1997. The Journal of Arachnology 25:11-19 NOTES ON THE REPRODUCTIVE BIOLOGY AND SOCIAL BEHAVIOR OF TWO SYMPATRIC SPECIES OF PHILOPONELLA (ARANEAE, ULOBORIDAE) Deborah R. Smith: Department of Entomology, Haworth Hall, University of Kansas, Lawrence Kansas 66045 USA ABSTRACT. Populations of the facultatively communal species Philoponella oweni (Chamberlin 1924) and Philoponella arizonica (Gertsch 1936) (Uloboridae) occur sympatrically in the Chiricahua mountains of southeastern Arizona. This study compares reproductive biology, structure of communal groups, and feeding rates of the two species, and documents differences in their phenology, webs, web construction sites, egg-cases and spiderlings. I suggest environmental factors that may select for different reproductive strategies in the two species. Many members of the spider family Ulo- lived than females and disappear from the boridae have been observed living in groups population during the course of the summer. (Opell 1979; Muma & Gertsch 1964). For Females can lay eggs throughout the summer only a few of these species has the nature of and may survive until early autumn, but in the their group-living behavior been investigated: populations studied no adult females overwin- the facultatively communal Philoponella ow- tered for a second breeding season. eni (see Smith 1982, 1983) and P. semiplu- Immatures hatch and emerge from the egg- mosa (Simon 1893)(see Lahmann & Eberhard case during the summer. As is true of other 1979); P. republicana (Simon 1891), with its uloborid spiders, the young can spin webs in instar large, semi-permanent colonies (Smith 1985; the first post-emergence (Szlep 1961; Eberhard The Binford & Rypstra 1992); and a west African 1977). newly emerged spider? lack a functional and the orb Philoponella Mello-Leitao which was ob? lings cribellum; webs are served in a very large colony (Breitwisch they produce distinctive, containing hundreds of radial threads without a 1989). many Later instars a functional This study compares reproductive biology, sticky spiral. possess cribellum and webs that are similar feeding rates and group-living behavior of two produce or identical to those of adults in form (Szlep sympatric populations of group-living Philo? 1961; Eberhard 1977). ponella, P. oweni (Chamberlin) and P. arizon- The young remain with the female for a ica (Gertsch) (Uloboridae). Notes on the nat? variable length of time and attach their orbs ural history of the two species are also to her web; during the course of the summer presented, including structure of the egg- some or all of them disperse out of the ma? cases, the structure of the webs and substrates ternal web and build independent webs (Smith used for web-building in the two species. 1982). Young spiders over-winter as sub- of Philoponella oweni and P. Populations adults or younger immatures, and emerge the arizonica are in the south- broadly sympatric following spring to form the next generation western United States and northeastern Mex? of reproductives. ico & Gertsch The (Muma 1964; Opell 1979). Both species are facultatively communal; basic life of the two are similar cycles species that is, adult females of both species can be in southeastern Arizona. Field observations found living in small communal groups or as indicate that both are annual; sub-adults solitary individuals (Smith 1982, 1983). In the emerge from overwintering sites in the spring communal groups each female constructs her (early April to early June, depending on ele- own orb web and defends it against other vation). Mating takes place in late spring and adult females. The orbs are joined by their early summer. In general, males are shorter- support lines and space webbing. 11 12 THE JOURNAL OF ARACHNOLOGY Reproductive biology: When a female pro? duced an egg-case it was given a color mark corresponding to that of the mother. To deter? mine mean clutch size (number of eggs per egg-case) for each species I collected the egg- cases after the young had emerged and examined their contents with a dissecting mi- croscope. As is true of most spiders, the young of Philoponella do not emerge from the egg-case immediately upon hatching from the egg; they remain in the egg-case for one in- Figure 1.?Egg-cases of Philoponella oweni star, molt, and emerge as second instar spi? (above) and Philoponella arizonica (below). derlings leaving behind both empty egg-shells and cast-off exoskeletons. In healthy I used the number of shells as Both species suffer from egg-parasitism by egg-cases egg a measure of the number of laid in the the chalcidoid wasp Arachnopteromalus dasys eggs A Gordh (Pteromalidae) (Gordh 1976), which egg-case (Smith 1982). parasitized egg-case can be the absence of exoskel? greatly affects reproductive