Proboscis Mobility and Floral Handling

The insect proboscis is a deployable structure. During nectar feeding, the position of the proboscis ranges from being directed anteriorly or held perpendicular to the main body axis (Figure 9.1). When not in use, the proboscis is stowed, probably to reduce body drag during flight and possible force asymmetries generated during flight maneuvers (Table 9.3; Figures 9.3A, 9.4A, and 9.6). In many Diptera and Hymenoptera, the proboscis is flexed under the head and body where the tip projects anteriorly or posteriorly. In most taxa, this flexion is accompanied by partial or complete retraction of the proboscis into the labium or head capsule. A number of unique resting positions correspond with these myriad proboscis morphologies. Long-tongued pollen wasps have evolved a rather unique and extreme solution to the problem of proboscis storage. In contrast to short-tongued pollen wasps where the glossa is flexed outside and in front of the head, long-tongued pollen wasps possess a modified basal glossa joint, which allows a double 90° flexion, effectively retracting the glossa in a backward loop under the basal labium sclerite (Figure 9.6). This strongly arched mouthpart sclerite forms a pouchlike formation wherein the folded glossal rod fits and structures forming the food canal are retracted. In extremely long-tongued pollen wasps, the labium actually forms a saclike protrusion posterior to the head wherein the retracted glossa lies [36]. The spirally coiled resting position of the lepidopteran proboscis (Figure 9.3A) is unique among nectar-feeding insects. This space-saving posture may be one reason why the longest proboscides evolved in this group. Recoiled primarily by intrinsic galeal musculature [21, 22], the proboscis fits under the head and between the labial palps, where it locks itself

TABLE 9.3

Resting Positions in Selected Nectar-Feeding Insects with Long Proboscides

TABLE 9.3

Resting Positions in Selected Nectar-Feeding Insects with Long Proboscides

Resting Position of Proboscis

Representative Taxa

Ref.

Flexed under body, tip pointing

Nemognatha, Leptopalpus

36, 38

backward

(Coleoptera: Meloidae)

Flexed under body and partly retracted,

Long-tongued Apoidea

42, 47

tip pointing backward

(Hymenoptera)

Prosoeca (Diptera: Nemestrinidae)

N.U. Szucsich,

personal

communication

Flexed under head, tip pointing forward

Corizoneura (Diptera: Tabanidae)

37

Folded under the head and partly

Rhingia (Diptera: Syrphidae)

34, 60

retracted, tip pointing forward

Bombyliidae (Diptera)

35

Conopidae (Diptera)

59

Tachinidae (Diptera)

59

Fully retracted loop in labium, tip

Masarina (Hymenoptera: Vespidae)

32

pointing forward

Spiral of three to seven coils under head

Glossata (Lepidoptera)

45

using the elasticity of the spirally coiled galeae without the need of further muscle action [45].

The time an insect spends deploying the proboscis and handling floral structures decreases foraging profitability, and a number of adaptations allow nectar feeders to minimize floral-handling time. Hummingbirds, nectar-feeding bats, and certain insects frequently hover when probing flowers, probably reducing floral access times [78] while simultaneously reducing possible predation risks [79]. Many long-pro-boscid insects partially extend their proboscis before landing, but others extend it after landing, thus making proboscis extension a rather cumbersome process. In bees, cranial muscles of the labiomaxillary complex unfold the proboscis by moving basal components anteriorly [80], a design that requires a substantial amount of space. In bumblebees, long proboscides may be a hindrance owing to the need to rear the head backward prior to proboscis insertion into the corollae [81]. In long-tongued euglossine bees, this process reaches comical proportions as they fumble to extend their ungainly tongues while barely hanging onto the petals of a Costus flower. By contrast, long-tongued pollen wasps are able to immediately extend their proboscis into narrow corolla tubes after landing since the glossa is propelled forward from its internally looped resting position [32].

Proboscis movements are well-studied in butterflies. After uncoiling the proboscis with a hydraulic mechanism [45,82,83], the proboscis assumes a flexed position during feeding that permits easy adjustment to various corolla lengths. Probing movements are controlled by this hydraulic mechanism in addition to high cuticular flexibility, proboscis musculature, and accompanying sensory equipment [45,55]. Elevation of the entire proboscis, combined with extension and flexion of the distal parts, leads to rapid and precise probing movements without whole body movements. These probing movements are likely to be advantageous in handling inflorescences [45,64].

The comparison of bombyliid flies with short and long proboscides indicates that the same principal mechanisms govern their proboscis movements. One remarkable innovation in long-proboscid bombyliid species is their ability to take up nectar from laterally open flowers with the proboscis directed anteriorly but without fully extending it or spreading the labellae [19,35].

Nectar-feeding insects are typically generalist pollinators, and there is little evidence to support the partitioning of floral resources on the basis of proboscis length alone [11,14-16]. Not surprisingly, animals with longer mouthparts are able to access deeper flowers, but the specificity of these relationships often depends on other aspects of plant and pollinator morphology [84-86]. In hummingbirds, foraging efficiency is influenced by the match between corolla and bill morphologies [70-72], and in bumblebees, there is some evidence to suggest that efficiency is maximized when foragers visit flowers matching their tongue length [14,17,18]. Unfortunately, because of a lack of comparative foraging studies, there are few data to address the relationship between handling time, feeding modality, and proboscis length in other insects. However, because insects with long proboscides tend to follow foraging traplines on a few nectar-rich resources [87], fluid-handling times may be more significant than probing times.

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