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. [Illustration: Fig. 26.--Mouth-parts of Bug. Copied from Landois, Zeits. f. wiss. Zool., Bd. XVIII., taf. xi., fig. 3.] [Illustration: Fig. 26A.--Diagram of Mouth-parts of Bug.] Comparing the four kinds of suctorial mouths, of which the Bee, the Moth, the Fly, and the Bug furnish examples, we observe that the sucking-tube is formed in the Moth out of the two maxillæ, in the other three out of the labium. Of these last the Bee has the edges of the labium turned _down_, so that the siphon becomes _ventral_; in the Bug and Fly the edges are turned _up_, and the siphon becomes _dorsal_. The more specialised flies have the simple arrangement of the Bug complicated by a system of branching tubes, which are probably a special modification of the salivary duct. Similar as the mouth-parts of the four types may be in regard to their mode of working, they cannot be reduced to any common plan which differs materially from that presented by the jaws of the Cockroach. _Composition of Head._