Fig. 64.—Maxilla of Eriocephala calthella: l, lacinia; g, galea; mx.p, maxillary palpus; st, stipes; c, cardo.—After Walter.
The second maxillæ are very much differentiated and vary greatly in the different orders, being especially modified in the haustellate or suctorial orders, notably the Hymenoptera and Diptera. In the mandibulate orders, particularly the Orthoptera, where they are most generalized and primitive in shape and structure, they consist of the following parts: the gula (a postgula is present in Dermaptera), submentum (lora of Cheshire, i, p. 91), mentum, palpifer, the latter bearing the palpi; the lingua (ligula) and paraglossæ, while the hypopharynx or lingua is situated on the upper side. The labial palpi are of the same general shape as those of the first maxillæ, but shorter, with very rarely more than three joints, though in Pteronarcys there are four. Leon has detected vestigial labial palpi in several Hemiptera (Fig. 73). As to the exact nature and limits of the gula, we are not certain; it is not always present, and may be only a differentiation of the submentum, or the latter piece may be regarded as a part of the gula.
We are disposed to consider the second maxillæ as morphologically nearly the exact equivalents of the first pair of maxillæ, and if we adopt this view it will greatly simplify our conception of the real nature of this complicated organ. The object of the fusion of the basal portion appears to be to form an under-lip, in order both to prevent the food from falling backwards out of the mouth, and, with the aid of the first pair of maxillæ, to pass it forward to be crushed between the mandibles, the two sets of appendages acting somewhat as the tongue of vertebrates to carry and arrange or press the morsels of food between the teeth or cutting edges of the mandibles.
The spines often present on the free inner edges of the first and second maxillæ (Figs. 54, 62) form rude combs which seem to clean the antennæ, etc., often aiding the tibial combs in this operation.
The submentum and mentum, or the mentum when no submentum is differentiated (with the gula, when present), appear to be collectively homologous with the cardines of the first pair of maxillæ, together with the palpifers and the stipites.[[15]] These pieces are more or less square, and have a slightly marked median suture in Termitidæ, the sign of primitive fusion or coalescence.
The most primitive form of the second maxillæ occurs in the Orthoptera and in the Termitidæ. The palpifer is either single (Periplaneta, Diapheromera, Gryllidæ) or double (Blatta orientalis, Locustidæ). In Prisopus the single piece in front of the palpifer is in other forms divided, each half (Blatta, Locustidæ, Acrydidæ) bearing the two “paraglossæ,” which appendages in reality are the homologues of the lacinia and galea of the first maxillæ.[[16]] In the Termitidæ (Fig. 65) the lingua is not differentiated from the palpifer, and the two paraglossæ (or the lamina externa and interna of some authors) with the palpus are easily seen to be the homologues of the three lobes of the first maxillæ. In the Perlidæ (Pteronarcys, Fig. 66) the palpifer is divided, while the four paraglossæ arise, as in Prisopus and Anisomorpha, from an undivided piece, the lingua not being visible from without. In the Neuroptera the lingua or ligula is a large, broad, single lobe, without “paraglossæ,” and the palpifer is either single (Myrmeleon, Fig. 67), or divided (Mantispa, Fig. 68). In Corydalus (Fig. 29) the palpifer forms a single piece, and the lingua is undivided, though lobed on the free edge.
Fig. 65.—Second maxillæ of Termopsis angusticollis: li, the homologue of the lacinia; le, galea.
In the metabolic orders above the Neuroptera the lingua is variously modified, or specialized, with no vestiges of the lacinia or galea, except in that very primitive moth, Eriocephala, in which Walter found a minute free galea, me, and an inner lobe (Figs. 76, 77), the lacinia.
