Fluid Pumps

Fluid pumps (Figures 9.6 and 9.7) create the pressure gradient required for imbibing nectar through the slender proboscis. In series with the food canal, these pumps are located in the head and are formed mainly by the cibarium. In Diptera, however, fluid feeding involves an interplay of successive suction pumps that enlarge subsequent sections of the food pathway through the mouthparts and the foregut [19,60-62]. Fluid pumps are not restricted to obligatory nectar-feeding insects because all fluid-feeding insects possess similar pump organs to consume liquid nutrients.

The functional anatomy of suction pumps has been studied in detail in butterflies (Figure 9.7) [20,63]. Contractions of dilator muscles enlarge the cibarium, and at the same time, a ring of muscles in the foregut closes the connection into the pharynx. When the pump lumen is enlarged, nectar is drawn in from the food tube. Subsequently, the entrance of the pump is sealed by a flaplike valve structure, and circularly arranged muscles, which form the wall of the cibarial pump, contract, thus forcing fluid into the opened pharynx. Based on video analysis of air bubbles in the food canal, the dilation-contraction cycle in a pierid butterfly occurs approximately once per second [64]. In addition, electrophysiological measurements have shown that contraction frequencies range from 4 Hz in the nectar-feeding ant, Camponotus mus [65], to 6 Hz in a hematophagous bug, Rhodnius prolixus [66].

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