Cephalic Labial Gland Secretions Research Articles

Resolving the species status of overlooked West‐Palaearctic bumblebees

AbstractMultisource approaches in taxonomy gather different lines of evidence in order to draw strongly supported taxonomic conclusions and constitute the basis of integrative taxonomy. In the case of overlooked taxa with disjunct distributions for which sampling is more challenging, integrative approaches help to propose stable hypotheses at the species and subspecies levels. Here, based on genetic and semio‐chemical traits, we performed an integrative taxonomic analysis to evaluate species delimitation hypotheses within a monophyletic group of bumblebees (Hymenoptera, Apidae, Bombus) including the formerly recognised subgenera Eversmannibombus, Laesobombus and Mucidobombus which are now included in the subgenus Thoracobombus. Our results demonstrate the conspecificity of several polytypic taxa, and we formally recognise the subspecies Bombus laesus aliceae comb. nov. Cockerell, 1931, endemic to North Africa, based on its allopatry, unique mitochondrial haplotype and divergent cephalic labial gland secretions. This highlights the need to maintain studying polytypic complexes of bumblebee taxa for which phylogenetic relationships could be still entangled and eventually implement conservation strategies for taxonomically differentiated lineages.

The arctic and alpine bumblebees of the subgenus Alpinobombus revised from integrative assessment of species’ gene coalescents and morphology (Hymenoptera, Apidae, Bombus)

The bumblebees of the subgenus Alpinobombus of the genus Bombus are unusual among bees for specialising in many of the most northerly vegetated arctic habitats on Earth. Most named taxa in this group (37 available names from a total of 67 names) were described originally from differences in the colour patterns of the hair. Previous revisions have shown unusually little agreement, recognising a range of 6‒9 species, in part because of pronounced intraspecific variation in both skeletal morphology and in the colour patterns of the hair. Here we examine variation among 4622 specimens from throughout the group's global range. Bayesian inference of the gene tree for the fast evolving mitochondrial COI gene combined with Poisson-tree-process analysis of this tree shows support for 10 gene lineages as candidates for being putative species lineages. Integrative assessment shows that the interpretation of these results is not straightforward. Evidence from the fast evolving mitochondrial 16S ribosomal RNA gene supports two of the COI gene alleles (from the samples B. kluanensis s. str. and 'unnamed2') as being associated with just one 16S allele. Double COI bands on the PCR gels for these individuals and double peaks on sequence traces (in one case with both COI alleles sequenced from one individual) identifies this as a likely case of COI paralogy that has resulted in mitochondrial heteroplasmy. Evidence from morphology also supports only the remaining nine lineages as separate. Evidence from extracts of cephalic labial gland secretions (CLGS, with components believed to function as sex pheromones) reported by others shows small diagnostic differences between all of the candidate species examined (although B. kluanensis s. l. was not examined) and shows larger differences between all of the species pairs that we find are likely to have co-occurred at least in the past, revealing a likely limitation to the CLGS approach in cases of recent and continuously allopatric species. Consequently we infer nine species in the subgenus Alpinobombus (so that B. kluanensis s. str. and 'unnamed2' are interpreted as conspecific, as B. kluanensis s. l.). We provide distribution maps and identification keys for the nine species. The morphology of the male of B. kluanensis is described for the first time, including a unique, unusually dense pad of short hair on the mandible that may have a function involving CLGS in mate-searching behaviour. In seeking to identify the valid names for these species, seven new lectotypes are designated and support is provided for synonymizing 10 names as proposed in a recent summary table of names. The prevailing usage of Bombus balteatus Dahlbom is maintained as valid by proposing Bombus nivalis Dahlbom and Bombus tricolor Dahlbom as nomina oblita and by proposing Bombus balteatus Dahlbom as a nomen protectum. The prevailing usage of Bombus hyperboreus Schönherr is maintained as valid by supporting Apis arctica Quensel as a nomen oblitum and by supporting Bombus hyperboreus Schönherr as a nomen protectum. We then use sequence data from COI and 16S together with nuclear PEPCK and opsin genes to estimate dated phylogenetic relationships among the nine species, allowing for incongruent gene trees with *BEAST. If crown-group divergence within the subgenus Alpinobombus coincided with the global climate cooling and with the growth of the northern ice sheets at the end of the Miocene at ca 7.2 Ma, then divergences between each of the three pairs of sister species are likely to have coincided with fluctuations in vegetated land connections across the Bering Strait after ca 2.5 Ma.

