among eusocial insects , paper wasps ( hymenoptera : vespidae : polistinae ) are notable for the diversity of their nest architecture ( jeanne , 1975 ; wenzel , 1991 ) . many swarm - founding polistine wasps , including most species of the neotropical tribe epiponini , construct envelopes around their brood combs . comparative and experimental studies suggest that , among other functions , nest envelopes reduce rates of predation and parasitism on wasp brood ( london and jeanne , 1998 ; smith et al . , 2001 ) . however , little is known about the adaptive significance of variation in the materials that wasps use to construct nest envelopes ( hansell 1984 ; wenzel , 1991 ; cole et al . , 2001 ) . polybia emaciata is unusual among the polistine wasps because it uses mud , rather than wood pulp or plant fibers , as the main raw material for nest construction ( schremmer , 1984 ) . the genus polybia includes 56 described species ; of these , only the other three species in the subgenus pedothoeca , and one species in the subgenus furnariana , share the derived trait of mud nest construction ( richards , 1978 ; jeanne , 1991 ; cooper , 1993 ) . no other swarm - founding polistine wasps are known to build primarily mud nests , although small amounts of inorganic material are sometimes incorporated into the nest paper of other species ( wenzel , 1991 ) . the nest of p. emaciata is otherwise typical of the genus ( jeanne , 1975 ; wenzel , 1991 ) , being phragmocyttarus ( comprising horizontal layers of comb that are conjoined and enclosed by a continuous envelope ) with a single small entrance hole near the bottom of the nest . numbers of adults in mature colonies are relatively small for the genus , ranging from fewer than 100 up to about 500 adults ( richards , 1978 ; chadab , 1979 ; strassmann et al . , 1992 ) . polybia emaciata nests are hard and much more durable than most paper nests of similar size and shape ( rau , 1933 ; richards , 1978 ) . for example , skutch ( 1971 ) noted that nests of this species in costa rica were unlike paper nests in that they persisted for several years . the high durability of p. emaciata construction material suggests that the nest itself may provide greater brood protection than paper nests . if so , we hypothesized that the wasps ' defensive responses to vertebrate attacks have changed to take advantage of nest durability . we predicted that the nests ' protective properties reduced the need for behavioral defenses by p. emaciata workers , relative to paper - nesting polybia species with similar colony and nest sizes . we tested this prediction by disturbing p. emaciata colonies , and comparing the workers ' responses to those of simultaneously observed paper - nesting p. occidentalis colonies in the same location . chadab ( 1979 ) surveyed the defensive responses of swarm - founding paper wasps to vertebrate - like disturbances , including tapping on the nest and/or on the supporting vegetation . the most frequently observed response was for dozens to hundreds of workers to rush out onto the nest envelope , sometimes adopting aposematic postures ( see also o'donnell et al . , response was seen in the genera angiopolybia , charterginus , leipomeles , parachartergus , polybia , pseudopolybia , and synoeca , and accords with our observations of what appears to be typical epiponine defensive behavior . weaker exit - nest responses ( either requiring several taps , or involving fewer wasps ) were seen in species of leipomeles , metapolybia , and protopolybia . interspecific variation in mature colony size was not a good predictor of the type of defensive response , as species with colony sizes smaller and larger than p. emaciata performed nest exiting defensive behavior . an attack response often coincides with , or quickly follows , the nest exit response and involves workers flying from the nest , striking the intruder , and attempting to sting ( hermann and blum , 1981 ) . when paper - using polybia nests are tapped , some of the workers on the nest surface may immediately perform attacks , while large numbers of workers rush out of the nest entrance and stand in alert posture on the envelope ( jeanne , 1981 ; jeanne et al . , 1992 ) . further tapping elicits additional attacks . breathing on the nest surface or into the nest results in immediate attack by several to dozens of workers . we discuss the possible adaptive significance of deviations from the typical nest exit that have been documented in several paper wasp species , including p. emaciata . this study was conducted in and near the town of gamboa , panama province , republic of panama ( 0907n 7942w ; 50 m elevation ) . observations and manipulations were conducted during the wet season from 1 august to 15 october 1990 and from 3 to 5 july 1998 . surveys of eusocial wasp colonies during these periods indicated that p. emaciata was among the most abundant swarm - founding wasps in terms of nest density ( s.o'd . , unpublished data ) . most of the data presented were collected from four colonies of p. emaciata and five colonies of p. occidentalis that had been moved approximately 0.5 km to 1 km from their natural nesting sites to artificial supports to facilitate observation . all subject colonies were moved into a small area ( < 0.25 ha ) , with similar amounts of tree cover ( one p. occidentalis colony was additionally sheltered by building eaves ) . all colonies