“You are what you eat”: Diet modifies cuticular hydrocarbons and nestmate recognition in the Argentine ant, Linepithema humile

 Nestmate recognition plays a key role in the behavior and evolution of social insects. We demonstrated that hydrocarbons are the chemical cues used in Argentine ant, Linepithema humile, nestmate recognition, and that these hydrocarbons can be acquired from insect prey. Consequently, Argentine ant cuticular hydrocarbon patterns reveal the same hydrocarbons present in their diet. Diet alters both the recognition cues present on the cuticular surface and the response of nestmates to this new colony odor, resulting in aggression between former nestmates reared on different insect prey. Received: 16 June 2000 / Accepted: 14 August 2000     

Postmating changes in cuticular chemistry and visual appearance in <Emphasis Type="Italic">Ectatomma tuberculatum</Emphasis> queens (Formicidae: Ectatomminae)

In the ectatommine ant Ectatomma tuberculatum, the visual appearance of queens changes after mating and ovarian development in that their cuticle turns from shiny to matte. In this study, we have shown that this change seems to be caused by 15-fold accumulation of hydrocarbons, in particular heptacosane that covers the multiple grooves present on the cuticular surface creating a wax coat in mated fully fertile queens. Analyses of the scrapped wax revealed that it is composed largely of heptacosane. Peak-by-peak comparison of the cuticular hydrocarbon (CHC) composition of mated, virgin with developed ovaries and virgin with nondeveloped ovaries revealed significant differences between the queen groups. Although the total amount of the CHC of virgin queens with developed ovaries was not higher than virgin queens that did not have developed ovaries, the composition showed a shift toward the mated queen. While it is possible that the large accumulation of hydrocarbons may give extra physical and chemical protection to queens, we propose that the switch in the relative abundance of heptacosane and nonacosane and perhaps of other components is indicative of being a mating and fertility cue. This is the first report in social insects where external chemical changes are accompanied by changes in visual appearance.     

The ladybird<Emphasis Type="Italic"> Thalassa saginata</Emphasis>, an obligatory myrmecophile of<Emphasis Type="Italic"> Dolichoderus bidens</Emphasis> ant colonies

The larvae and pupae of the ladybird Thalassa saginata develop inside colonies of the dolichoderine ant Dolichoderus bidens. This association is the first specific and obligatory relationship recorded between ants and ladybirds. The ants provide shelter and protection to the larvae but the diet of the latter remains unclear. The integration of T. saginata larvae into the ant colonies is achieved by mimicking the cuticular patterns of the ants brood. Moreover, the larvae secrete substances from their hairs and anal gland that are likely to enhance their attractiveness.     

Reclaiming the crown: queen to worker conflict over reproduction in <Emphasis Type="Italic">Aphaenogaster cockerelli</Emphasis>

In many social taxa, reproductively dominant individuals sometimes use aggression to secure and maintain reproductive status. In the social insects, queen aggression towards subordinate individuals or workers has been documented and is predicted to occur only in species with a small colony size and a low level of queen–worker dimorphism. We report queen aggression towards reproductive workers in the ant species Aphaenogaster cockerelli, a species with a relatively large colony size and a high level of reproductive dimorphism. Through analysis of cuticular hydrocarbon profiles, we show that queens are aggressive only to reproductively active workers. Non-reproductive workers treated with a hydrocarbon typical for reproductives are attacked by workers but not by queens, which suggests different ways of recognition. We provide possible explanations of why queen aggression is observed in this species.     

Surface lipids of queen-laid eggs do not regulate queen production in a fission-performing ant

In animal societies, most collective and individual decision making depends on the presence of reproductive individuals. The efficient transmission of information among reproductive and non-reproductive individuals is therefore a determinant of colony organization. In social insects, the presence of a queen modulates multiple colonial activities. In many species, it negatively affects worker reproduction and the development of diploid larvae into future queens. The queen mostly signals her presence through pheromone emission, but the means by which these chemicals are distributed in the colony are still unclear. In several ant species, queen-laid eggs are the vehicle of the queen signal. The aim of this study was to investigate whether queen-laid eggs of the ant Aphaenogaster senilis possess queen-specific cuticular hydrocarbons and/or Dufour or poison gland compounds, and whether the presence of eggs inhibited larval development into queens. Our results show that the queen- and worker-laid eggs shared cuticular and Dufour hydrocarbons with the adults; however, their poison gland compounds were not similar. Queen-laid eggs had more dimethylalkanes and possessed a queen-specific mixture of cuticular hydrocarbons composed of 3,11?+?3,9?+?3,7-dimethylnonacosane, in higher proportions than did worker-laid eggs. Even though the queen-laid eggs were biochemically similar to the queen, their addition to experimentally queenless groups did not prevent the development of new queens. More studies are needed on the means by which queen ant pheromones are transmitted in the colony, and how these mechanisms correlates with life history traits.     