Thoracobombus from Mexico: a description of the male species-specific cephalic labial gland secretions

The male cephalic labial gland secretions of bumblebees are known to be species-specific semiochemicals. These secretions that are involved in bumblebee pre-mating recognition provide efficient diagnostic characters for species delimitation. The subgenus Thoracobombus is the largest group of bumblebees and is found in the Palearctic, Nearctic, and Neotropical regions. Here, the cephalic labial gland secretion (CLGS) composition of six Mexican Thoracobombus bumblebee species are analyzed: Bombus diligens, B. medius, B. mexicanus, B. pensylvanicus, B. trinominatus, and B. weisi. Our results suggest the presence of two new potential species into the formerly recognized B. weisi as well as one new potential species in the taxon presently identified as B. pensylvanicus. Moreover, the male of B. pensylvanicus, known to congregate at nest sites awaiting the emergence of virgin queens, is characterized by low concentrations of the C16 component. This observation raises the possibility that courtship behavior as well as environmental constraints could affect the role of the male bumblebees’ CLGS.

Adding attractive semio-chemical trait refines the taxonomy of Alpinobombus (Hymenoptera: Apidae)

Species taxonomy of bumblebees (Bombus Latreille, 1802) is well known to be problematic due to a potentially high intra-specific variability of morphological traits while different species can converge locally to the same color pattern (cryptic species). Assessing species delimitation remains challenging because it requires to arbitrarily select variable traits whose accuracy continues to be debated. Integrative taxonomic approach seems to be very useful for this group as different independent traits are assessed to propose a rational taxonomic hypothesis. Among operational criteria to assess specific status, the reproductive traits involved in the pre-mating recognition (i.e., the male cephalic labial gland secretions, CLGS) have been premium information. Since these secretions are supposed to be species-specific, these chemical traits can bring essential information where species delimitation is debated. Here, we describe and compare the CLGS of 161 male specimens of nine Alpinobombus taxa: alpinus, balteatus, helleri, hyperboreus, kirbiellus, natvigi, neoboreus, polaris, and pyrrhopygus. We aim also to test the congruence between this new information (reproductive traits) and published genetic dataset. Our results emphasized six distinct groups with diagnostic major compounds: (a) alpinus + helleri with hexadec-9-en-1-ol; (b) polaris + pyrrhopygus with two major compounds hexadec-9-en-1-ol and hexadec-9-enal; (c) balteatus with tetradecyl acetate; (d) kirbiellus with geranyl geranyl acetate; (e) hyperboreus + natvigi with octadec-11-en-1-ol; (f) neoboreus with octadec-9-en-1-ol. Based on this new information, we can confirm the species status of B. alpinus, B. balteatus, B. hyperboreus, B. kirbiellus, B. neoboreus, and B. polaris. We also confirm the synonymy of helleri (Alps) with alpinus (Sweden). However, the specific status of natvigi (Alaska) and pyrrhopygus (Sweden) is questionable and these taxa do not have specific CLGS composition.

Variability in Sexual Pheromones Questions their Role in Bumblebee Pre-Mating Recognition System.

