1996. The Journal of Arachnology 24:58–67
FORAGING ACTIVITY AND BURROW DISTRIBUTIO N IN THE SYDNEY BROWN TRAPDOOR SPIDER (MISGOLAS RAPAX KARSCH: IDIOPIDAE )
Richard A . Bradley : Ohio State University, 1465 Mt . Vernon Ave ., Marion, Ohi o 43302 US A
ABSTRACT. Burrow-associated behavior of Misgolas rapax was observed in the field and laboratory. Spider density was estimated on 16 replicate plots at one study site in Brisbane Water National Park , NSW, Australia . These estimates were compared to habitat features including vegetation, topographic slop e and soil penetrance . Spider burrow density varied significantly across the study plots and was significantl y aggregated . There were no detectable patterns of relationship between habitat features and variation in M. rapax density . Misgolas rapax density and activity patterns were not related to natural variation in spatia l and temporal patterns of prey-abundance . Field and laboratory experiments were conducted to determine the influence of proximate feeding history on activity in adult females . Foraging activity decreased afte r food supplementation in the field and was lower among the high-food treatment females in the laboratory . Despite the fact that M. rapax behavior is influenced by feeding history, these spiders are apparentl y unable to adjust their behavior to the unpredictable fluctuations in prey availability .
The Sydney brown trapdoor spider (Mis- at the same time on the same study plots as golas rapax Karsch 1878) is abundant in a similar work on the common orb-weaving spi- variety of dry sclerophyll woodland habitat s der Argiope keyserlingi Karsch 1878 . In that in SE coastal New South Wales, Australia . I t species, density was related to features of veg- builds a burrow lacking a trapdoor, but the etation that provided favored web sites (Brad - entrance is occasionally sealed with silk an d ley 1993). A second goal of the study was to covered with leaves . Prey are captured fro m examine the burrow density and dispersion ambush at the burrow entrance, and adult s patterns of M. rapax and their relationship to rarely wander more than a few centimeter s features of the habitat. from the burrow. The sole exception to thi s pattern is the mate-searching behavior o f METHOD S males . One striking aspect of the biology of Study site M. rapax is that an occupied burrow is either .—The study area was located i n open or closed (sealed with silk) for long pe- Brisbane Water National Park (33 33 S , 151 18 E) at the SE corner of the town o riods of time (weeks to months) . Individual s f of Misgolas rapax can be found at the en- Pearl Beach, New South Wales, Australia . A trance to their burrows waiting for prey in an y set of sixteen 0.023 ha plots (15m X 15 m ) month of the year. This pattern contrasts with was marked with wood corner posts . The plots other trapdoor spiders which aestivate for lon g were established on one hillside in a patch o f periods during the summer months (Buchl i dry sclerophyll woodland dominated by Ca- 1969 ; Marples Marples 1972) or dry peri- suarina torulosa (70% by frequency) suc h ods (Main 1982). The current study was ini- that no two plots were closer to each othe r tiated to determine if proximate environmenta l than 5 m . Consult Bradley (1993) for a de - factors, or the abundance of prey, might influ- tailed description of the study plots and gen- ence the foraging activity patterns of this spe- eral study site . Other large burrowing spiders cies . Another pattern in this species is the ten- known from the study area included Atrax ro- dency for many burrows to be located in som e bustus Cambridge 1877, Geolycosa godeffroy i areas and few in adjacent, apparently similar, (L. Koch) 1865, Hadronyche sp .?, Lycosa fur- areas . This work on M. rapax was conducted cillata L . Koch 1867, Lycosa pictiventris L . 58
BRADLEY—BURROW-ASSOCIATED BEHAVIOR OF MISGOLAS RAPAX 59
Koch 1877 and Misgolas gracilis (Rainbow Spider activity measurement.—Burrow Pulleine 1918). entrances were marked with numbered woo d Environmental measurement.—Weathe r tongue depressors placed 30 cm S of the en - records (rainfall, temperature) were obtaine d trance (n = 429) . Regular (approximately bi- from the Crommelin Biological Research Sta- weekly, on the same nights as prey samples) tion, a station within 500 m of the study area . observations of marked burrows were made t o Soil temperature (surface, 15 cm and 30 cm) , determine if they were open and whether the air temperature, and relative humidity wer e spider was near the entrance . At each visit t o continuously recorded with a Weathertronic s a marked burrow, the temperature and relative thermograph and hygrothermograph located in humidity were also measured. Spider popula- a ventilated wooden box on the study area. tion density was estimated during Decembe r Temperature and humidity were measured at 1985 by counting the number of occupied bur - the burrow entrance for each spider activity rows on each plot . observation using a Novasina MIK-2000 por- Prey abundance measurement .