Selective olfactory attention of a specialised predator to intraspecific chemical signals of its prey

Prey-specialised predators have evolved specific cognitive adaptations that increase their prey searching efficiency. In particular, when the prey is social, selection probably favours the use of prey intraspecific chemical signals by predatory arthropods. Using a specialised ant-eating zodariid spider, Zodarion rubidum, which is known to prey on several ant species and possesses capture and venom adaptations more effective on Formicinae ants, we tested its ability to recognise chemical cues produced by several ant species. Using an olfactometer, we tested the response of Z. rubidum towards air with chemical cues from six different ant species: Camponotus ligniperda, Lasius platythorax and Formica rufibarbis (all Formicinae); and Messor structor, Myrmica scabrinodis and Tetramorium caespitum (all Myrmicinae). Z. rubidum was attracted to air carrying chemical cues only from F. rufibarbis and L. platythorax. Then, we identified that the spiders were attracted to airborne cues coming from the F. rufibarbis gaster and Dufour's gland, in particular. Finally, we found that among several synthetic blends, the decyl acetate and undecane mixture produced significant attraction of spiders. These chemicals are produced only by three Formicine genera. Furthermore, we investigated the role of these chemical cues in the communication of F. rufibarbis and found that this blend reduces their movement. This study demonstrates the chemical cognitive capacity of Z. rubidum to locate its ant prey using chemical signals produced by the ants. The innate capacity of Z. rubidum to olfactory detect different ant species is narrow, as it includes only two ant genera, confirming trophic specialisation at lower than subfamily level. The olfactory cue detected by Zodarion spiders is probably a component of the recruitment or trail pheromone.     

In-nest environment modulates nestmate recognition in the ant<Emphasis Type="Italic"> Camponotus fellah</Emphasis>

Multiple behavioral and chemical studies indicate that ant nestmate recognition cues are low-volatile substances, in particular hydrocarbons (HCs) located on the cuticular surface. We tested the hypothesis that in the ant Camponotus fellah, nest environment, in particular nest volatile odors, can modulate nestmate-recognition-mediated aggression. Workers were individually confined within their own nest in small cages having either a single mesh (SM = limited physical contact permitted) or a double mesh (DM = exposed to nest volatiles only) screen. Individual workers completely isolated outside their nest (CI) served as control. When reintroduced into a group of 50 nestmates, the CI workers were attacked as alien ants after only 2 weeks of separation, whereas the SM workers were treated as nestmates even after 2 months of separation. Aggression towards DM ants depended on the period of isolation. Only DM workers isolated for over 2 months were aggressed by their nestmates, which did not significantly differ from the CI nestmates. Cuticular HC analyses revealed that the profile of the non-isolated ants (NI) was clearly distinct from that of CI, SM and DM ants. Profile differences matched the aggressive response in the case of CI ants but were uncorrelated in the case of SM or DM ants. This suggests that keeping the ants within the nest environment affected nestmate recognition in additional ways than merely altering their HC profile. Nest environment thus appears to affect label–template mismatch by modulating aggressive behavior, as well as the direction at which cuticular HCs diverged during the separation period.     

Infanticide by a solitary koinobiont ichneumonid ectoparasitoid of spiders

When encountering an already parasitized host, a parasitoid’s optimal choices (superparasitism, host rejection, host feeding or infanticide) seem to depend on the individual species’ life history, because the same choice may have different fitness consequences. We demonstrate infanticide under laboratory conditions by a polysphinctine, Zatypota albicoxa, which is a solitary koinobiont ectoparasitoid of spiders. The female always removed any previously attached egg or larva from the body of the host spider, Parasteatoda tepidariorum, with a rubbing behaviour. She rubbed her ovipositor back and forth toward the undersurface of the attached egg or of the saddle under the attached larva to pry it off and laid an egg after removal. When removing a larva, the infanticidal female engaged exclusively in unfastening the ‘saddle’ which fastens the larva to the body of the spider. All larvae were removed with the ‘saddle’ attached to the ventral surface of the body. The female invested more time to remove the medium second and the large penultimate instar larvae than to remove eggs and first instar larvae because of the labour involved in unfastening the saddle. Oviposition with infanticide of the medium second and the penultimate instar larvae imposed more time upon the female than that on an unparasitized host. Removal of any previous occupant in spite of the associated labour costs suggests that infanticide will always be adaptive, no matter the time costs to Z. albicoxa, because so much is invested in attacking the host and because the parasitoid cannot detect whether the spider is already parasitized until she achieves subjugation.     