Sex-specific chemical secretions have been widely used as diagnostic characters in chemotaxonomy. The taxonomically confused group of bumblebees has reaped the benefit of this approach through the analyses of cephalic labial gland secretions (CLGS). Most of currently available CLGS descriptions concern species from the West-Palearctic region but few from the New World. Here, the CLGS of four East-Palearctic species Bombus deuteronymus, B. filchnerae, B. humilis, and B. exil (subgenus Thoracobombus) are analysed. Our results show high levels of variability in the major compounds in B. exil. In contrast, we describe a low differentiation in CLGS compounds between B. filchnerae and its phylogenetically closely related taxon B. muscorum. Moreover, the chemical profiles of B. filchnerae and B. muscorum are characterized by low concentrations of the C16 component, which is found in higher concentrations in the other Thoracobombus species. This raises the possibility that courtship behavior as well as environmental constraints could affect the role of the bumblebee males' CLGS.

Chemical reproductive traits of diploid Bombus terrestris males: Consequences on bumblebee conservation.

The current bumblebee decline leads to inbreeding in populations that fosters a loss of allelic diversity and diploid male production. As diploid males are viable and their offspring are sterile, bumblebee populations can quickly fall in a vortex of extinction. In this article, we investigate for the first time a potential premating mechanism through a major chemical reproductive trait (male cephalic labial gland secretions) that could prevent monandrous virgin queens from mating with diploid males. We focus our study on the cephalic labial gland secretions of diploid and haploid males of Bombus terrestris (L.). Contrary to initial expectations, our results do not show any significant differentiation of cephalic labial gland secretions between diploid and haploid specimens. Queens seem therefore to be unable to avoid mating with diploid males based on their compositions of cephalic labial gland secretions. This suggests that the vortex of extinction of diploid males could not be stopped through premating avoidance based on the cephalic labial gland secretions but other mechanisms could avoid mating between diploid males and queens.

First Chemical Analysis and Characterization of the Male Species-Specific Cephalic Labial-Gland Secretions of South American Bumblebees.

The evolution of signals and reproductive traits involved in the pre-mating recognition has been in focus of abundant research in several model species, such as bumblebees (genus Bombus). However, the most-studied bumblebee reproductive trait, the male cephalic labial gland secretions (CLGS), remains unknown among bumblebee species from South America. In this study, the CLGS of five South American bumblebees of the subgenera Thoracobombus (Bombus excellens and B. atratus) and Cullumanobombus (B. rubicundus, B. hortulanus, and B. melaleucus) were investigated, by comparing the chemical compositions of their secretions to those of closely related European species. The results showed an obvious interspecific differentiation in both subgenera. The interspecific differentiation among the species of the Thoracobombus subgenus involved different compounds present at high contents (main compounds), while those of the Cullumanobombus subgenus shared the same main components. This suggests that among the species of the Cullumanobombus subgenus, the differentiation in minor components could lead to species discrimination.

Methods for species delimitation in bumblebees (Hymenoptera, Apidae, Bombus): towards an integrative approach

Delimitation of closely related species is often hindered by the lack of discrete diagnostic morphological characters. This is exemplified in bumblebees (genus Bombus). There have been many attempts to clarify bumblebee taxonomy by using alternative features to discrete morphological characters such as wing shape, DNA, or eco‐chemical traits. Nevertheless each approach has its own limitations. Recent studies have used a multisource approach to gather different lines of speciation evidence in order to draw a strongly supported taxonomic hypothesis in bumblebees. Yet, the resulting taxonomic status is not independent of selected evidence and of consensus methodology (i.e. unanimous procedure, majority, different weighting of evidence). In this article, we compare taxonomic conclusions for a group of taxonomically doubtful species (the Bombus lapidarius‐group) obtained from the four commonly used lines of evidence for species delimitation in bumblebees (geometric morphometric of wing shape, genetic differentiation assessment, sequence‐based species delimitation methods and differentiation of cephalic labial gland secretions). We ultimately aim to assess the usefulness of these lines of evidence as components of an integrative decision framework to delimit bumblebee species. Our results show that analyses based on wing shape do not delineate any obvious cluster. In contrast, nuclear/mitochondrial, sequence‐based species delimitation methods, and analyses based on cephalic labial gland secretions are congruent with each other. This allows setting up an integrative decision framework to establish strongly supported species and subspecies status within bumblebees.