were in place at least 2 weeks before observations on defense behavior were initiated , and all continued to forage and to raise brood ( visible through the nest entrance in the lower combs ) throughout the observation period . the p. emaciata observation colonies had nests ranging in size from 9 cm long to 22 cm long . the numbers of adult wasps and colony age probably varied positively with nest size , though we did not measure these variables . we also collected data opportunistically from p. emaciata colonies in situ in 1990 and again in 1998 . these nests were located in the gamboa area on thin tree branches from 1.5 m to 5 m above the ground . we disturbed p. emaciata and p. occidentalis colonies in an effort to evoke defensive behavior , using stimuli that elicited nest exit and attack responses in costa rican p. occidentalis ( s.o'd . and we disturbed the polybia nests by first tapping on the side of the outer envelope near the middle of the nest for 30 sec while wearing a full bee suit , and we noted the responses of on - nest and in - nest workers . if there was no attack response to tapping , we blew into the nest entrance for 30 sec . each polybia observation colony was disturbed 10 times over the course of the study period in 1990 . the in situ nests of p. emaciata were tested once each in a similar fashion . we used a non - parametric wilcoxon test for species differences in the probability of immediate attack responses using the four p. emaciata and five p. occidentalis observation colonies . trials were pooled within colonies and each colony was used as a single data point in the analysis . all observation and in situ polybia colonies contained brood ( larvae and/or pupae were visible in the lower combs ) when they were tested . based on forager traffic and on the numbers of wasps exiting the nests during attacks polybia emaciata only rarely ( 5 occasions out of 40 trials on the observation nests and 2 occasions out of > 15 trials on in situ nests ) exhibited a typical polybia attack response within 30 sec when the nest was tapped . polybia emaciata workers never rushed out of the nest and onto the nest envelope . in all trials where the wasps did not attack in response to the initial mechanical disturbance ( within the first 30 sec ) , some of the workers on the nest surface flew off the nest and departed ; the remainder rapidly entered the nest . after the initial disturbance , the nest surface and the lowest layer of comb ( partially visible through the nest entrance , fig . breathing into the nest entrance did not elicit rapid ( within 10 sec ) attack responses . after continued disturbance within a trial ( breath and tapping ) , p. emaciata workers often attacked violently . in approximately 75% of trials , several dozen individuals rapidly exited the nest , took flight , and struck and attempted to sting the observer 's bee veil . the p. emaciata nests were not abandoned following these attacks , and workers later returned to the nests . in contrast , all 50 nest - tapping trials with p. occidentalis resulted in immediate ( within 1 sec ) exit - nest and attack responses . the species difference in the per - colony probability of nest exit and attack within 30 sec of disturbance was significant ( fig . 2 ; wilcoxon 2-tailed test , z=2.60 , p<0.01 ) . breathing in or on p. occidentalis nests always elicited immediate attack responses , therefore , habituation to tapping and breathing stimuli was not evident over the course of the study in p. occidentalis . the four p. emaciata observation nests suffered no damage while exposed to nearly daily heavy rains for 3 weeks ; four of the p. occidentalis observation nests in the same location were visibly damaged by rain during the same period . even during heavy downpours , the mud nests repelled water much longer than nearby p. occidentalis nests . during one rain shower , the mud nests did not appear waterlogged until 15 min of rainfall had elapsed , while p. occidentalis nests stopped shedding water and became waterlogged after < 5 min . the nest fell 2 m to the ground , with only minor damage to its envelope , and the nest was still occupied by adult wasps when it was collected within 12 h of falling . rather than exiting and attacking within a few sec of vibrational disturbance , p. emaciata workers usually either departed or fled into the nest interior . unlike many other paper wasps ( s.o'd . and r.l.j . , personal observation ) , p. emaciata failed to respond aggressively to human breath , even when we blew directly into the nest entrance . polybia emaciata behavioral responses to nest disturbance were unusual among their congeners , and among paper - nesting epiponini with similar colony sizes ( s.o'd and r.l.j . personal observation ; chadab , 1979 ) . however , a few species of paper wasps in other genera exhibit similar responses to human disturbance , which chadab ( 1979 ) labeled richards ( 1978 ) collected a p. emaciata colony in brazil , and the wasps did not exit the nest until 1.5 h after it was collected . residents of costa rica from several rural communities reported collecting and moving active p. emaciata colonies without being stung , by placing a finger over the nest entrance and plucking the nest from its attachment point . chadab ( 1979 ) noted that one colony of p. emaciata in ecuador had  durable  nest paper and exhibited defensive retreat responses like those that