Social Hackers: Integration in the Host Chemical Recognition System by a Paper Wasp Social Parasite

 Obligate social parasites in the social insects have lost the worker caste and the ability to establish nests. As a result, parasites must usurp a host nest, overcome the host recognition system, and depend on the host workers to rear their offspring. We analysed cuticular hydrocarbon profiles of live parasite females of the paper wasp social parasite Polistes sulcifer before and after usurpation of host nests, using the non-destructive technique of solid-phase micro-extraction. Our results reveal that hydrocarbon profiles of parasites change after usurpation of host nests to match the cuticular profile of the host species. Chemical evidence further shows that the parasite queen changes the odour of the nest by the addition of a parasite-specific hydrocarbon. We discuss the possible role of this in the recognition and acceptance of the parasite and its offspring in the host colony. Received: 18 November 1999 / Accepted in revised form: 22 December 1999     

Decision rules for egg recognition are related to functional roles and chemical cues in the queenless ant Dinoponera quadriceps

The capacity to distinguish colony members from strangers is a key component in social life. In social insects, this extends to the brood and involves discrimination of queen eggs. Chemical substances communicate colony affiliation for both adults and brood; thus, in theory, all colony members should be able to recognize fellow nestmates. In this study, we investigate the ability of Dinoponera quadriceps workers to discriminate nestmate and non-nestmate eggs based on cuticular hydrocarbon composition. We analyzed whether cuticular hydrocarbons present on the eggs provide cues of discrimination. The results show that egg recognition in D. quadriceps is related to both age and the functional role of workers. Brood care workers were able to distinguish nestmate from non-nestmate eggs, while callow and forager workers were unable to do so. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Scent of a queen—cuticular hydrocarbons specific for female reproductives in lower termites

In social insects, it is assumed that signals of the queen inform nestmates about her reproductive status. Thus, workers forego their own reproduction if the queen signals high fertility. In hemimetabolous termites, little is known about reproductive inhibition, but evidence exists for a royal-pair control. Workers of lower termites exhibit a high developmental flexibility and are potentially able to become reproductives, but the presence of a fertile reproductive restrains them from reaching sexual maturity. The nature of this control, however, remains unknown. Here, we report on qualitative differences in cuticular hydrocarbon profiles between queens and workers of the basal drywood termite Cryptotermes secundus. Queens were characterized by a shift to long-chained and branched hydrocarbons. Most remarkably, similar chemical patterns are regarded as fertility cues of reproductives in social Hymenoptera. This might suggest that both groups of social insects convergently evolved similar chemical signatures. The present study provides deeper insights into how termites might have socially exploited these signatures from sexual communication in their cockroach-like ancestor. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Nestmate recognition in ants is possible without tactile interaction

Ants of the genus Camponotus are able to discriminate recognition cues of colony members (nestmates) from recognition cues of workers of a different colony (non-nestmates) from a distance of 1 cm. Free moving, individual Camponotus floridanus workers encountered differently treated dummies on a T-bar and their behavior was recorded. Aggressive behavior was scored as mandibular threat towards dummies. Dummies were treated with hexane extracts of postpharyngeal glands (PPGs) from nestmates or non-nestmates which contain long-chain hydrocarbons in ratios comparable to what is found on the cuticle. The cuticular hydrocarbon profile bears cues which are essential for nestmate recognition. Although workers were prevented from antennating the dummies, they showed significantly less aggressive behavior towards dummies treated with nestmate PPG extracts than towards dummies treated with non-nestmate PPG extracts. In an additional experiment, we show that cis-9-tricosene, an alkene naturally not found in C. floridanus' cuticular profile, is behaviorally active and can interfere with nestmate recognition when presented together with a nestmate PPG extract. Our study demonstrates for the first time that the complex multi-component recognition cues can be perceived and discriminated by ants at close range. We conclude that contact chemosensilla are not crucial for nestmate recognition since tactile interaction is not necessary.     

