Iacopo Petrocelli

Facial markings in the hover wasps: quality signals and familiar recognition cues in two species of Stenogastrinae

Both recognition and conventional signals are widespread in the animal kingdom. Chemical communication plays a major role in invertebrates, and especially in social insects. In the last decade, observational and experimental evidence has shown the existence of visual quality signals and individual recognition cues in Polistes paper wasps, meaning that visual communication might also be common in insect societies. Here we show that two species of facultatively eusocial hover wasps (Vespidae, Stenogastrinae) use the visual channel for social communication. By combining morphoanatomical measurements and behavioural assays, we found that the size of the dark facial markings was related to reproductive status and dominance in colonies of Liostenogaster vechti, thus representing a badge of status. By contrast, no correlation between facial coloration and reproductive status was found in Liostenogaster flavolineata, which instead used facial markings as familiar visual recognition cues. Our results reveal that visual communication in social insects might be more widespread than previously thought and has evolved independently in distinct wasp taxa facing similar selection pressures.

Speed and accuracy in nest-mate recognition: a hover wasp prioritizes face recognition over colony odour cues to minimize intrusion by outsiders

Social insects have evolved sophisticated recognition systems enabling them to accept nest-mates but reject alien conspecifics. In the social wasp, Liostenogaster flavolineata (Stenogastrinae), individuals differ in their cuticular hydrocarbon profiles according to colony membership; each female also possesses a unique (visual) facial pattern. This species represents a unique model to understand how vision and olfaction are integrated and the extent to which wasps prioritize one channel over the other to discriminate aliens and nest-mates. Liostenogaster flavolineata females are able to discriminate between alien and nest-mate females using facial patterns or chemical cues in isolation. However, the two sensory modalities are not equally efficient in the discrimination of ‘friend’ from ‘foe’. Visual cues induce an increased number of erroneous attacks on nest-mates (false alarms), but such attacks are quickly aborted and never result in serious injury. Odour cues, presented in isolation, result in an increased number of misses: erroneous acceptances of outsiders. Interestingly, wasps take the relative efficiencies of the two sensory modalities into account when making rapid decisions about colony membership of an individual: chemical profiles are entirely ignored when the visual and chemical stimuli are presented together. Thus, wasps adopt a strategy to ‘err on the safe side’ by memorizing individual faces to recognize colony members, and disregarding odour cues to minimize the risk of intrusion from colony outsiders.

Hydrocarbon rank signatures correlate with differential oophagy and dominance behaviour in Polistes dominulus foundresses.

SUMMARY Social life offers animals increased fitness opportunities. However, the advantages are not evenly distributed and some individuals benefit more than others. The ultimate advantage of reaching the highest rank in a dominance hierarchy is the achievement of reproduction monopoly. In social insects, dominant individuals and queens keep their reproductive control through differential oophagy of unwanted eggs (egg policing). Egg recognition is the main proximate mechanism for maintaining reproductive dominance. In the social wasp Polistes dominulus, subordinate queens often lay eggs in the presence of the dominant individual. Combining gas chromatography–mass spectrometry (GC-MS) analysis and laboratory bioassays, we found that chemical differences between eggs of subordinate and dominant foundresses can explain the differential success in oophagy enjoyed by dominant individuals. We propose that dominance behaviour is an investigative behaviour as well as a ritualized agonistic behaviour. In fact, the frequency of dominance acts increases with the chemical similarity of the surfaces of dominant- and subordinate-laid eggs. Therefore, dominant individuals probably perform dominance behaviour to test the cuticular signatures of subordinates and so better assess the chemical profiles of subordinate eggs. Finally, we provide evidence that in particular social contexts, subordinate Polistes foundresses can develop ovaries as large as those of dominant individuals but nevertheless lay very few eggs. The subordinates probably lay a limited number of eggs to avoid unnecessary energy loss, as a result of efficient queen policing, but will start laying eggs as soon as the queen fails.

Incipient morphological castes in Polistes gallicus (Vespidae, Hymenoptera)

The occurrence of a pre-imaginal caste determination represents a sort of “point of no return” to eusociality. In some social insect taxa, including Polistes species, the occurrence of distinct queen and worker castes is still debated. Before this report, no clear morphological differences between reproductive and non-reproductive individuals were known. Here we show that several pre-imaginal morphological differences of the Van der Vecht organ occur between foundresses (queens) and workers. Geometric morphometrics revealed that queens are characterized by shape deformations of this organ, which is responsible for a typical allometric growth of the secretory area. This organ is predicted to be larger in foundresses compared to workers because its secretion is involved in defense against ants, in nestmate recognition and in preventing workers from challenging for direct reproduction. The results presented here indicate the existence of an incipient morphological caste determination Polistes gallicus and suggest that this species may have passed the “point of no return” for eusociality.

Social biology of Parischnogaster striatula (Hymenoptera: Stenogastrinae)

The social biology of the wasp Parischnogaster striatula has been studied in Peninsular Malaysia. This species shows the main characteristics of hover wasps (Vespidae, Stenogastrinae) which set them aside from the other social Vespidae. These include the use of an abdominal secretion in brood rearing, the three-phase egg deposition and the presence of only four larval instars. Colonies are invariably small sized and on average 1.92 females on the nests present developed ovaries. Colony life revolves around a dominant female, which has developed ovaries and spends most of her time on the nest. Since visual cues were recently found in two different species of Liostenogaster (Stenogastrinae), we also looked for a possible badge of status in these wasps but with negative results.

Competition between the native and the introduced hornets Vespa crabro and Vespa velutina: a comparison of potentially relevant life‐history traits

1. Invasive alien species are a major threat to biodiversity. In addition to predation and parasitism, native species might suffer from competition when invasive alien species occupy a similar ecological niche.