success of indi? recognized by etons from first instar and vidual females. The female wasp oviposits in spiderlings presence of skins. all the a uloborid spider's egg-case, and the larvae wasp pupal Although spider are killed in a the consume the contents of the spider's eggs, eggs parasitized egg-case, are still visible and were used to leaving behind empty spider egg shells. The egg-shells infer the clutch size. wasp larvae then pupate inside the spider egg- original structure: I noted the size and or- case and emerge as adults. If an egg-case is Colony of all communal formed parasitized, all of the spider eggs inside are ganization groups by each in several locations: P. oweni in killed (Smith 1982). species Cave Creek Canyon (1977), South Fork Can- METHODS yon (1979, 1980) and Herb Martyr (1980); P. Observations and collections were made at arizonica in South Fork Canyon (1979, 1980). several sites in the Chiricahua Mountains in Feeding rates: From 21-27 August 1979, I southeastern Arizona: the Southwestern Re? compared feeding rates of solitary and com? search Station of the American Museum of munal females of the two species in South Natural History; South Fork Canyon, Cave Fork Canyon using a trapline census method. Creek Canyon, and Herb Martyr Reservoir in Censuses were carried out from 0600-1800 h. the Coronado National Forest; and the town Daily census periods were 4-6 h long, for a of Portal (Cochise County). total of 29 h of observation. Every hour I vis- Population censuses: Adult females and ited each female in the study area and record? some males were individually color-marked ed whether or not she was feeding or engaged with dots of fast-drying enamel paints (Tes- in prey capture. tor's model airplane paint) and censused 2-5 Web structure: In July and August of 1980 times per week. From 25 April-20 August I measured the webs of all adult females 1977 I censused a marked population of P. found in the South Fork Canyon study area: oweni in Cave Creek Canyon at an elevation 22 webs of P. oweni and 19 webs of P. ari? of approximately 1695 m. This population zonica. I measured longest diameter of the orb was destroyed by a flood in 1978. From 3 webs and deviation (to the nearest 10?) of the June-18 September 1979 I censused marked plane of the orb webs from horizontal, and populations of P. oweni and P. arizonica in counted number of radii and number of spiral South Fork Canyon. The P. arizonica popu? turns along the longest radius. I also noted the lation was located in the lower part of the can? amount and position of "space webbing" (ir- yon (1525-1660 m) while the P. oweni pop? regular tangles of threads), the presence or ab? ulation was in the upper part of the canyon sence of a stabilimentum and the general ap- (1630-1730 m). At each census visit I noted pearance of the webs. the presence of adult females, males, imma? Habitat: To evaluate the distribution of the tures or egg-cases. two species with respect to elevation I located SMITH?NOTES ON PHILOPONELLA 13 * Table 1.?Phenological data for Philoponella oweni and Philoponella arizonica. Still active when field observations ended. all collection sites (eight for P. oweni, six for Table 3. Both tables show that P. oweni fe? P. arizonica) on a topographical map. I also males produce fewer egg-cases per female and recorded the substrates used for web attach? lay more eggs per egg-case than do P. arizon? ment by members of each species. In July and ica females. The behavior of females with August 1977 I recorded the substrates used for egg-cases also differs between the two spe? 25 P. oweni webs in Cave Creek Canyon, and cies. in August 1979 I recorded the substrates of 36 A female P. oweni about to construct an P. oweni and 31 P. arizonica webs in South egg-case leaves her prey-capture orb and Fork Canyon (in each case, this represented moves to the retreat area, a protected area near all adult webs present in the study sites). the orb usually under a rock or log, and con- Egg-cases: In July 1980 I collected 24 emp- structs the egg-case there. There she remains, ty P. oweni egg-cases and 33 empty P. ari? holding the egg-case and (presumably feeding zonica egg-cases and measured maximum little or not at all) until the young emerge, a width and length to the nearest mm using dial period of approximately 20 days. After the calipers, and noted their color, shape and or- young emerge the mother discards the egg- namentation. case, leaves the retreat, and spins a new prey capture-orb. P. oweni females usually have RESULTS one or at most two egg-cases at a time. The Population censuses: Table 1 gives the time interval between successive egg-cases dates of first sightings of age and sex classes produced by a female is more than a week, of both species.