paired sampling scheme. I placed a Heliconia leaf trap next to each of seven colonies at the same height as the colony's prey capture surface, and a second trap at an arbitrarily chosen site 5 m due north, at the same height. Two trap sizes were used in different trials?10 X 20 and 15 X 30 cm. The traps in any paired comparison were the same size. In most cases the traps were examined after 24 hrs, but in some cases pairs were examined after 48 or 72 hours. I analyzed these data with the Wilcoxon signed rank test for paired comparisons (Seigel 1956) to allow for the variation in size and time among pairs. When traps were examined I recorded the number of insects captured, their size (length to the nearest mm; insects less than 1 mm were placed in one of two size classes: those less than 0.25 mm, and those greater than 0.25 mm and less than 0.5 mm), and taxonomic order. I also compared insect abundance in the three forest types. I placed five Heliconia leaf traps with a 10 X 20 cm capture area in each forest type. Points for trap placement were randomly selected by laying a 50 m forester's tape along a trail passing through the appropriate forest type, and selecting two numbers from a random number table. The first number dictated how many meters I moved along the meter tape, the second how many meters I moved into the forest perpendicular to the tape, alternating left and right of the tape. I collected data on the number of insects captured as above, every 24 hours for five days. Not all insects captured in sticky traps are potential prey for P.

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