Kairomones from scarabaeid grubs and their frass as cues in below-ground host location by the parasitoids Tiphia vernalis and Tiphia pygidialis

Cues used in below-ground host-searching behaviour and host discrimination were examined for Tiphia vernalis Rohwer and Tiphia pygidialis Allen (Hymenoptera: Tiphiidae), ecto-parasitoids of root-feeding larvae of the Japanese beetle, Popillia japonica Newman, and masked chafers, Cyclocephala spp. (Coleoptera: Scarabaeidae), respectively. Response to potential stimuli was compared in dual choice tests in an observation chamber filled with soil. Each wasp showed species-specific, directed movement along residual body odor trails made by dragging its respective host through the soil. Presence of a grub was not necessary for wasps to follow such trails. Frass from either host- or non-host grubs elicited trail-following, but each Tiphia species followed frass trails from its respective host when a choice was presented. Frass trails elicited stronger responses than body odor trails. The combination of host frass and body odor elicited the strongest trail-following responses. Our results suggest that once in the soil, Tiphia spp. locate their hosts using contact kairomones present in grub body odor trails and frass.     

Effects of Transect Selection and Seasonality on Lemur Density Estimates in Southeastern Madagascar

I investigated how transect type (trails vs. cut transects) and seasonality influenced density estimates for 5 lemur taxa (Avahi laniger, Cheirogaleus major, Eulemur rubriventer, Hapalemur griseus griseus, and Microcebus rufus) in the Vohibola III Classified Forest in SE Madagascar. I surveyed tree height and diameter and lemur populations from June 1 to December 28, 2004 along 2 1250-m trails local people used and 2 1250-m transects cut parallel to the trails in primary rain forest. Despite dendrometric variations within and between trails and transects, only density estimates of Hapalemur griseus griseus differed significantly by transect type. The spatial variation may be a result of removal by local people of giant bamboo, which is the main food for Hapalemur griseus griseus, along trails. Conversely, seasonality influenced density estimates for Cheirogaleus major, Eulemur rubriventer, Hapalemur griseus griseus, and Microcebus rufus. The temporal variations may be related to seasonal torpor for Cheirogaleus major and increased detection probabilities during periods of fruit exploitation for Eulemur rubriventer, Microcebus rufus, and Hapalemur griseus griseus. Transect type and seasonality did not affect density estimates for Avahi laniger, which may be related to the highly folivorous and low-energy diet of the nocturnal lemur. Researchers surveying lemurs along line transects should be aware that transect selection may influence density estimates for Hapalemur griseus griseus and that seasonality may influence density estimates for Cheirogaleus major, Eulemur rubriventer, Hapalemur griseus griseus, and Microcebus rufus.     

The predatory snail Euglandina rosea successfully follows mucous trails of both native and non‐native prey snails

Mucous trail following is a primary means by which many predatory snails locate prey. I compared the ability of individuals of Euglandina rosea to follow mucous trails of two groups of gastropods: those found within its native habitat (southeastern USA), and those found outside its native range (Kansas). Members of E. rosea followed trails for both species found inside and outside its native range equally well. In contrast to previous studies, I found that the predatory snails consistently followed trails in the direction in which they were laid. I quantified the kinematics of trail‐following behavior using inter‐tentacle angle as the primary metric. In both prey groups, there were significant differences in the predator's inter‐tentacle angle when tracking a trail versus not, and when successfully following (in the direction the trail was laid) versus unsuccessfully following (opposite the direction that the trail was laid) trails. In addition, in both prey groups, there were significant differences in the predator's velocity when tracking a trail versus not, and when successfully versus unsuccessfully following trails. This study confirmed that members of E. rosea are robust generalist predators, capable of successfully tracking native and non‐native snails, and should not be introduced as biologic control agents. These results may be useful to managers, as they provide insight into how trail following could be used to trap members of this invasive species.     

Arboreal Quadrupedism and Forelimb Articular Anatomy of Red Howlers

I describe the patterns of forelimb movements that facilitate adult red howling monkeys (Alouatta seniculus) to advance quadrupedally on arboreal trails in directions ± 45° from the horizontal, the articular surface morphology at the scapulohumeral and elbow joints, and the structure of the trails. Results indicate (1) that red howlers splay their forelimbs when advancing on arboreal trails of mesh-like substrates, but they do not splay them to proceed on unilineal arboreal trails, and (2) that the articular surfaces are adapted to enable the scapulohumeroulnar bony alignments that result in both splayed and nonsplayed forelimb motions. These results have implications for the locomotor habits of several fossil catarrhines.     