—Tw o table hygrothermograph (Novasina AG, Zu- measures of prey abundance were made. Sam- rich). Soil penetrance was measured with a ples of leaf-litter were collected adjacent to Soiltest CL-700 penetrometer ; 100 measure- the study plots by removing all litter down to ments were made on each plot using a strati- bare soil from a 0 .25 m2 area. Potential prey fied random sampling design . This device pro- organisms were extracted using a modifie d vides an indication of surface resistance , Tullgren funnel (Southwood 1966) . Two row s which has been implicated as an importan t of five pitfall traps were placed on each plot ; factor in the choice of burrowing site in stud- pits were separated by 2 m, and rows were 3 ies of other arachnids (Lamoral 1978 ; Bradle y m from edge of plot . Pits 20 cm deep were 1986; Polis McCormick 1986) . Average to- constructed of 5 mm thick polyvinyl chloride pographic slope was estimated for each plo t tubing with an inside diameter of 11 cm bur- by estimating total elevation change fro m ied flush with the soil surface . A removable highest to lowest point on the plot . Twelve sleeve was used within pits fitted on the bot- vegetation variables (Bradley 1993) were sub- tom with 0 .5 mm nylon mesh for drainage . jected to principal components analysis . The Pits were covered when not in use but were principal component scores for each plot left dry and open during sampling . Samples along the first two axes were compared to were collected over 24-hour periods on 1 8 measures of M. rapax density using Pearson dates between August 1994–February 1996 . product-moment correlation. The dispersion Potential prey captured from litter or pitfal l pattern of burrows was analyzed using th e samples were identified (usually to family ) variance/mean ratio method (Southwood and measured to nearest 1 mm (body length) . 1966) . Significance of this ratio was assessed Dry-weight biomass was estimated using re- by Kershaw s method of calculating t (Ker- gression equations appropriate for each taxon shaw 1973) . (Rogers et al. 1976, 1977) . In cases where no Burrow characteristics .—Burrows of 48 appropriate regression equation was available , adult M. rapax were excavated (a large hole one was calculated from specimens captured was dug beside the burrow, which was then in the study area . Potential-prey samples (lit- approached from the side) ; spiders were col- ters, pits) were compared to temporal pattern s lected from 34 of these burrows and the mid - of M. rapax activity using Pearson product- den of rejected prey remains (rejectamenta ) moment correlation analysis of prey variable s was collected from 32 burrows . All rejecta- against the proportion of M. rapax burrow s menta were identified and a minimum number that were open with an active spider near the of prey of each taxon was estimated from the entrance. Prey samples were also compared t o parts. Burrow depth, diameter at entrance (just M. rapax density spatially (across plots) usin g below flared opening at point where burro w ANOVA (repeated-measures design) . becomes cylindrical), orientation of opening , Laboratory feeding experiment .—Twen- depth of protective silk-closure, and depth o f ty-four adult female M. rapax were installed midden were measured . Spider fang length in individual 30 cm (tall) X 30 cm (deep) X (right), carapace length and the length of tibia 7 cm (wide) clear acrylic plastic ("Plexiglas" ) I (right) were measured to 0.1 mm. containers . Each container was first filled to
60 THE JOURNAL OF ARACHNOLOGY
within 5 cm of the top with natural soil fro m the Casuarina-needle litter at the entrance . near the study area. The spiders readily dug There were few attached radially-arrange d burrows in the containers and lined them wit h leaf or twig lines, which are common feature s silk. The soil was kept damp with regular of other trapdoor spider (Aganippe O. Pick- misting of distilled water. All spiders were ard-Cambridge 1877 and Idiosoma Ausserer housed in a room illuminated on a 12 :12 cycle 1871) burrows found in areas with a substan- and maintained between 21–29 C . Spiders tial leaf litter (Main 1984) . Burrows were rel- were randomly divided into two groups . One atively vertical silk-lined tubes with a mea n group (high-food) was fed either one adult depth of 33 .6 cm (SD = 8 .1, n = 47) and cricket (Gryllus sp.), one ultimate-instar meal- mean entrance diameter of 22 .3 mm (SD = worm larva (Tenebrio molitor Linnaeus) or 2.6, n = 48). Only 10% of the burrows con- one adult mealworm beetle every other day. tained a defensive-sock closure (mean depth All prey items were weighed to the nearest 0 . 