Morphology and structure of the tarsal glands of the stingless bee Melipona seminigra

Parasitoid attachment ability and the host surface wettability

Climbing animals such as geckos and arthropods developed astonishing adhesive mechanisms which are fundamental for their survival and represent valuable models for biomimetic purposes. A firm adhesion to the host surface, in order to successfully lay eggs is necessary for the reproduction of most parasitoid insects. In the present study, we performed a comparative investigation on the attachment ability of four parasitoid species (the egg parasitoid Anastatus bifasciatus (Eupelmidae), the aphid parasitoid Aphidius ervi (Braconidae), the fly pupal ectoparasitoid Muscidifurax raptorellus (Pteromalidae) and the pupal parasitoid of Drosophila Trichopria drosophilae (Diapriidae)) with hosts characterized by a surface having different wettability properties. The friction force measurements were performed on smooth artificial (glass) surfaces showing different contact angles of water. We found that attachment systems of parasitoid insects are tuned to match the wettability of the host surface. Sexual dimorphism in the attachment ability of some tested species has been also observed. The obtained results are probably related to different microstructure and chemical composition of the host surfaces and to different chemical composition of the parasitoid adhesive fluid. The data here presented can be interpreted as an adaptation, especially in the female, to the physicochemical properties of the host surface and contribute to shed light on the coevolutionary processes of parasitoid insects and their hosts.

Mechanoecology: biomechanical aspects of insect-plant interactions

Plants and herbivorous insects as well as their natural enemies, such as predatory and parasitoid insects, are united by intricate relationships. During the long period of co-evolution with insects, plants developed a wide diversity of features to defence against herbivores and to attract pollinators and herbivores' natural enemies. The chemical basis of insect-plant interactions is established and many examples are studied, where feeding and oviposition site selection of phytophagous insects are dependent on the plant's secondary chemistry. However, often overlooked mechanical interactions between insects and plants can be rather crucial. In the context of mechanoecology, the evolution of plant surfaces and insect adhesive pads is an interesting example of competition between insect attachment systems and plant anti-attachment surfaces. The present review is focused on mechanical insect-plant interactions of some important pest species, such as the polyphagous Southern Green Stinkbug Nezara viridula and two frugivorous pest species, the polyphagous Mediterranean fruit fly Ceratitis capitata and the monophagous olive fruit fly Bactrocera oleae. Their ability to attach to plant surfaces characterised by different features such as waxes and trichomes is discussed. Some attention is paid also to Coccinellidae, whose interaction with plant leaf surfaces is substantial across all developmental stages in both phytophagous and predatory species that feed on herbivorous insects. Finally, the role of different kinds of anti-adhesive nanomaterials is discussed. They can reduce the attachment ability of insect pests to natural and artificial surfaces, potentially representing environmental friendly alternative methods to reduce insect pest impact in agriculture.

Attachment devices and the tarsal gland of the bug Coreus marginatus (Hemiptera: Coreidae)

The present ultrastructural investigation using scanning and transmission electron microscopy as well as light and fluorescence microscopy describes in detail the attachment devices and tarsal gland of the bug Coreus marginatus (L.) (Hemiptera: Coreidae). In particular, the fine structure of pulvilli reveals a ventral surface rich with pore channels, consistent with fluid emission, and a folded dorsal surface, which could be useful to enhance the pulvillus contact area during attachment to the substrate. The detailed description of the tarsal gland cells, whose structure is coherent with an active secretory function, allows us to consider the tarsal gland as the plausible candidate for the adhesive fluid production. Scolopidia strictly adhering to the gland cells are also described. On the basis of the fine structure of the tarsal gland, we hypothesise a fluid emission mechanism based on changes of the hydraulic pressure inside the gland, due to the unguitractor tendon movements. This mechanism could provide the fluid release based on compression of the pad and capillary suction, as demonstrated in other insects. The data here reported can contribute to understanding of insect adhesive fluid production, emission and control of its transport.