Chemical trail systems,orientation, and territorial interactions in the antLasius neoniger

Foraging and territoriality in the ant Lasius neonigerinvolves a series of trails which channel foragers away from adjacent colonies. Experimental studies suggest that the trails are composed of colony-specific, persistent orientation components of hindgut material that accumulate on trails during foraging. A less durable component of the hindgut trail pheromone regulates recruitment. Foraging directionality and the use of a trail could be modified by experimentally arranging confrontations with conspecifics. The orientation of foragers is mediated by visual as well as chemical cues. Components of the foraging and territorial system of L. neonigerappear to include (1) a network of subnests which change in position seasonally within each polydomous nest; (2) a series of trails emanating from each subnest that adjusts search toward resource patches and away from aggressive, neighboring conspecifics; and (3) trail communication involving an ephemeral component of the hindgut trail pheromone that regulates the organization of cooperative prey retrieval and a more persistent component that serves as an orientation guide.     

Trail Marking by the Larva of the Madrone Butterfly Eucheira socialis and the Role of the Trail Pheromone in Communal Foraging Behavior

The larva of the Madrone butterfly Eucheira socialis (Lepidoptera: Pieridae) secretes a trail pheromone from the ventral surface of the posterior tip of its abdomen. Caterpillars mark trails by bringing the secretory site into brief contact with the substrate during a locomotive cycle. Foragers mark most heavily when they move onto new branches and little, if at all, when they move over established trails or when they return to the communal shelter after feeding. The caterpillars make careful comparisons of alternative pathways at choice points and select newer and stronger trails over older and weaker trails. Differential marking of new and established trails during nightly forays, coupled with sensory discrimination of trails by strength and age, leads colonies to abandon old trails in favor of new trails. When applied at a rate as low as 2.5 × 10 ^–10 g/mm, caterpillars followed synthetic trails prepared from 5-cholestane-3-one, a trail pheromone previously reported from the tent caterpillars (Malacosoma spp.). Although both Eucheira and Malacosoma mark with the tip of the abdomen and have near-identical sensitivites to 5-cholestane-3-one, our study shows that Eucheira employs a relatively unsophisticated system of trail-based communication and does not recruit to food. The trail-based communication system of Eucheira appears to represent an early stage in the evolution of cooperative foraging that is derived from, and motivationally linked to, conflict behavior.     

Adaptations of Coccinella magnifica Redtenbacher, a Myrmecophilous Coccinellid, to Aggression by Wood Ants (Formica rufa Group). I. Adult Behavioral Adaptation, Its Ecological Context and Evolution

Coccinella magnifica is an obligate associate of Formica rufa-group ants. The congener Coccinella septempunctata can serve as a model for its nonmyrmecophilous ancestor. Formica rufa behavior toward these two ladybirds, and their behavior, was compared. Although C. magnifica was rarely attacked on ant trails, it was usually attacked on tended aphid colonies. Coccinella septempunctata was more readily attacked. The two ladybirds' behavior was similar on trails, but C. magnifica used more defense and C. septempunctata more rapid escape behavior on aphid colonies. Only C. magnifica fed upon tended aphids. Chemical adaptation to overcome ant aggression probably exists in C. magnifica, but it possesses almost no novel behaviors to counter ant aggression. Instead, modifications have occurred in the expression of behaviors present in C. septempunctata.     