Subspecific differentiation in male reproductive traits and virgin queen preferences, in Bombus terrestris

Many species display local variations in pre-mating signals and in mating preferences. This may lead to discrimination against potential foreign mates that may ultimately lead to reproductive isolation. However, the extent to which population differentiation in mating cues affects the species recognition has received little empirical support. Here, we investigate the consequence of geographic differentiation in male reproductive traits on female preferences to these traits in Bombus terrestris. We characterise (1) the geographic differentiation in male cephalic labial gland secretions (CLGS), a key trait for mate attraction, and (2) the preference of virgin females to the CLGS of different subspecies. Our results show geographic CLGS differences parallel with divergences in female preferences for these secretions. This geographic CLGS differentiation in males, along with female preference for sympatric males, could lead to or reflect a pre-mating isolation among subspecies.

An integrative taxonomic approach to assess the status of Corsican bumblebees: implications for conservation

AbstractMany islands are biodiversity hotspots that host numerous endemic species. Unfortunately, insular faunas suffer from high rates of extinction and endangerment, and numerous conservation plans have been developed for their protection. These conservation plans are often assessed on the basis of occurrence and proportion of endemic taxa. However, delimitations of species and subspecies are still confusing and controversial. From a practical point of view, these disagreements make it difficult for government agencies and non‐governmental organizations to initiate conservation measures. The present study develops a pragmatic integrative taxonomic approach on the basis of molecular and eco‐chemical criteria. This method is applied to the insular bumblebee fauna of Corsica. For each taxon, the differentiation of Corsican taxa from the nearest related allopatric parents is characterized using genetic markers and the chemical composition of cephalic labial gland secretions. Phylogenetic analyses, Bayesian implementation of the general mixed Yule‐coalescent approach, and comparative chemical studies show that two Corsican taxa can be considered as endemic species while five others can be considered as subspecies. Regardless of the taxonomic assessment the method facilitates diagnosis of evolutionarily significant units and rank taxa according to their distinctiveness. International Union for Conservation of Nature red lists are reconsidered according to the new taxonomic hypothesis for Corsican bumblebees. Modifications in species assessments are proposed. The present approach provides useful data sets for policy‐makers and conservation organizations.

Cephalic labial gland secretions of males as species recognition signals in bumblebees: are there really geographical variations in the secretions of the Bombus terrestris subspecies? (Hymenoptera: Apidae: Bombus).

Um künstliche Kolonien zu züchten und frische Männchen zu bekommen, wurden Frühjahrsköniginnen der Unterarten von Bombus terrestris gefangen (B. terrestris audax, B. t. canariensis, B. t. maderensis, B. t. terrestris and B. t. dalmatinus). Zusätzlich wurden Männchen von B. t. lusitanicus in Portugal, B. t. terrestris in Südfrankreich und Deutschland, B. t. xanthopus auf Korsika und B. t. sassaricus auf Sardinien im Freiland gefangen. Die männlichen Labialdrüsensekrete von 18 Proben wurden mittels Gaschromatographie / Massenspektrometrie untersucht. Etwa 70 Substanzen wurden identifiziert, eine Mischung azyklischer Sesqui- und Diterpene (Alkohole, Aldehyde, Azetate und Ester) und verschiedene geradkettige Fettsäurederivate (Alkohole, Aldehyde, Azetate, Ester und gesättigte sowie ungesättigte Kohlenwasserstoffe C21 - C29). Die Hauptkomponente in allen Proben war 3,7,11-Trimethyldodeca-(6E),10-dien-1-ol (2,3-Dihydrofarnesol), einige wenige Freilandproben enthielten zum Teil beträchtliche Mengen an Estern (wohl im Zusammenhang mit der Alterung der Männchen), vor allem 3,7,11-Trimethyldodeca-(6E),10-dien-1-yl-dodecanoat, und da diese wohl nicht für die Kommunikation geeigneten Substanzen die prozentuale Peak Fläche der übrigen Substanzen absenkt und so den Vergleich zwischen den verschiedenen Proben erschwert, wurden alle Substanzen mit einer Molmasse größer als Heptacosan (C27 Kohlenwasserstoff) bei der Auswertung ausgeschlossen. Mit Ausnahme der Proben B. t. xanthopus von Korsika und B. t. sassaricus von Sardinien, denen das in den übrigen Proben mit etwa 10 % Peak Fläche enthaltene Ethyldodecanoat vollständig fehlt, gibt es keine bemerkenswerte geographische Variation. Die Variabilität in der Zusammensetzung der Labialdrüsensekrete von (1) einzeln gemessenen Männchen aus derselben Kolonie, von (2) Proben geographisch benachbarter Fundorte und von (3) Proben geographisch weit entfernter Fundorte ist in derselben Größenordnung. Diese Befunde werden im Vergleich mit abweichenden veröffentlichten Befunden diskutiert. Wegen ihrer geringen Variabilität auch über große geographische Distanz sind die Labialdrüsensekrete von B. terrestris als Art-Erkennungs-Signale gut geeignet.