The Stingless Bee Melipona solani Deposits a Signature Mixture and Methyl Oleate to Mark Valuable Food Sources

Stingless bees foraging for food improve recruitment by depositing chemical cues on valuable food sites or pheromone marks on vegetation. Using gas chromatography/mass spectrometry and bioassays, we showed that Melipona solani foragers leave a mixture composed mostly of long chain hydrocarbons from their abdominal cuticle plus methyl oleate from the labial gland as a scent mark on rich food sites. The composition of hydrocarbons was highly variable among individuals and varied in proportions, depending on the body part. A wide ratio of compounds present in different body parts of the bees elicited electroantennogram responses from foragers and these responses were dose dependent. Generally, in bioassays, these bees prefer to visit previously visited feeders and feeders marked with extracts from any body part of conspecifics. The mean number of visits to a feeder was enhanced when synthetic methyl oleate was added. We propose that this could be a case of multi-source odor marking, in which hydrocarbons, found in large abundance, act as a signature mixture with attraction enhanced through deposition of methyl oleate, which may indicate a rich food source.

Pretarsus structure in relation to climbing ability in the ants Brachyponera sennaarensis and Daceton armigerum

We studied the external and internal pretarsus structure of the ants Brachyponera sennaarensis and Daceton armigerum in relation to their very different climbing ability. B. sennaarensis is a ground-dwelling species that is not able to climb vertical smooth walls. They have a pair of straight pretarsal claws with an average claw tip angle of 56 degrees, while the ventral tarsal surface lacks fine hairs that touch the substrate. They have no adhesive pad on the vestigial arolium, while the arolium gland is very small. D. armigerum, on the other hand, is an arboreal and thus well-climbing species with a very strong grip on the substrate. Their pretarsal claws are very hooked, with a claw tip angle around 75 degrees. They have dense arrays of fine hairs on the ventral tarsal surface, a well-developed arolium and arolium gland. These clearly different morphological characteristics are in line with the opposite climbing performance of both species.

Stingless bees (Melipona scutellaris) learn to associate footprint cues at food sources with a specific reward context

Foraging insects leave chemical footprints on flowers that subsequent foragers may use as indicators for recent flower visits and, thus, potential resource depletion. Accordingly, foragers should reject food sources presenting these chemical cues. Contrasting this assumption, experimental studies in stingless bees (Apidae, Meliponini), so far, demonstrated an attractive effect of footprints. These findings lead to doubts about the meaning of these chemical cues in natural foraging contexts. Here, we asked whether foragers of stingless bees (Melipona scutellaris) use footprints according to the previously experienced reward level of visited food sources. Bees were trained to artificial flower patches, at which the reward of a flower either decreased or, alternatively, increased after a visit by a forager. Individuals were allowed a total of nine foraging bouts to the patch, after which their preference for visited or unvisited flowers was tested. In the choice tests, bees trained under the decreasing reward context preferred unvisited flowers, whereas individuals trained under the increasing reward context preferred visited flowers. Foragers without experience chose randomly between visited and unvisited flowers. These results demonstrate that M. scutellaris learns to associate unspecific footprint cues at food sources with differential, specific reward contexts, and uses these chemical cues accordingly for their foraging decisions.

Exocrine glands in the legs of the social wasp Vespula vulgaris

This study brings a survey of the exocrine glands in the legs of Vespula vulgaris wasps. We studied workers, males, virgin queens as well as mated queens. A variety of 17 glands is found in the different leg segments. Among these, five glands are novel exocrine structures for social insects (trochanter-femur gland, ventrodistal tibial gland, distal tibial sac gland, ventral tibial gland, and ventral tarsomere gland). Most leg glands are present in the three leg pairs of all castes. This may indicate a mechanical function. This is likely for the numerous glands that occur near the articulation between the various leg segments, where lubricant production may be expected. Other possible functions include antenna cleaning, acting as a hydraulic system, or pheromonal. Further research including leg-related behavioural observations and chemical analyses may help to clarify the functions of these glandular structures in the legs.

Mechanisms of fluid production in smooth adhesive pads of insects

Insect adhesion is mediated by thin fluid films secreted into the contact zone. As the amount of fluid affects adhesive forces, a control of secretion appears probable. Here, we quantify for the first time the rate of fluid secretion in adhesive pads of cockroaches and stick insects. The volume of footprints deposited during consecutive press-downs decreased exponentially and approached a non-zero steady state, demonstrating the presence of a storage volume. We estimated its size and the influx rate into it from a simple compartmental model. Influx was independent of step frequency. Fluid-depleted pads recovered maximal footprint volumes within 15 min. Pads in stationary contact accumulated fluid along the perimeter of the contact zone. The initial fluid build-up slowed down, suggesting that flow is driven by negative Laplace pressure. Freely climbing stick insects left hardly any traceable footprints, suggesting that they save secretion by minimizing contact area or by recovering fluid during detachment. However, even the highest fluid production rates observed incur only small biosynthesis costs, representing less than 1 per cent of the resting metabolic rate. Our results show that fluid secretion in insect wet adhesive systems relies on simple physical principles, allowing for passive control of fluid volume within the contact zone.