1 of closure 16.0 cm; SD = 3.1; n = 5). Main mg. The second group (low-food) was fed a (1984) also mentions the inconsistency of de- similar meal once per 14 days. Spiders were fensive-sock closures in M. rapax burrows . only fed if they were in the "foraging posi- Most of the spiders located in excavated bur- tion" within 3 cm of the top of the burrow rows were found at the bottom; some wer e entrance. The food item was presented to the found just below the silk closure, holding it spider by gently rubbing the silk near the en - closed with their chelicerae . The mean burro w trance to the burrow until the spider graspe d depth was correlated with mean entrance di- the item. If a prey item was not eaten, it was ameter (r = +0.26, P < 0.05). Burrow di- removed the following day . Each spider was ameter was correlated with spider carapace weighed at the beginning (27 October 1984 ) length (r = +0 .50, P < 0.01) and fang length and end of the experimental period (2 Septem- (r = +0.49, P < 0.01). The burrow opening ber 1985) . For the test of differences in activ- of most M. rapax burrows was tilted at a n ity (proportion of nights in foraging position) , angle. There was no significant bias to the ori- the proportions were subjected to the arcsin entation of the openings with respect to eithe r transformation before analysis . slope or compass direction. Many burrows Field food-supplementation experi- contained a midden of uneaten prey remains ment .—Seventy-three active (in foraging po- near the bottom of the burrow (mean depth sition) adult female M. rapax were located 19.8 mm ; SD = 10.5; n = 26). and their burrows marked . Spiders were ran- Density and habitat relationships .—Mis- domly divided into two groups . Spiders in one golas rapax density varied by a factor of 6 .4 group (food-supplementation, n = 30) were across the study plots . The mean density es- fed one seventh (ultimate) instar mealwor m timate based on occupied burrows in Decem- larva on four successive nights (13–16 Feb- ber 1995 for all 16 plots combined was 391 / ruary 1986). The mealworm was presented to ha (n = 16, SD = 236) . Vegetation, elevation, the spider by gently rubbing the silk near th e and slope characteristics varied significantly entrance to the burrow until the spider graspe d across the plots (Bradley 1993) but there were the mealworm. Spiders in the second grou p no correlations with M. rapax density pat- (unmanipulated, n = 43) were checked for ac- terns . There was significant variation in soil tivity by rubbing the silk, but they were not penetrance across plots (ANOVA ; F = 12.9; fed. No attempt was made to restrict natural df = 15, 1584 ; P << 0.001), but this feature capture of prey . All spiders were checked sub- was not correlated with M. rapax density (r = sequently on 20 February and 10 March 1986 ; – 0.35, ns) . The mean distance between bur- the spider was scored for activity and for rows was 5 .4 m (SD = 9 .0). The dispersion whether the burrow entrance was open or of M. rapax burrows was significantl y closed (with silk) . clumped with a variance/mean ratio of ran- dom point-to-burrow distances of 14 .8 (t = RESULTS 13.9, P < 0.01). This may reflect the tendency Burrow characteristics .—Burrows of M. for young spiders to disperse relatively shor t rapax were found in the loosely consolidate d distances from the maternal burrow befor e sandy soils across the entire study area . Bur- constructing their own burrows . rows were lined with silk that was attached to Activity patterns in field.—to assess "ac-
BRADLEY—BURROW-ASSOCIATED BEHAVIOR OF MISGOLAS RAPAX 6 1
So
40
30 20
10
0 JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DE C JAN FEB MAR APR MAY JUN JUL AUG SEP OCT NOV DE C (119) (62) (121) (86) (51) (50) (69) (36) (83) (73) (178) (125 ) (I19) (62) (121) (86) (51) (50) (69) (36) (83) (73) (178) (125)
Figure 1 .-Annual activity patterns of Misgolas Figure 2.-The proportion of Misgolas rapax bur- rapax. The values in the figure represent the per - rows that were discovered newly opened during the centage of marked burrows which were open (ac - month indicated . The sample size of burrow s tive) and known to contain a live spider. The sampl e checked is included in parentheses below th e size of burrows checked is included in parenthese s month. below the month.