Male labial gland secretions as species recognition signals in species of Bombus

Abstract Spring queens of Bombus cryptarum from Europe, Bombus burjaeticus from the Russian Transbaikal region and Bombus moderatus from North America were collected at different localities to grow artificial colonies and obtain fresh males. Furthermore, males of Bombus florilegus , B. moderatus , Bombus magnus and Bombus occidentalis were collected in the field. Cephalic labial gland secretions of 29 specimens from these different taxa were analysed by gas chromatography/mass spectrometry (GC/MS). About 50 compounds were identified, including a mixture of straight-chain fatty acid derivatives (alkenoles, acids, acetates and esters), and the usual pattern of saturated and unsaturated hydrocarbons C 21 –C 29 . The secretions of all specimens investigated were very similar: ethyl dodecanoate was identified as the main component (90–97% peak area). Since the sum of percentage peak areas for the hydrocarbons amounted up to 3–6%, most of the remaining compounds (8 alkenoles, 6 acids, 6 acetates and 20 esters) could only be detected at very low levels ( B. florilegus , each taxon had a sympatric bumblebee taxon that also used ethyl dodecanoate as the main component: B. magnus , sympatric with B. cryptarum in Europe, Bombus patagiatus , sympatric with B. burjaeticus in the Russian Transbaikal and B. occidentalis , sympatric with B. moderatus in North America. Male cephalic labial gland secretions of 20 specimens from these sympatric taxa were also investigated to see how far their pattern of compounds differed from that of the cryptarum complex taxa ( cryptarum – burjaeticus – florilegus – moderatus ). Besides some quantitative differences in ethyl dodecanoate and ethyl octadec-9-enoate, the occurrence of octadeca-9,12-dien-1-ol, octadeca-9,12,15-trien-1-ol and octadec-11-en-1-ol was an essential qualitative difference that separated the labial gland secretions of B. magnus , B. patagiatus and B. occidentalis from those of their sympatric counterparts. A principal component analysis confirmed this separation. Due to their low variability and differences in pattern of compounds compared to those of closely related sympatric bumblebees, the labial gland secretions of B. cryptarum , B. burjaeticus, B. florilegus and B. moderatus distributed across Europe, the Russian Far East and North America are very well suited to function as ‘species recognition signal’. The taxonomic consequences for the cryptarum complex taxa and their relationship to Bombus albocinctus from the Russian Far East are discussed in context of recent genetic investigations.