Imperfect chemical female mimicry in males of the ant Cardiocondyla obscurior

Winged and wingless males coexist in the ant Cardiocondyla obscurior. Wingless (“ergatoid”) males never leave their maternal colony and fight remorselessly among each other for the access to emerging females. The peaceful winged males disperse after about 10 days, but beforehand also mate in the nest. In the first 5 days of their life, winged males perform a chemical female mimicry that protects them against attack and even makes them sexually attractive to ergatoid males. When older, the chemical profile of winged males no longer matches that of virgin females; nevertheless, they are still tolerated, which so far has been puzzling. Contrasting this general pattern, we have identified a single aberrant colony in which all winged males were attacked and killed by the ergatoid males. A comparative analysis of the morphology and chemical profile of these untypical attacked winged males and the tolerated males from several normal colonies revealed that normal old males are still performing some chemical mimicry to the virgin queens, though less perfect than in their young ages. The anomalous attacked winged males, on the other hand, had a very different odour to the females. Our study thus exemplifies that the analysis of rare malfunctioning can add valuable insight on functioning under normal conditions and allows the conclusion that older winged males from normal colonies of the ant C. obscurior are guarded through an imperfect chemical female mimicry, still close enough to protect against attacks by the wingless fighters yet dissimilar enough not to elicit their sexual interest.     

Nest- and colony-mate recognition in polydomous colonies of meat ants (Iridomyrmex purpureus)

Workers of polydomous colonies of social insects must recognize not only colony-mates residing in the same nest but also those living in other nests. We investigated the impact of a decentralized colony structure on colony- and nestmate recognition in the polydomous Australian meat ant (Iridomyrmex purpureus). Field experiments showed that ants of colonies with many nests were less aggressive toward alien conspecifics than those of colonies with few nests. In addition, while meat ants were almost never aggressive toward nestmates, they were frequently aggressive when confronted with an individual from a different nest within the same colony. Our chemical analysis of the cuticular hydrocarbons of workers using a novel comprehensive two-dimensional gas chromatography technique that increases the number of quantifiable compounds revealed both colony- and nest-specific patterns. Combined, these data indicate an incomplete transfer of colony odor between the nests of polydomous meat ant colonies.     

Colony disassociation following diet partitioning in a unicolonial ant

Discriminating nestmates from alien conspecifics via chemical cues is recognized as a critical element in maintaining the integrity of insect societies. We determined, in laboratory experiments, that nestmate recognition in an introduced population of the Argentine ant, Linepithema humile, is modified by hydrocarbons acquired from insect prey, and that workers from spatially isolated colony fragments, each provided with prey that possessed distinct cuticular hydrocarbons, displayed aggressive behavior towards their former nestmates. Isolation for 28 days or more between colony fragments fed different prey was sufficient to prevent re-establishment of inter-nest communication for at least an additional 28 days through the introduction of a bridge between the nests. Ants possessed intrinsic cuticular hydrocarbons plus only those hydrocarbons from the prey they received during the isolation period. Colony fragments which were isolated for less than 28 days reunited with workers possessing both prey hydrocarbons. Therefore, L. humile nestmate recognition may be dynamic, being in part dependent on the spatio-temporal distribution of prey, along with physical factors permitting or restricting access of subcolony units to those prey.     

The role of tending ants in host plant selection and egg parasitism of two facultative myrmecophilous butterflies

Ovipositing adult females of myrmecophilous lycaenids are expected to select plants based on ant presence in order to maximize the survivorship of immature stages. Usually, larvae feed ants with honey-like solutions and, in turn, ants ward off parasitoids. Nonetheless, a rarely investigated approach is whether ant partners can also extend their protective behavior towards lycaenids eggs. Here, we investigated the ant-related oviposition pattern of Allosmaitia strophius and Rekoa marius; then, we compared egg parasitism according to the presence of ants. Lycaenid oviposition and egg parasitism (in percent) were experimentally compared in ant-present and ant-excluded treatments. The study plant, Heteropterys byrsonimifolia, is an extrafloral nectaried shrub which supports several ant species. We sampled 280 eggs, of which 39.65 % belonged to A. strophius and 60.35 % to R. marius. Both lycaenids eggs were significantly more abundant on branches with ants, especially those with Camponotus crassus and Camponotus blandus, two ant species known to attend to lycaenids. A. strophius and R. marius parasitism was 4.5- and 2.4-fold higher, respectively, in ant-present treatments, but the results were not statistically significant. Our study shows that ant-mediated host plant selection in lycaenids might be much more widespread than previously thought, and not restricted to obligate myrmecophilous species. Tending ants may be inefficient bodyguards of lycaenid eggs, because unlike larvae which release sugared liquids, eggs do not offer obvious rewards to ants. Ants can ward off parasitoids of larvae, as observed elsewhere, but our findings show that positive ant–lycaenid interactions are conditional and depend on immature ontogeny.     