Occurrence and structural organization of the exocrine glands in the legs of ants

Apart from their obvious locomotory function and hence the presence of muscle fibres, ant legs are also endowed with an astonishing variety of exocrine glands. This paper reviews the presence and structural variety of the 20 different glands that have so far been found in the legs of ants. Four of these glands are described for the first time in this paper. Glands have been described in the three leg pairs, although considerable differences may exist. Glands occur in the various leg segments. A number of glands, especially those located in the hindlegs, may have a function in the production of trail pheromones. Other possible functions that have been reported deal with antenna cleaning, production of lubricant substances and sex pheromones.

Foraging scent marks of bumblebees: footprint cues rather than pheromone signals

In their natural habitat foraging bumblebees refuse to land on and probe flowers that have been recently visited (and depleted) by themselves, conspecifics or other bees, which increases their overall rate of nectar intake. This avoidance is often based on recognition of scent marks deposited by previous visitors. While the term 'scent mark' implies active labelling, it is an open question whether the repellent chemicals are pheromones actively and specifically released during flower visits, or mere footprints deposited unspecifically wherever bees walk. To distinguish between the two possibilities, we presented worker bumblebees (Bombus terrestris) with three types of feeders in a laboratory experiment: unvisited control feeders, passive feeders with a corolla that the bee had walked over on its way from the nest (with unspecific footprints), and active feeders, which the bee had just visited and depleted, but which were immediately refilled with sugar-water (potentially with specific scent marks). Bumblebees rejected both active and passive feeders more frequently than unvisited controls. The rate of rejection of passive feeders was only slightly lower than that of active feeders, and this difference vanished completely when passive corollas were walked over repeatedly on the way from the nest. Thus, mere footprints were sufficient to emulate the repellent effect of an actual feeder visit. In confirmation, glass slides on which bumblebees had walked on near the nest entrance accumulated hydrocarbons (alkanes and alkenes, C23 to C31), which had previously been shown to elicit repellency in flower choice experiments. We conclude that repellent scent marks are mere footprints, which foraging bees avoid when they encounter them in a foraging context.

The use of heterospecific scent marks by the sweat bee Halictus aerarius

To forage effectively amongst flowers, some bee species utilize olfactory cues left by previous visitors in addition to direct assessment of visual cues to identify rewarding flowers. This ability can be more advantageous if the bees can recognize and use scent marks left by heterospecifics, not just marks left by members of their own species. We conducted field experiments to investigate whether the sweat bee Halictus aerarius avoids visiting flowers of trailing water willow Justicia procumbens emptied by other bee species. We found that H. aerarius rejected the flowers visited by both heterospecifics and conspecifics. They also rejected visited flowers artificially replenished with nectar. Our results demonstrate that social bees outside the Apidae can detect marks left on flowers by heterospecifics but that (on this plant species) they are unable to discriminate against flowers by directly detecting nectar volume. H. aerarius exhibited different rejection rates according to the identity of the previous bee species. We suggest that the frequency of rejection responses may depend on the amount of chemical substances left by the previous bee. In general, the use of scent marks left by previous visitors is almost certainly advantageous, enabling foragers to avoid flowers with depleted nectar levels and thereby improving their foraging efficiency.

Tracing pollinator footprints on natural flowers

Many insects are known to leave lipid footprints while walking on smooth surfaces. Presumably, the deposited substances improve tarsal adhesion. In bumblebees, footprint hydrocarbons also function as scent marks that allow detection and avoidance of recently depleted flowers. I used GC-MS to detect hydrocarbons deposited by bumblebee (Bombus pascuorum) on flowers of Lamium maculatum. In addition to the plants' own cuticular lipids, extracts of corollas that had been visited by bumblebees contained odd-numbered alkenes. The amount of pentacosenes (C25H50) on corollas was linearly related to the number of bumblebee visits, with workers depositing approximately 16 ng per visit (extrapolated to a total of 65 ng of bumblebee cuticular hydrocarbons). Pentacosenes were retained on visited flowers without loss for 2 hr, and probably longer. This and results from flight cage experiments suggest that flower epicuticles retain a chemical record of pollinator visitation, including information on visiting bee species. Continuous footprint accumulation necessitates new explanations concerning the reversibility of "repellent scent marks" of bumblebees.