In insects that feed on fluids, the first section of the alimentary canal is often adapted to pump fluids out of the food and into the insect. The robber flies, (Asilidae) feed on other insects by piercing them with sharp mouthparts, injecting digestive fluid and sucking in the digested and liquified body tissues. The robber flies have a dual pump mechanism. The pre-oral cavity (cibarium) is musculated and adapted for the initial pumping. (Mosquitoes also use their cibarium to pump a blood meal.) Muscles in the head compress and expand the cavity in a way that pumps food. The second pump is a modification of the pharynx, the first section of the alimentary canal. The pharynx is a swelling in the alimentary canal that is heavily musculated. Powerful circular muscles ring the outside of the pharynx. These muscles contract the pharynx to force food into and through the esophagus. Longitudinal muscles on the outside of the pharynx cause the pharynx to expand and draw another round of liquid. The cibarium and the pharynx function as a two stage pump. The cibarial pump lifts food out of the prey and into a cavity in reach of the pharyngeal pump. The pharynx pumps the food into the insect.
Why do robber flies have a dual pump when many other sucking insects have only one? Extra-oral digestion by robber flies may be incomplete and contain particles of soft tissue as well as liquid that require an extra boost going through the narrow food canal into the crop.