for the spider to be in the active foraging po- tive" spiders, I analyzed observations o f sition . marked burrows . If a marked burrow was dis- Relationship to potential-prey phenolo- covered closed and was never observed open gy.—Prey remains analyzed from the midden s again during the study, it was assumed that in excavated M. rapax burrows reveal that this the spider had died or abandoned the burrow , spider eats substantial numbers of the large and it was removed from the sample . Of th e and aggressive bulldog ant Myrmecia gulosa . 429 marked burrows, activity data wer e Unfortunately, the remains in middens un- scored for 1053 observations from 253 bur - doubtedly contain a biased sample of prey ; rows known to contain a living M . rapax in- large and hard-bodied prey are probably over- dividual . These observations were made be- represented (Table 1) . tween September 1984–February 1986 . The A total of 2715 potential prey items was pattern of annual activity as indicated by the captured in the pitfall samples (this total ex- proportion of open burrows that were know n cludes species known to be rejected by M. ra- to contain a living M. rapax is not strongly pax in the laboratory) . Prey availability (num- seasonal (Fig . 1). Similarly there is no obvious seasonal pattern to the proportion of previ- ously-closed burrows which were opened dur- Table l .—Identified prey remains found in Mis- ing a particular month (Fig . 2). Burrows tend- golas rapax burrows (n = 287 prey; n = 32 mid- ed to remain open for long periods ()I = 8 3 dens) . days, SD = 65, range 1-397 days) . After a spider sealed the entrance to its burrow with Per- Per- centag e centage silk, it typically remained closed for an ex - of of all tended interval (x = 76, SD = 60, range 6– Prey category burrows prey 332 days) . On any given visit a burrow had a probability of 49.6% of being found open . Ac- Myrmecia (Formicidae) 78 46 Carabidae tive M. rapax are usually visible near the en - 50 18 Scarabaeidae 41 5 trance to the burrow (within three cm of the Formicidae (others) 34 5 opening) . The number of spiders in this for - Coleoptera 28 4 aging position was assessed during 722 ob- Curculionidae 19 2 servations at open burrows . Significantl y Hemiptera 13 2 more spiders were observed in the entrance o f Blattodea 13 1 open burrows at night (18%) than during th e Camponotus (Formicidae) 9 2 day (7%) (t = 2 .3 ; P < 0.05). There were n o Cerambycidae 9 < 1 significant correlations between surface tem- Hymenoptera (others) 9 < 1 perature, soil temperature or surface humidity Scorpionida 3 1 and the tendency for a burrow to be open, or others nla 12
62 THE JOURNAL OF ARACHNOLOG Y
Table 2 .—Results of the laboratory feeding experiment. The sample size reflects differences in number of spiders surviving at the time of measurement . 2The proportion of nights when a spider was seen in th e "feeding position" within 3 cm of the top of the burrow.