Labial gland marking secretions of male Bombus lucorum bumblebees from Europe and China reveal two separate species: B. lucorum (Linnaeus 1761) and Bombus minshanicola (Bischoff 1936)

Abstract Differences in male bumblebee labial gland secretions can be used to separate species pairs. Cephalic labial gland secretions from male Bombus lucorum Linnaeus bumblebees from Europe, and male ‘ B. lucorum ’ bumblebees from China, were analyzed by gas chromatography/mass spectrometry (GC/MS). In the European B. lucorum L., ethyl tetradec-9-enoate was identified as the major compound (58% peak area), along with a complex mixture of straight-chain alkenols, acetates, hydrocarbons, and wax type esters. In contrast, the main component of the labial gland secretions of the Chinese ‘ B. lucorum ’, ethyl dodecanoate (31%), did not dominate the secretion, which additionally contained large amounts of 3,7,11-trimethyldodeca-6,10-dien-1-ol (=2,3-dihydrofarnesol, 24% peak area), ethyl octadec-9-enoate (15% peak area) and a mixture of acyclic diterpenes (alcohols, aldehydes, and acetates). Furthermore, in B. lucorum , 3,7,11-trimethyldodeca-6,10-dien-1-ol was only detected in trace amounts (0.05%) and ethyl octadec-9-enoate corresponded to only 0.8% of the mixture. These results indicate that ‘ B. lucorum ’ from China is a separate taxon from B. lucorum L. Further analysis revealed that ‘ B. lucorum ’ from China has been previously described as Bombus minshanicola Bischoff 1936 from Gansu/China. Differences in the chemical composition of male bumblebee labial gland secretions are discussed in comparison with other known bumblebee species pairs.

Age-dependent attractivity of males’ sexual pheromones in Bombus terrestris (L.) [Hymenoptera, Apidae

Males of Bombus terrestris (L.) adopt a patrolling behaviour during their nuptial parade using cephalic labial gland (CLG) secretions containing sexual pheromones to attract conspecific virgin queens. The changes in chemical composition of their CLG secretions with age are quite well known. In this study, we investigate the evolution of CLG secretions with age in greater detail and compare behavioural reactions of conspecific virgin queens to the secretions. We show that compounds of CLG secretions follow two profiles. Most of the compounds increase from the first day after emergence until the bees are 15-days-old and then decrease. Others are less abundant in 1 to 15-day-old males and then increase (e.g. tricosane, tricosene, henicosane, tetradecanoic acid, pentacosene, pentacosane, heptacosene, heptacosane, nonacosene and geranylcitronellyl tetradecanoate). Differences in secretion composition lead to preferences of virgin queens for males according to the male’s age. Virgin queens prefer the pheromonal gland secretions of bees of the following ages in decreasing order; 1 day = 3 days < 7 days = 30 days < 15 days < 10 days. The virgin queens are strongly attracted by secretions containing high amounts of 2,3-dihydrofarnesol, 2,3-dihydrofarnesal, ethyl dodecanoate and hexadecanol. On the contrary, geranylcitronellol is more abundant in 30-day-old males.

A simplified protocol for bumble bee species identification by cephalic secretion analysis

Studies of bumble bee male sex pheromones are often based on hexane extraction of cephalic labial gland secretions. Here, we propose an alternative extraction procedure based on whole head extractions. To test this method, we compared cephalic parts of the labial glands (CLG) to whole head extractions using high-sensitivity gas chromatography coupled to mass spectrometry (GC–MS) analysis. Two species were analyzed: Bombus terrestris dalmatinus and Bombus ignitus ignitus. We used correlation coefficients, cluster analysis and the two-group k-means method to test the similarity between the two extractions of the two species. We also used the indicator value (IndVal) method to characterize the indicator compounds of each procedure. Statistical analyses reveal a similar compound extraction using both methods. This simplified sample preparation gives reliable results and is more suitable for broad-scale sampling both by specialists and by amateur collectors. Using this method, we provide the first description of the male pheromonal secretions of B. ignitus ignitus and demonstrate that these secretions are very similar to the pheromonal blend of B. terrestris dalmatinus where 44/51 compounds are shared.

