Anatomical Considerations

Mouthpart elements that make up the proboscis vary considerably among insect taxa. In Hymenoptera, where nectar feeding has evolved independently multiple times, proboscis morphology is similarly diverse. Most frequently, the hymenopteran proboscis is formed by basally linked maxillary and/or labial components, known as the labiomaxillary complex. In the "long-tongued" bees (Apidae + Megachilidae), the proboscis is composed of the elongated galeae and labial palps that together form the food canal surrounding the long and hairy glossa (Figure 9.2) [41]. In some

TABLE 9.1

Principal Composition and Maximal Reported Proboscis Length of the Proboscides of Selected Nectar Feeders

Taxon

Coleoptera

Meloidae (blister beetles) Nemognathinaea

Hymenoptera

Apidae Bombini (bumblebees) Bombus hortorum

Euglossini (orchid bees) Eufriesea ornata

Colletidae ("short-tongued" bees) Niltonia virgili

Vespidae Masarinae (pollen wasps) Ceramius metanotalis

Lepidoptera

Sphingidae (hawkmoths) Amphimoea walkerib

Riodinidae (metalmark butterflies) Eurybia lycisca

Diptera

Tabanidae (horseflies)

Corizoneura longirostris

Nemestrinidae (tangle-veined flies) Moegistorynchus longirostris

Bombyliidae (beeflies) Bombylius major

Syrphidae (hoverflies) Rhingia campestris

Chiroptera

Phyllostomidae (leaf-nosed bats) Choeronycteris mexicana

Proboscis Components

Galeae or maxillary palps

Galeae, glossa, labial palps

Galeae, glossa, labial palps Labial palps

Glossa

Galeae Galeae

Labrum/epipharynx, hypopharynx, mandible stylets, lacinia, labium; distally labium alonec Labrum/epipharynx, hypopharynx, lacinia, labium; distally labium alone Labrum/epipharynx, hypopharynx, maxillary structures, labium Labrum/epipharynx, hypopharynx, maxillary structures, labium

Length (mm)

280 45

12.5

10.5

Ref.

133 43

H.W. Krenn, unpublished

Tongue

(continued)

TABLE 9.1 (CONTINUED)

Principal Composition and Maximal Reported Proboscis Length of the Proboscides of Selected Nectar Feeders

Length

Taxon Proboscis Components (mm) Ref.

Aves

Trochilidae (hummingbirds)

Ensifera ensifera Mandibles and tongue 91d 136

a No detailed studies are available. b World record holder in proboscis length. c Piercing blood feeding and nectar feeding in females. d Functional proboscis length may exceed reported bill length.

TABLE 9.2

Evolutionary Transitions to Specialize Suction Feeding in Some Nectar-Feeding Insect

Taxon

Coleoptera

Meloidae

Ancestral Feeding Mode

Biting/chewing on various floral food sources

Derived Taxon Ref.

Nemognatha, Leptopalpus 36

Hymenoptera

Apidae Vespidae

Lapping nectar feeding Euglossini

Lapping nectar feeding Masarinae

Lepidoptera

Glossata

Diptera

Culicidae

Nemestrinidae

Tabanidae

Bombyliidae

Empididae

Syrphidae

Suction feeding of nonfloral plant fluid

Piercing blood feeding females Unknown

Piercing blood feeding females Mopping up fluid feeding Predatory insect feeding Nectar and pollen feeding

Eulepidopteraa

Toxorhynchites

Nemestrinidaeb

Corizoneurac

Bombylius

Empis

Rhingia a Secondarily nonfeeding in several taxa. b Unknown whether all are suction-feeding flower visitors. c Proboscis of females specialized to both nectar and blood feeding.

39, 137

FIGURE 9.2 (A) Head and extended proboscis of Melipona sp. (Hymenoptera: Apidae); proboscis consists of galeae (ga), labial palps (lp), and glossa (gl). (B) Close up of the glossal tip.