The Effect of Snowmobile Trails on Coyote Movements Within Lynx Home Ranges

Abstract: Coyotes (Canis latrans) and Canada lynx (Lynx canadensis) are sympatric throughout much of the lynx's southern range. Researchers and managers have suggested that the presence of compacted snowmobile trails may allow coyotes to access lynx habitat from which they were previously excluded by deep, unconsolidated snow. This could then allow coyotes to more effectively compete with lynx for snowshoe hares (Lepus americanus), the lynx's primary prey. We investigated how coyotes interacted with compacted snowmobile trails by conducting carnivore track surveys and by snow tracking adult coyotes (4 M, 8 F) in areas of western Montana, USA, with both documented lynx presence and recreational snowmobile use. Coyotes remained in lynx habitat having deep snow throughout the winter months. They used compacted snowmobile trails for 7.69% of their travel distance and traveled on them for a median distance of 124 m. Coyotes used compacted forest roads (5.66% of total travel) and uncompacted forest roads (4.62% of total travel) similarly. Coyotes did not travel closer to compacted snowmobile trails than random expectation (coyote x̄ distance from compacted trails = 368 m, random expectation = 339 m) and the distance they traveled from these trails did not vary with daily, monthly, or yearly changes in snow supportiveness or depth. However, they strongly selected for naturally shallower and more supportive snow surfaces when traveling off compacted snowmobile trails. Coyotes were primarily scavengers in winter (snowshoe hare kills composed 3% of coyote feed sites) and did not forage closer to compacted snowmobile trails than random expectation. The overall influence of snowmobile trails on coyote movements and foraging success during winter appeared to be minimal on our study area. The results of this study will allow land managers to better assess the effects of snow-compacting activities on coyotes and lynx.     

Acanthocephalan (Oligacanthorhynchidae) parasitism of the Caribbean termite <Emphasis Type="Italic">Nasutitermes acajutlae</Emphasis>: implications for reproductive success

Summary Few studies have examined the impact of parasitism on free-living social insects. We documented the association between an acanthocephalan parasite and survival, reproduction and growth of a Caribbean termite (Nasutitermes acajutlae). We measured these parameters yearly in 100 termite colonies for 4 years. In 2001 and 2002 we also compared the rate at which parasitized and unparasitized colonies rebuilt damaged foraging trails because trails protect foragers from predation as well as fluctuations in temperature and humidity. Although there were no significant differences in growth or survival between parasitized and unparasitized colonies, parasitized colonies reproduced significantly less often and rebuilt foraging trails significantly more slowly than unparasitized colonies. The average parasitized colony may sustain a loss of alate production =30% in the year in which it is parasitized, and a loss of up to 19% if it is parasitized once in a 5-year period. We conclude that the acanthocephalan parasite is associated with significant alteration of fitness parameters of individual N. acajutlae colonies. However, prevalence of parasitism is low (6.8%) and alate production by non-parasitized colonies is likely to compensate for losses due to parasitism. Therefore, it seems unlikely that this parasite affects N. acajutlae at the population level.Received 5 October 2003; revised 19 November 2003; accepted 3 December 2003.     

Trail‐sharing among tropical ants: interspecific use of trail pheromones?

1. Trail‐sharing between different ant species is rare and restricted to a small number of species pairs. Its underlying mechanisms are largely unknown. For trail‐sharing to occur, two factors are required: (i) one or both species must recognise the other species or its pheromone trails and (ii) both species must tolerate each other to a certain extent to allow joint use of the trail. A species that follows another's trails can efficiently exploit the other's information on food sources contained in the pheromone trails. Hence, food competition and thus aggressive interactions between a species following another's trail and the species being followed, seem likely. 2. In the present study, we investigated interspecific trail following and interspecific aggression in trail sharing associations (i) among Polyrhachis ypsilon, Camponotus saundersi, and Dolichoderus cuspidatus, and (ii) among Camponotus rufifemur and Crematogaster modiglianii. We tested whether trail‐sharing species follow each other's pheromone trails, and whether the ants tolerated or attacked their trail‐sharing partners. In both associations, we confronted workers with pheromone trails of their associated species, and, for the former association, measured interspecific aggression among the trail‐sharing species. 3. In our assays, D. cuspidatus and C. rufifemur regularly followed heterospecific pheromone trails of P. ypsilon and C. modiglianii, respectively. However, only few workers of the remaining species followed heterospecific pheromone trails. Thus, shared trails of P. ypsilon and C. saundersi cannot be explained by interspecific trail‐following. 4. Interspecific aggression among P. ypsilon, C. saundersi, and D. cuspidatus was strongly asymmetric, C. saundersi being submissive to the other two. All three species differentiated between heterospecific workers from the same or another site, suggesting habituation to the respective trail‐sharing partners. We therefore hypothesise that differential tolerance by dominant ant species may be mediated by selective habituation towards submissive species and this way determines the assembly of trail‐sharing associations.     

How Specific are Host-Produced Kairomones to Host-Seeking Ixodid Ticks?