we recorded ; two other p. emaciata colonies had  fragile  nests and performed exit - nest and attack behavior when jarred . we did not observe qualitative variation in p. emaciata nest durability in panama ( > 50 colonies observed ; not all were tested for defensive responses ) . predation on swarm - founding wasp nests by bats ( jeanne , 1970a ) , birds ( skutch , 1971 ; windsor , 1976 ) , and primates ( vecht , 1967 ) has been documented . we hypothesize that the unusual defensive response of p. emaciata is a behavioral adaptation to its use of mud in nest construction , which may make the nest more resistant than paper to entry by vertebrate predators . it appears that the wasps initially rely on the nest itself , rather than on exit and attack behavior , to thwart vertebrate predators . we suggest that this represents a special type of architectural defense ( hermann and blum , 1981 ) , where the defensive behavior of a species has been modified to exploit the properties of its nest material skutch ( 1959 ) observed a red throated caracara ( daptrius americanus ) removing combs from a p. emaciata nest and feeding on the brood in costa rica . our subject nests often responded with attack behavior after extended disturbances , suggesting that attack can be effective for this species . when attack did come it was sudden and massive , suggesting that it was coordinated by an alarm pheromone , as has been demonstrated for p. occidentalis ( jeanne , 1981 ) . although army ants are among swarm - founding wasps ' most frequent predators in wet tropical habitats , preliminary tests on the reaction to eciton army ants did not suggest that p. emaciata 's mud nest is effective in resisting these predators ( chadab , 1979 ; s.o'd . personal observation ) . the use of mud as a nesting material may have evolved in response to predation pressure , particularly by vertebrate enemies . similar advantages against hornet ( vespa ) predation may have favored the evolution of mud construction in hover wasps of the genus liostenogaster ( hansell 1984 ; turillazzi 1991 ) . however , mud construction appears to provide other benefits . our observations suggest that p. emaciata mud nests were more water repellent , and more resistant to mechanical damage , than similarly sized paper nests . even if increases in general durability were the original selective advantage driving the evolution of mud construction , the workers ' defensive behavior has apparently been secondarily modified . there is variability in average colony size and in the details of nest architecture among species within the mud - nesting polybia subgenus pedothoeca , and in the independently evolved mud - nester p. furnaria ( richards , 1978 ; carpenter et al . , 2000 ) . of special value to our understanding of the evolutionary significance of the mud - nesting habit will be field studies on the defensive behaviors of the other mud - nesting polybia species . in particular , such studies could establish whether retreat behavior in response to vertebrate disturbance always accompanies the use of mud in nest building . useful comparisons of defensive behavior could also be made to epiponine paper - nesters with relatively strong nest paper , such as epipona and chartergus . retreat behavior in response to vertebrate disturbance is not restricted to mud - nesting species of epiponini . similar responses to human approach or nest vibration have been observed in paper- and secretion - nesting species ( protopolybia emortualis , protopolybia exigua , and protopolybia ( formerly pseudochartergus ) fuscatus ) and in cavity - nesting agelaia cajennensis ( jeanne , 1970b ; chadab , 1979 ; carpenter et al . , 2001 ) . paper - nesting polybia jurenei colonies retreated when their nest tree was rubbed , but exited and attacked when the substrate was jarred ( s. o'd . these patterns suggest that retreat behavior has evolved convergently in several lineages of epiponini , perhaps in response to particular types of predators . photograph of a mud nest of polybia emaciata collected july 1998 in gamboa , panama . box plots showing the range of probabilities of response to nest disturbance with attack within 30 sec by polybia emaciata and p. occidentalis colonies .
the swarm - founding wasp polybia emaciata is unusual among eusocial vespidae because it uses mud , rather than wood pulp , as its primary nest construction material . 
 polybia emaciata nests are more durable than similarly sized paper nests . 
 we tested the hypothesis that the defensive behavior of this wasp may have been modified to take advantage of their strong nests in defense against vertebrate attacks . 
 we simulated vertebrate disturbances by tapping on , and breathing in , p. emaciata nests and similarly sized p. occidentalis paper nests in the same location at the same time . 
 polybia emaciata responses to disturbance were qualitatively different from those of p. occidentalis . 
 the latter exit the nest and attack , while p. emaciata workers typically fled or entered the nest , attacking only after repeated and extended disturbances . 
 we conclude that durable nest material may permit predator avoidance behavior in p. emaciata . 
 we compare the defensive responses of p. emaciata workers with those of other swarm - founding vespidae , and discuss several selective forces that could cause the evolution of species variation in nest defense behavior .