High-food Low-food Significanc e Variable z SD n' x SD n' t P Number of meals 14 (9.1) 11 8 (2.9) 12 2.09 ns Initial spider mass 2.38 (0.77) 12 2.47 (0.74) 12 -0.29 ns Total food mass (g) 2.60 (1.44) 11 1.22 (0.43) 12 3 .05 <0.05 Final spider mass (g) 2.83 (0.72) 11 2.36 (0.38) 12 -2.02 <0.05 Change in mass (g) 0.56 (0.73) 11 -0.11 (0.56) 12 -2.51 <0.05 Prop. nights active2 0.15 (0.13) 11 0.27 (0.16) 12 -2.13 <0.05
bers trapped) appears to vary significantl y food consumed by the spiders in the two treat- through the season as assessed by pitfall-tra p ments differed. The high-food treatment spi- samples (ANOVA F = 2.17; df = 15, 272 ; P ders ate significantly more and were signifi- < 0.01) . Numbers and biomass of potential - cantly heavier at the end of the experimen t prey both peak during spring or early summer (Table 2) . High-food treatment females gaine d (late September through December) . Result s 24% and low-food treatment females lost 4 % from the litter samples appear less seasona l of initial body weight. High-food treatment fe- (ANOVA ns). Using proportion of spiders ac- males were also significantly less active (les s tive (open burrow with spider near entrance ) often in foraging position) than low-food fe- as an index of M. rapax temporal foraging males (Table 2) . Spider growth was positivel y phenology, there was a weak correlation with correlated with the number of meals eaten ( r total potential-prey numbers captured in the = +0.87 ; P < 0.01). When this relationship pitfall-trap samples (r = +0 .57 ; P < 0.05); is plotted, the "break-even point" or the num- this correlation disappears when the sample i s ber of meals eaten over the 11 months tha t restricted to the larger potential prey that com- resulted in no weight change was approxi- prise most of the diet of M. rapax (r = +0.38 ; mately four meals (Fig . 3). ns). Similarly, there is no seasonal correlation Field experiment results .—Of the 43 spi- with potential-prey biomass (r = +0 .42; ns). ders in the unmanipulated group, most kep t There were no signficant correlations between their burrow open through 20 February, bu t M. rapax foraging phenology and potential 38% had closed their burrow by 10 Marc h prey in the litter samples . (Fig. 4A). On 10 March, 59% were still active Spatially, potential-prey abundance varie d at the top of their burrow. Among the 30 spi- signficantly across the study plots for number s ders in the food-supplementation group, onl y (F = 2.17; df = 15, 272 ; P < 0.01) but not 45% had open burrows by 10 March (Fig . biomass (ANOVA F = 0.71 ; df = 15, 272 ; 4B). Fewer of the food-supplementation group ns). There was no spatial correlation betwee n remained active ; by 20 February this propor- potential-prey patterns of numbers or biomas s tion of active individuals was significantly and M. rapax density (r = +0.0008 ns; r = lower than among the unmanipulated group 0.36 ns) . (Fig. 4). On 10 March, only 7% of the food- Laboratory experiment results .—The supplementation females were actively forag- feeding behavior of M. rapax confounded the ing, while 60% of the unmanipulated female s treatment effect in this experiment. After eat- were active (Fig. 4). ing the first few (2–4) successive meals, man y of the high-food treatment spiders ceased DISCUSSIO N feeding . They often remained at the bottom o f Density and dispersion .—One clear result their burrow and would not respond for day s of this study is that the burrows of M. rapax or weeks . Even though they were offered are clumped. Clumped dispersion patterns food, they did not feed. This diluted the dif- have been observed in a variety other myga- ferences in food consumed between the tw o lomorph species (Main 1982, 1987 ; Marple s treatments (Table 2) . Nevertheless, the total Marples 1972 ; Coyle Icenogle 1994) and
BRADLEY—BURROW-ASSOCIATED BEHAVIOR OF MISGOLAS RAPAX 63
140 •
120 r +.87 ** *
10 0
8 0
4 0
N 20