Ixodid ticks respond to host-produced substances (kairomones) that influence the ticks’ host-finding behavior. In the laboratory adult blacklegged ticks, Ixodes scapularis Say, lone star ticks, Amblyomma americanum L., and American dog ticks, Dermacentor variabilis (Say) became akinetic on residues rubbed from their principal hosts (deer for the former two species and dogs for the latter). However, arrestment also occurred when adults of these species were tested using the same method bioassay, but with host substances reversed (i.e., I. scapularis and A. americanum against canine substances, and D. variabilis against deer gland substances). Although adult D. variabilis exhibited arrestant responses to deer substances and are often found along trails used by deer, they apparently make little use of deer as hosts. It is unclear whether responding to deer-produced kairomones may have disadvantages for D. variabilis. Until the active components of host-produced arrestment kairomones are isolated, identified and evaluated in behavioral tests, this host-finding strategy remains only partially understood. This revised version was published online in July 2006 with corrections to the Cover Date.     

Social Cues and Following Behavior in the Forest Tent Caterpillar

Colonies of the social caterpillar Malacosoma disstria Hubner (Lepidoptera: Lasiocampidae) travel in groups following silk trails marked with pher-omone. This study examined first, the cues involved in following behavior and second, the responses to these cues at different larval stadia. Both second and fourth instar larvae discriminated between fresh and older trails, and travelled faster in the presence of trails. In addition to trail following, young caterpillars exhibited leader following, which might be particularly important in exploring unmarked territory. Indeed, second instar caterpillars were more likely to travel together when trails were absent. Fourth instar larvae exhibited greater independent locomotion in the absence of trails than did younger larvae. These findings help explain patterns of social behavior observed in forest tent caterpillar colonies in the field.     

内蒙古典型草原区狭颅田鼠集群数量与被捕食风险的关系

2005年9月15日—30日,在内蒙古锡林郭勒盟西乌珠穆沁旗的典型草原上研究了狭颅田鼠(Microtus gregalis)冬季集群与来自艾虎(Mustela eversmanni)的捕食风险。采用鼠洞口数量作为狭颅田鼠集群大小的指标,分析了艾虎对不同大小狭颅田鼠集群的捕食优先选择偏好。对实验样地内狭颅田鼠的洞群洞口数进行了调查统计(样地内总共涉及102个狭颅田鼠有效洞群),并计数此期间的艾虎粪便堆数以及攻击掘开狭颅田鼠洞群的情况。运用非参数的Mann-Whitney U检验法进行数据分析,结果表明:从艾虎遗留的粪便痕迹来看,没有被艾虎访问过的田鼠洞群与被艾虎访问过的洞群之间差异达到极显著水平;另外,从狭颅田鼠洞口被艾虎掘开的痕迹来分析,没有被艾虎挖掘的洞群与被艾虎挖掘的洞群之间的差异也达到极显著的水平;另外,随着田鼠洞群洞口数量的增加,出现艾虎粪便和掘洞的频次和概率就越高。表明艾虎对狭颅田鼠集群洞口数的选择差异性非常显著,明显倾向于选择在洞口数量多的狭颅田鼠洞群停留,同时也更倾向于掘开洞口数量较高的狭颅田鼠洞群作为捕食对象。     

Rediscovery of the workerless inquiline antPogonomyrmex colei and additional notes on natural history (Hymenoptera: Formicidae)

Summary Pogonomyrmex colei is a workerless inquiline ant known only from nests ofP. rugosus, its closest relative. Ten of 776 (1.3%) host nests were parasitized at a site in central Arizona, while none of 1499 potential host colonies were parasitized at two other locales. Colonies ofP. colei are perennial, and host alate females in 9 of 10 colonies demonstrates that host queens survive parasitism. Three of 10 colonies died over 19 colony years of observation, while only 1 of 601 colonies became newly parasitized. Mating occurs in morning for up to 2–3 days following summer and fall rains and in afternoon during cool fall days. Mating is intranidal just outside the nest entrance, with males returning to the natal nest. MaleP. colei may be flightless because their wing area is reduced compared to host males. Females fly from the nest and locate potential host colonies by following trunk trails. Workers are the largest barrier to nest establishment, as they removed over 90% ofP. colei females placed in trunk trails or that entered host nests. Males and females ofP. colei andP. anergismus, the only other congeneric inquiline species, are diminutive compared to their hosts, with females 30% lighter than host workers. Fat content is lower and water content is higher inP. colei andP. anergismus females than in their hosts.     

