Fig. 25.—Mouth-parts of Gad-fly (Tabanus).
Fig. 25A.—Diagram of Mouth-parts of Gad-fly.
In Diptera both piercing and sucking parts are usually present. The Gad-fly (fig. 25) is typical. Here we recognise the labrum (Lbr), mandible (Mn), and maxilla (Mx′) of the Cockroach transformed into stylets. The maxillary palp (Mxp) is still sensory. A pointed process, stiffened by chitinous ribs, is developed from the back of the labrum. This is the epipharynx (Ep), a process undeveloped in the Cockroach, though conspicuous in some Coleoptera. All these parts are overtopped by the suctorial labium (Lm), which has a two-lobed expansion at the end. In the more specialised Diptera this becomes a kind of cupping-glass. The Gad-fly is intermediate between the Gnat, in which all the mouth-parts are converted into piercing organs of extraordinary length and sharpness, and such flies as the House-fly and Blow-fly, where the sucking labium forms an organ of the most elaborate kind, the piercing organs undergoing a marked reduction. Except where the labium is short, it is doubly or trebly hinged, so that it can be readily tucked away under the chin.
In Hemiptera the long four-jointed labium (Lm) forms a sheath for the stylets. When not in use the whole apparatus is drawn up beneath the head and prothorax. The mandibles (Mn) are sharp at the tip, and close like a pair of forceps, enclosing the maxillæ (Mx). These are of unequal length, only one reaching the end of the mandibular case. Both have saw teeth on the free edge. Palps are entirely wanting.