Chemistry of the Cephalic Labial Gland Secretions of Male Bombus morrisoni and B. rufocinctus, Two North American Bumblebee Males with Perching Behavior

The labial gland secretions from males of the North American bumblebees Bombus morrisoni Cresson and B. rufocinctus Cresson were analyzed by gas chromatography/mass spectrometry. In both species, 3,7,11,15-tetramethyl-2,6,10,14-hexadecatetraenyl acetate was found as the major compound of a complex mixture of alkenols, acetates, hydrocarbons, and wax-type esters. In addition to a mixture of saturated and mono-unsaturated straight chain hydrocarbons, the labial gland of both species contained the isoprenoid hydrocarbons (6E, 10E)-3,7,15-trimethyl-3-methylene-hexadeca-1,6,10,14-tetraene [beta-springene], three isomers of 3,7,11,15-tetramethyl-hexadeca-1,3,6,10,14-pentaene [alpha-springene], and two further unidentified cyclic diterpenes. In B. morrisoni, 3,7,11-trimethyl-2,6,10-dodecatrien-1-ol and 3,7,11,15-tetramethyl-6,10,14-hexadecatrien-1-ol were detected as characteristic alcohols, as well as small amounts of 9-hexadecenol and hexadecanol. Furthermore, a large peak of hexadecyl dodecanoate and minor amounts of 9-hexadecenyl, 9-octadecenyl, and eicosenyl 9-tetradecenoate were found as typical esters in this species. In B. rufocinctus, 9-hexadecenol, hexadecanol, and 9-octadecenol were present in considerable amounts, with their acetates and 9-tetradecenoic, tetradecanoic, hexadecanoic, and 11-octadecenoic acids. The chemical composition of cephalic labial glands in male bumblebees with perching behavior is discussed.

Atypical Secretions of the Male Cephalic Labial Glands in Bumblebees: The Case of Bombus (Rhodobombus) mesomelasGerstaecker (Hymenoptera, Apidae)

In bumblebees, the male secretion of the cephalic labial gland is species-specific. It is highly involved in the nuptial behavior, acting as a sexual attracting pheromone. Therefore, it is also used to accurately identify the species. In contrast to this common scheme, the secretions of Bombus mesomelas are found to be strongly reduced and do not include the most volatile compounds that are present in the secretions of all the other studied bumblebee species. These secretions correspond to cuticular hydrocarbons that can be found in all bumblebee species. This was also the case for another bumblebee species from the same Rhodobombus subgenus: Bombus pomorum. This atypical composition of the male cephalic labial gland secretions seems to indicate that, at least for these two species of Rhodobombus, these secretions are not used to attract virgin females from a long distance, as it is the case for all the other bumblebee species studied.

Reduced cephalic labial glands in the male bumblebees of the subgenus Rhodobombus Dalla Torre (Hymenoptera: Apidae: Bombus Latreille)

Until now, all males of bumblebees are known to attract conspecific females by marking places with a chemical signal secreted by their cephalic labial gland. The specific combination of patrolling flight and scentmarking is of outmost importance to bumblebees as it is their main species specific recognition system. We report here the lack of that recognition system in species of the subgenus Rhodobombus by comparing the morphology and histology of the cephalic labial glands of Bombus (Rhodobombus) mesomelas with those of a well known species, B. (Bombus) terrestris (L.) The cephalic labial glands are much smaller in B. mesomelas than in other bumblebees species and most likely non-functional. This morphology is also observed in B. pomorum and B. brodmanni which also belong to the subgenus Rhodobombus. Our morphological observations are consistent with the chemical analyses of the secretions in B. mesomelas and B. pomorum which are very limited and of most unusual composition for a bumblebee. In addition, whereas other species are thought to use their barbae mandibularis to spread their secretion onto the substrate, these structures are absent in all Rhodobombus. All these observations would mean that the males of Rhodobombus do not attract females from far away by the use of their cephalic labial gland secretions.