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2. Mouth Parts
These have labrum, mandibles, maxillae, labium and hypopharynx all well developed. Though of an omnivorous feeder, they are more particularly adapted for biting and tearing.
Labrum. Figs. 6, 7, 8. Borne on the ante-clypeus to which its outer surface is attached by the clypeo-labral suture (Fig. 7, Cl), it consists of a broad flat lobe with the distal margin notched in the mid line and bearing several rows of sensory papillae (Fig. 8. Sp); laterally the margin is strongly chitinized and provided with a fringe of setae (Fig. 8, Se); the outer surface has a number of basiconic sensillae scattered over it. The inner surface is thickly clothed with setae and has a median groove towards which nearly all the setae are directed; on either side of this groove, and anterior to it, is a dense mass of
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Text-fig. 2. Mouth Parts.—Fig. G—View of labrum, inner surface. Fig. 7—View of labrum, outer surface. Fig. 8—Labrum enlarged. Left half shows inner surface and right half outer surface. Fig. 9—Maxilla. Fig. 10—View of mandible, outer surface. Fig. 11—View of mandible, inner surface. Fig. 12—Hypophaivnx Fig. 13—Labium
Bt, bifid tooth; C, cardo, Cd condyle, Cl, clypeo-labial suture, Ct. G-cusp tooth, Dsp, dendritic sensory papillae, G, galea, Gj, ginglymus joint Gl; glossa; L; lacina; Lg. ligula; M, mentum; P, palp; Pa, palpiger; Pf, palpifer; Pg, bristles and basiconic sensillae; Pgl, para-glossa; Pm, prementum; S, stipes; Se, setae; Sg, subgalea, Sm; submentum; Sp, sensory papillae; St, single sub-apical tooth; T, torma.
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dendritic sensory papillae (Fig. 8, Dsp); dendritic papillae are also situated near the distal margin and are probably concerned with taste. The inner surface is continuous with the inner membranous surface of the clypeus and forms the epipharynx. The inner surface of the ante-clypeus is less thickly clothed with hairs than the labrum, and is separated from it by two small chitmous bars, the tormae (Fig. 6, T).
Mandibles. Figs. 10, 11. Very strongly chitimzed; situated one on either side of and behind the labrum; strongly toothed with an apical bifid tooth (Bt) extending one-third down, a single subapical tooth (St) extending from one-third to one-half down, and a sub-basal tooth (Ct) with six cusps between one-fifth and two-thirds up. The proximal surface carries a thick clothing of setae and a blunt projection from the posterior surface. Mandibular sclerite small, continuous with the post-clypeus, separated from the gena by the invagination forming the anterior arm of the tentorium of its side, and the subgenal suture. The mandible articulates with the head-capsule by an anterior ginglymus (Gj) and a posterior condyle (Cd). The ginglymus articulates with a concave process situated at the point where the post-clypeus and mandibular sclerite are joined. The condyle articulates with an acetabulum on the ventral margin of the occiput.
Maxillae. Fig. 9. Behind the mandibles are paired maxillae which, excluding their palps (P), are slightly larger than the labium. The cardo (C) is triangular in shape, with the length to the width as 1.1:1, and articulates with the rim bordering the lateral margin of the occipital foramen. Distal to each cardo is the stipes (S), twice as long as broad and a third as long again as the cardo. Externally each stipes carries a five-segmented maxillary palp (P), which is thickly clothed with setae and very sensitive to touch and chemical senses. Each palp arises from a palpifer (Pf), formed from the outer distal portion of the stipes by constriction from the rest as a separate lobe. First two palpal segments short, last three long, of which the terminal segment is club-shaped. The relationship of the length of each segment to the palp is 7:7:20:20:44. Proximally the stipes carries a lacinia (L) and galea (G) which are both weakly chitinised. At the tip of the lacinia is a bifid spine, and beneath this spine an articulated spine. The internal surface of the lacinia has numerous long setae. The galea consists of two joints, a proximal one, the sub-galea (Sg) which is short and slightly swollen, and a distal one which is elongated and at its tip bears an area of sensory papillae (Sp) behind which is a group of setae.
Labium. Fig. 13. The submentum (Sm), mentum (M), prementum (Pm), palp (P), ligula (Lg), glossa (Gl) and paraglossa (Pgl) are all present. The submentum is large, length related to width as 1:1:2; it articulates with the rim at the anterior margin of the occipital foramen and also with the small mentum. Mentum wider than long, as 2.3:1. The prementum, which is wider than long as 1:4:1, lies in front of the mentum and is cleft anteriorly for a third of its length. Submentum mentum prementum are related in length as 2.2:1:1.5. On each side of the prementum is a palpiger (Pa) which bears a three-segmented labial palp, the ratio of the length of each segment to the total length of the palp being 21:30:50. Both palps are clothed with setae; last segment of the palp is club-shaped and more heavily clothed with setae towards its apex, where the setae form a series of whorls all directed inwards towards the apex. Anterolateral to the prementum are two small ligulae, each of which bears a large thick
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paraglossa; at the tip of the paraglossa is an area of sensory papillae. Paired glossae are borne on the anterior border of the prementum, and are clothed with setae. They taper towards their tips, and, as in the paraglossae, have an area of sensory papillae at their anterior ends.
Hypopharynx. Fig. 12. This is an unpaired tongue-like organ which forms the lower border of the pharynx proper. It is greatly swollen, soft and spongy and is strengthened by a chitinous framework. Its posterior wall is reflected into the ventral wall of the labium at the base of the prementum, from where it is produced forwards into a lobe which lies above the glossae and paraglossae; beneath this lobe is the opening of the salivary duct. Near the base of the hypopharynx on each side is a lobe bearing bristles and basiconic sensillae (Pg). According to Snodgrass (1928) these setae are supposed to be pervious to chemical stimulae and are regarded as receptors of taste or odour.