PERIODICAI. CICADA
the tongue (Hphy) to pass to either side of the latter organ, but the channel between them here becomes con- tinuous with a groove on the middle of the forward sur- face of the tongue. When the mouth-opening is closed, as it always is in the fully matured insect, the tongue groove is converted into a tube which leads upward from the channel between the second bristles into the inner cavity of the mouth. It is through this minute passage that the cicada obtains its liquid food; but obviously there must be a pumping apparatus to furnish the sucking force. The sucking mechanism is the mouth cavity and its muscles. The mouth cavity, as seen in a section of the head (Fig. ? 22, Pmp), is a long, oval, thick-walled capsule having its roof, or anterior wall (e), ordinarily bent inward so far as almost to fill the cavity. Upon the midline of the roof is inserted a great mass of muscle fibers (Pmp:$lcl«) that have their other attachment on the striated plate of the face (Clp). The contraction of these muscles lifts the roof, and the vacuum thus created in the cavity of the mouth sucks up the liquid food. Then the muscles relax, and the elastic roof again collapses, but the lower end comes down first and forces the liquid upward through the rear exit of the mouth cavity into the pharynx, a small muscular-walled sac (P]zy) lying in the back of the head. From the pharynx, the food is driven into the. tubular gullet, or oesophagus (OE), and so on into the stomach. The bases of both pairs of bristles are retracted into pouches of the lower head wall behind the tongue, and tapon each bristle base are inserted sets of protractor and retractor muscle fibers. By means of these muscles, the bristles can be thrust out from the tip of the beak or withdrawn, and the bristles of the stronger first pair are probably the chier organs with which the insect punctures the tissues of the plant on which it feeds. As the bristles enter the wood, the sheath of the beak can
INSECTS
