Tip Region

The presence of a fluid-tight food tube requires a specially adapted tip, which must interact with the fluid surface. The tips of lapping and sucking mouthparts of many Hymenoptera are characterized by their hairy glossae (Figure 9.2A). In some long-tongued bees, the glossa is extended just beyond the food canal, and nectar is loaded between extendible hairs by capillary forces (see Section 9.3.2). The lapping movement of the glossa is mediated by muscles that originate on the basal sclerites of the labium and insert at the glossal base. When these muscles relax, the glossa extends because of the elasticity of the glossal rod [42,47,48]. Contraction of these muscles draws the proximal end of the glossal rod into an S-shaped position. As a result, the glossa retracts between the galeae and the labial palps [42]. It is unknown whether nectar is unloaded either by "squeezing" the glossa [49,50] or via suction pressure generated in the cibarial chamber [25]. For suction-feeding euglossine bees, the glossa no longer plays an active role in fluid transport [31]. In short-tongued pollen wasps, the glossa is employed in lapping, whereas in long-tongued taxa, the modified glossa serves as the actual suction tube (Figure 9.5C) [32]. In long-tongued pollen wasps, arched cuticle structures form an incomplete food canal in the bifurcated tip region of the glossa. More proximally, these flattened structures overlap to form a tightly closed food tube (Figure 9.5B) [32].

The flexible tip region of the lepidopteran proboscis has been modified to permit fluid uptake into the otherwise tightly closed food tube. Terminal ends of the galeae are characterized by rows of slits leading into the food canal (Figure 9.3B). There, the galeal-linking structures are arched and elongated, not tightly sealing the food canal; instead, they interlock only at their tips with those of the opposite galea. Because of their curved and extended shape, a slit is formed between consecutive structures. These slits are found on the dorsal side of the proboscis tip in a region that makes up 5 to 20% of the total proboscis length [39,51-53]. Because there is no apical opening into the food canal, the intake slits of the tip region must be immersed into the fluid prior to sucking. The tip region is further characterized by rows of combined contact chemomechanical sensilla [54-56]. Each of these sensilla consists of a variably shaped stylus and short apical sensory cone (Figure 9.3B). Their shape and arrangement are correlated to some extent with butterfly feeding ecology [51,53,57]. When the butterfly feeds from a surface, the fluid adheres to these structures, forming a droplet that is then ingested [58]. In Lepidoptera with particularly long proboscides (e.g., Papilio and Sphinx), these sensillae are short and barely extend over the surface [51], suggesting that they are adapted to work within the narrow confines of the tubular flowers these insects visit.

The proboscis tip region of brachyceran Diptera has paired movable and variously shaped labellae [34,59] that contact nectar on their inner surface; that surface is equipped with an elaborate system of tiny cuticular channels known as the pseudo tracheae (Figure 9.4B). Pseudotracheae distribute saliva over the labellae [60], helping to dissolve nutrients and dilute dried up nectar (see Section 9.3.3). In unspecialized flies, labellae tend to be broad and cushionlike, equipped with a comblike arrangement of pseudotracheae [34,59]. In nectar-feeding hoverflies and beeflies, the labellae are slender and elongate, and the number of pseudotracheal channels is reduced [19,34]. In other nectar-feeding flies (e.g., Conopidae), they are also short and slender, not exceeding the diameter of the labium (Figure 9.4) [59]. In all, the pseudotracheal system forms an extension of the food canal, and pure suction feeding is likely in all those that feed from tubular flowers where spreading of the labellae is impaired.

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