Hunting for moving hosts: Cephalonomia tarsalis, a parasitoid of free-living grain beetles

The parasitic wasp Cephalonomia tarsalis parasitizes larvae of the saw-toothed grain beetle Oryzaephilus surinamensis, which feed on wheat grains. In contrast to most other host–parasitoid systems studied so far, the grain beetles are highly mobile within their habitat, bulk of grains in grain stores. This should increase the wasps’ problem to locate the hosts. To study the host-finding strategy of C. tarsalis females, the reaction of wasps to different grain and host-derived odour sources was tested in a four-chamber-olfactometer. These experiments revealed that wasps were attracted by healthy grains and mechanically damaged grains. In direct comparison, healthy and mechanically damaged grains are equally attractive. Both potential sources of host-derived odours, host faeces and trail-traces of larvae on filter paper were attractive to the wasps. The response to trail-traces vanished 30 min after larvae had been removed from the filter paper. With respect to the specificity of the odours, it turned out that wasps were attracted to odours from the seed–host complexes from O. surinamensis and Oryzaephilus mercator and the non-host complex of larvae of the granary weevil Sitophilus granarius in wheat grains. Odours from the seed–host complex were preferred. From these results, we hypothesize that host habitat location in C. tarsalis is achieved by using grain-derived odours. Within the habitat, wasps search for kairomones from host faeces and host trails. Following these larval trails finally leads wasps to their hosts.     

Food and reproductive strategies of the myrmecophilous staphylinid beetleHomoeusa acuminata (aleocharinae) on the foraging trails of its host antLasius fuliginosus (Hymenoptera: Formicidae)

Summary The trail following behaviour of the adult myrmecophilous beetleHomoeusa acuminata on the foraging trails on the host antLasius fuliginosus was examined in June and all the observed behaviours were quantified. The beetle appears as a food robber, using the stowaway behaviour which consists of attaching itself to prey transported by ants to the nest. Foraging trails are also used by the beetles as a meeting place for mating. However, the beetle does not enter into the nest and appears as a symbiont poorly integrated in the social life of its host.     

The trail following behaviour of Yponomeuta cagnagellus

In this study the trail following behaviour of the caterpillar Yponomeuta cagnagellus (Hübner) (Lepidoptera, Yponomeutidae) is investigated. It is demonstrated that these caterpillars follow trails made by conspecifics. Ablation experiments show that at least part of this behaviour is directed by the tactile senses, but additional chemical cues cannot be excluded. In choice experiments using trails from different species, Y. cagnagellus strongly preferred conspecific trails over those from Malacosoma neustria, but did not prefer conspecific over other Yponomeuta trails. This lack of species-specificity within Yponomeuta is discussed and it is concluded that trail following is probably of little help in the elucidation of the evolutionary history of the genus.

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La façon dont Yponomeuta cagnagellus suit les pistes

Résumé Cette étude traite de la façon dont les chenilles de Y. cagnagellus Hübner (Lépido. Yponomeutidae) suivent des pistes. Elles suivent les pistes de leurs congénères. Par ablation, on peut montrer qu'une partie au moins de ce comportement est due à des perceptions tactiles, mais que l'intervention d'informations chimiques complémentaires ne peut être exclue. En présence de choix entre traces de différentes espèces, elles préfèrent nettement les traces des congénères à celles de Malacosoma neustria, mais ne choisissent pas entre les différentes espèces d'yponomeutes. L'absence de spécificité parmi le genre Yponomeuta montre que la sélection des pistes est probablement de peu d'utilité pour l'étude de l'histoire évolutive du genre.