The venom apparatus in stenogastrine wasps: subcellular features of the convoluted gland.

In the wasp venom apparatus, the convoluted gland is the tract of the thin secretory unit, i.e. filament, contained in the muscular reservoir. Previous transmission electron microscope investigation on Stenogastrinae disclosed that the free filaments consist of distal and proximal tracts, from/to the venom reservoir, characterized by class 3 and 2 gland patterns, respectively. This study aims to extend the ultrastructural analysis to the convoluted tract, in order to provide a thorough, subcellular representation of the venom gland in these Asian wasps. Our findings showed that the convoluted gland is a continuation of the proximal tract, with secretory cells provided with a peculiar apical invagination, the extracellular cavity, collecting their products. This compartment holds a simple end-apparatus lined by large and ramified microvilli that contribute to the processing of the secretory product. A comparison between previous and present findings reveals a noticeable regionalization of the stenogastrine venom filaments and suggests that the secretory product acquires its ultimate composition in the convoluted tract.

Ontogenic Caste Differences in the Van der Vecht Organ of Primitively Eusocial Neotropical Paper Wasps

Recent studies have reported incipient morphological caste dimorphism in the Van der Vecht organ size of some temperate Polistes paper wasps. Whether species other than the temperate ones show a similar pattern remains elusive. Here, we have studied some Neotropical Polistes species. By comparing females collected through the year, we showed caste related differences in the size of the Van der Vecht organ in P. ferreri (body size corrected Van der Vech organ size of queens = 0.45 ± 0.06, workers = 0.38 ± 0.07 mm2, p = 0.0021), P. versicolor (body size corrected Van der Vech organ size of queens = 0.54 ± 0.11, workers = 0.46 ± 0.09 mm2, p = 0.010), but not P. simillimus (body size corrected Van der Vech organ size of queens = 0.52 ± 0.05, workers = 0.49 ± 0.06 mm2, p = 0.238). Therefore, it seems that queens and workers of some Neotropical Polistes have diverged in their ontogenic trajectory of the Van der Vecht organ size, providing clear evidence for incipient morphological caste dimorphism. As Polistes are distributed mostly in the tropics, we propose that physical caste differences may be widespread in the genus. Also, we highlight that morphological divergence in the queen–worker phenotypes may have started through differential selection of body structures, like the Van der Vecht organ.

Antennal Protein Profile in Honeybees: Caste and Task Matter More Than Age

Reproductive and task partitioning in large colonies of social insects suggest that colony members belonging to different castes or performing different tasks during their life (polyethism) may produce specific semiochemicals and be differently sensitive to the variety of pheromones involved in intraspecific chemical communication. The main peripheral olfactory organs are the antennal chemosensilla, where the early olfactory processes take place. At this stage, members of two different families of soluble chemosensory proteins [odorant-binding proteins (OBPs) and chemosensory proteins (CSPs)] show a remarkable affinity for different odorants and act as carriers while a further family, the Niemann-Pick type C2 proteins (NPC2) may have a similar function, although this has not been fully demonstrated. Sensillar lymph also contains Odorant degrading enzymes (ODEs) which are involved in inactivation through degradation of the chemical signals, once the message is conveyed. Despite their importance in chemical communication, little is known about how proteins involved in peripheral olfaction and, more generally antennal proteins, differ in honeybees of different caste, task and age. Here, we investigate for the first time, using a shotgun proteomic approach, the antennal profile of honeybees of different castes (queens and workers) and workers performing different tasks (nurses, guards, and foragers) by controlling for the potential confounding effect of age. Regarding olfactory proteins, major differences were observed between queens and workers, some of which were found to be more abundant in queens (OBP3, OBP18, and NPC2-1) and others to be more abundant in workers (OBP15, OBP21, CSP1, and CSP3); while between workers performing different tasks, OBP14 was more abundant in nurses with respect to guards and foragers. Apart from proteins involved in olfaction, we have found that the antennal proteomes are mainly characterized by castes and tasks, while age has no effect on antennal protein profile. Among the main differences, the strong decrease in vitellogenins found in guards and foragers is not associated with age.

Female body size, weight and fat storage rather than nestmateship determine male attraction in the invasive yellow-legged hornet Vespa velutina nigrithorax

In the early stage of the invasion process, alien species may face costs linked to pioneer effect due to genetic bottleneck, drift and the consequential inbreeding depression. Thus, introduced species that show an immediate exponential growth in their invasive population should have some mechanisms to reduce such costs minimising the chance of inbreeding or allowing them to cope with them. The yellow-legged hornet Vespa velutina nigrithorax has been spotted in France in 2004; since then, the species has been invading Europe with a relentless pace. In their native range, males and reproductive females of a Chinese non-invasive sub-species of V. velutina seem to leave their nests to search for unrelated partners. However, previous studies showed a low genetic diversity and a high rate of diploid males in colonies of the invasive population, suggesting that mating could occur inside nests, where males should be able to discriminate between reproductive gynes and sterile workers. Here, we used laboratory behavioural assays to investigate the mating preferences of yellow-legged hornet males from the recent invasive population in Italy. We assessed the importance of nestmateship and female morpho-physiological traits, likely indicators of caste, in determining male attraction towards potential partners. Our results demonstrate that males are more attracted to bigger females with more abundant fat storage, good indicators of female reproductive caste in wasps, regardless of nestmateship. Our findings represent a first step in understanding the reproductive biology of V. velutina nigrithorax in its invasive range, providing a framework for future research in the field to prevent or reduce the species expansion.