"long-tongued" bees, even basal elements of the mouthparts have a significant influence on a bee's functional tongue length [42]. Remarkably, one group of "short-tongued" bees (Colletidae, Niltonia), which feeds on deep Jacaranda flowers in the New World tropics, has a proboscis that approaches its body length but is composed of the labial palps alone [43]. Another group of colletid bees has a proboscis formed mostly from the concave maxillary palps [27,44]. In long-tongued pollen wasps (Vespidae: Masarinae), the proboscis and food canal are formed from the glossa alone [36]. There are many other compositions found in various groups of Hymenoptera, including Braconidae, Sphecidae, and even in Tenthredinoidea. Overviews on the occurrence and principal compositions are given in Jervis [28], Jervis and Vilhelmsen [30], and Krenn, Plant, and Szucsich [27].

In contrast to mouthpart diversity exhibited by Hymenoptera, the proboscides of all "higher" Lepidoptera consist only of the two maxillary galeae enclosing the food canal (Figure 9.3) [20,39,40].

Most Diptera have sponging and sucking mouthparts that are similar in composition but with highly variable lengths. Their proboscis is complex, consisting of an elongated labrum-epipharynx unit and a hypopharynx, which, sometimes together with rodlike maxillary structures, form the food canal and are enclosed by the gutter-shaped labium. The paired labellae (a homologue to the labial palps of other insects) at the apical end protrude from the proboscis (Figure 9.4) [41]. Adaptations to nectar feeding include elongation of the whole functional unit, a simplified composition of the food canal formation, and a slender labellae [27,34].

The long suctorial proboscis of the typical nectar-feeding insect is characterized by a tightly sealed food canal (Figures 9.5A, 9.5B, and 9.5C), a specialized tip region

FIGURE 9.3 (A) Spirally coiled proboscis (p) of Vanessa cardui (Lepidoptera: Nymphalidae) in lateral view; tip region (tr). (B) Proboscis tip slits into food canal formed by extended galeal-linking structures; sensilla styloconica (s) are characteristic sensory organs of the lepidopteran proboscis.
FIGURE 9.4 (A) Head of Physocephala rufipes (Diptera: Conopidae) with proboscis (p) tip projecting forward in resting position. (B) Labella (la) of proboscis tip.
50 |m

FIGURE 9.5 Cross-sections of the feeding canals (fc) of some nectar feeding insects. (A) In Volucella bombylans (Diptera: Syrphidae), food canal is formed by groove and tongue junction of labrum-epipharynx unit (lb) and the hypopharynx (h); labium (l) surrounds the other proboscis components. (B) In Pieris brassicae (Lepidoptera: Pieridae) the galeae (ga) interlock on the dorsal and ventral margins to enclose the central food canal. Dorsal linkage (dl) consists of overlapping platelets sealed by gland cell (gc) substances; ventral linkage (vl) is formed by cuticular hooks. (C) Overlapping cuticular structures of the glossa (gl) form the food canal in Ceramius hispanicus (Hymenoptera: Vespidae: Masarinae). (D) Food canal is formed from the galeae (ga) and labial palps (lp) in Euglossa sp. (Hymenoptera: Apidae: Euglossini), and is disengaged in the resting position.

FIGURE 9.5 Cross-sections of the feeding canals (fc) of some nectar feeding insects. (A) In Volucella bombylans (Diptera: Syrphidae), food canal is formed by groove and tongue junction of labrum-epipharynx unit (lb) and the hypopharynx (h); labium (l) surrounds the other proboscis components. (B) In Pieris brassicae (Lepidoptera: Pieridae) the galeae (ga) interlock on the dorsal and ventral margins to enclose the central food canal. Dorsal linkage (dl) consists of overlapping platelets sealed by gland cell (gc) substances; ventral linkage (vl) is formed by cuticular hooks. (C) Overlapping cuticular structures of the glossa (gl) form the food canal in Ceramius hispanicus (Hymenoptera: Vespidae: Masarinae). (D) Food canal is formed from the galeae (ga) and labial palps (lp) in Euglossa sp. (Hymenoptera: Apidae: Euglossini), and is disengaged in the resting position.

(Figures 9.2B, 9.3B, and 9.4B), and a powerful suction pump (Figure 9.6 and Figure 9.7). These features are integral to the functioning of the proboscis and must be considered in detail before biomechanical generalizations can be developed.

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