Ant-lepidopteran associations along African forest edges

Working along forest edges, we aimed to determine how some caterpillars can co-exist with territorially dominant arboreal ants (TDAAs) in tropical Africa. We recorded caterpillars from 22 lepidopteran species living in the presence of five TDAA species. Among the defoliator and/or nectarivorous caterpillars that live on tree foliage, the Pyralidae and Nymphalidae use their silk to protect themselves from ant attacks. The Notodontidae and lycaenid Polyommatinae and Theclinae live in direct contact with ants; the Theclinae even reward ants with abundant secretions from their Newcomer gland. Lichen feeders (lycaenid; Poritiinae), protected by long bristles, also live among ants. Some lycaenid Miletinae caterpillars feed on ant-attended membracids, including in the shelters where the ants attend them; Lachnocnema caterpillars use their forelegs to obtain trophallaxis from their host ants. Caterpillars from other species live inside weaver ant nests. Those of the genus Euliphyra (Miletinae) feed on ant prey and brood and can obtain trophallaxis, while those from an Eberidae species only prey on host ant eggs. Eublemma albifascia (Erebidae) caterpillars use their thoracic legs to obtain trophallaxis and trophic eggs from ants. Through transfer bioassays of last instars, we noted that herbivorous caterpillars living in contact with ants were always accepted by alien conspecific ants; this is likely due to an intrinsic appeasing odor. Yet, caterpillars living in ant shelters or ant nests probably acquire cues from their host colonies because they were considered aliens and killed. We conclude that co-evolution with ants occurred similarly in the Heterocera and Rhopalocera.     

Chemically mediated group formation in soil-dwelling larvae and pupae of the beetle Trypoxylus dichotomus

Many insects form groups through interactions among individuals, and these are often mediated by chemical, acoustic, or visual cues and signals. In spite of the diversity of soil-dwelling insects, their aggregation behaviour has not been examined as extensively as that of aboveground species. We investigated the aggregation mechanisms of larvae of the Japanese rhinoceros beetle Trypoxylus dichotomus, which live in groups in humus soil. In two-choice laboratory tests, 2nd- and 3rd-instar larvae gathered at conspecific larvae irrespective of the kinship. The ablation of maxillae, which bear chemosensilla, abolished aggregation behaviour. Intact larvae also exhibited aggregation behaviour towards a larval homogenate. These results suggest that larval aggregation is mediated by chemical cues. We also demonstrated that the mature larvae of T. dichotomus built their pupal cells close to a mesh bag containing a conspecific pupal cell, which indicated that larvae utilize chemical cues emanating from these cells to select the pupation site. Thus, the larvae of T. dichotomus may use chemical cues from the conspecifics in two different contexts, i.e. larval aggregation and pupation site selection. Using conspecific cues, larvae may be able to choose suitable locations for foraging or building pupal cells. The results of the present study highlight the importance of chemical information in belowground ecology.     

Self-Injection of a Dipteran Parasitoid into a Spider

 A host invasion strategy hitherto unknown from other insect parasitoids was observed in the dipteran Acrocera orbicula (Fabricius) (Diptera: Acroceridae) parasitizing the wolf spider, Pardosa prativaga (L. Koch) (Araneida: Lycosidae). In laboratory experiments the free-living first instar acrocerid larvae attached themselves firmly to the spiders' integument by the mouthparts, cutting a tiny hole through the integument. No first instar larvae invaded the host. A week later the parasitoids molted, and a small, flexible, and glabrous second instar larva left each of the attached first instar exuviae and invaded the host through the attachment hole of the first instar larva. The novel host invasion pattern observed may reduce physical damage to the host in the initial phase of endoparasitism, enhancing parasitoid survival. Received: 14 April 1999 / Accepted in revised form: 2 July 1999     

Two pathways ensuring social harmony

Reproductive division of labour is a characteristic trait of social insects. The dominant reproductive individual, often the queen, uses chemical communication and/or behaviour to maintain her social status. Queens of many social insects communicate their fertility status via cuticle-bound substances. As these substances usually possess a low volatility, their range in queen–worker communication is potentially limited. Here, we investigate the range and impact of behavioural and chemical queen signals on workers of the ant Temnothorax longispinosus. We compared the behaviour and ovary development of workers subjected to three different treatments: workers with direct chemical and physical contact to the queen, those solely under the influence of volatile queen substances and those entirely separated from the queen. In addition to short-ranged queen signals preventing ovary development in workers, we discovered a novel secondary pathway influencing worker behaviour. Workers with no physical contact to the queen, but exposed to volatile substances, started to develop their ovaries, but did not change their behaviour compared to workers in direct contact to the queen. In contrast, workers in queen-separated groups showed both increased ovary development and aggressive dominance interactions. We conclude that T. longispinosus queens influence worker ovary development and behaviour via two independent signals, both ensuring social harmony within the colony.