     

Host-ant trail following by myrmecophilous larvae of Liphyrinae (Lepidoptera,Lycaenidae)

In this study we report a case of ant-trail following by lycaenid caterpillars. Euliphyra mirifica and E. leucyana caterpillars are involved in a commensal association with the weaver ant Oecophylla longinoda. The host nests are made with leaves which over the course of time dry out or are broken open by storms, forcing the ants to migrate and build a new nest elsewhere. Euliphyra caterpillars are stimulated by recruitment behaviour which triggers the migration of their host. They then follow the host trails leading to the new nesting site. Laboratory experiments showed that these caterpillars are able to follow host trails under varied conditions: (1) fresh trails actually used by workers, (2) fresh trails in the absence of workers, (3) heterocolonial, 2-month-old trails, and (4) fresh trails washed with water (to simulate the effect of tropical rains). They can also bridge trail gaps of more than 1 cm. Under natural conditions, the trails are frequently situated along thin twigs. The forward progress of the ants in such a situation is not impeded by the presence of large Euliphyra larvae. Workers just climb over the caterpillars, even on larger trails where there is enough room to pass alongside them. This suggests that an allomone is secreted on the dorsal part of the caterpillars. When crawling along heterocolonial trails, the caterpillars are not attacked, even if about 21% of the workers from the new colony spread their mandibles when encountering them. They are then adopted and are admitted to the nest of the new host colony of O. longinoda.     

The Role of Tactile and Chemical Stimuli in the Formation and Maintenance of the Processions of the Social Caterpillar Hylesia lineata (Lepidoptera: Saturniidae)

Colonies of the social caterpillar Hylesia lineata (Lepidoptera: Satumiidae) form long, single-file, head-to-tail processions as they move between their shelters and distant feeding sites. Although investigations of other processionary species have implicated a silk trail in the processionary process, silk plays little or no role in initiating or maintaining processions in H. lineata. Studies we report here implicate both tactile stimuli and a trail pheromone in the establishment and maintenance of processions. Processionaries elicit locomotion in the individual preceding them in line by brushing their heads against prominent sulci that project from the tips of their abdomens. Caterpillars mark their pathways with a pheromone deposited by brushing the ventral surfaces of their last abdominal segments against the substrate. The persistent pheromone is soluble in hexanes and appears to be secreted from glandular setae found on the proximal regions of the anal prolegs and the venter. In Y-choice tests, caterpillars selected newer trails over older trails and stronger trails over weaker trails. They did not distinguish between trials deposited by newly fed caterpillars and those deposited by starved caterpillars. Despite the unidirectional nature of processions, there is no indication that caterpillars can determine from the trail alone the direction in which the procession advanced. The significance of these findings to the foraging ecology of the caterpillars is discussed.     

Forager size and ecology of Acromyrmex coronatus and other leaf-cutting ants in Costa Rica

I compare forager size and foraging ecology of the leaf-cutting ant Acromyrmex coronatus (Fabricius) with published data on three other leaf-cutter species in Costa Rica, Atta cephalotes (L.), Acromyrmex octospinosus (Reich), and Acromyrmex volcanus Wheeler. Intra-and interspecific differences in forager size in these leaf-cutting ants appear to reflect the economics of harvesting different preferred resources. Ac. coronatus colonies have relatively small foragers (mean mass=3.4±1.4 mg) that cut almost exclusively the thin, soft leaves and other parts of small herbaceous plants. Similarly, small A. cephalotes colonies have small foragers (3.3±1.0 mg) that attack the leaves of small herbaceous plants. In contrast, mature A. cephalotes colonies have a wider sizerange of foragers (7.3±4.1 mg) that primarily attack the leaves of trees, with larger foragers cutting thicker, tougher leaves. In A. cephalotes, the match of forager size to leaf type (both ontogenetically and behaviorally) increases foraging efficiency. Extreme forager polymorphism in mature A. cephalotes colonies appears to broaden the diversity of tree species that they can exploit efficiently. Ac. octospinosus and Ac. volcanus both have large, relatively monomorphic foragers (13.3±4.2 mg and 30.6±4.3 mg, respectively) that typically scavenge for pieces of fallen vegetation, such as dead leaves, fruit, and flowers, in addition to cutting herbs. The large foragers of Ac. octospinosus and Ac. volcanus appear to be well suited as generalist foragers, able to cut or collect any desirable vegetation encountered. Ac. coronatus is similar to A. cephalotes in other ways. Both Ac. coronatus and A. cephalotes establish and maintain cleared trunk trails for foraging, and both have minima workers that hitchhike on the loads carried by foragers, apparently serving to protect the larger foragers from attack by phorid flies. Trunk trails and hitchhikers are not known for Ac. octospinosus and Ac. volcanus. That A. coronatus and A. cephalotes show little overlap in geographic distribution within Costa Rica may relate both to differences in habitat requirements and to interspecific competition.