Fig. 62.—Larva of Rhagium lineatum: lat, lateral view of head and thoracic segments; mx, first maxilla; ml, undifferentiated lacinia and galea; v, under side of head and pro- and meso- thoracic segments; v.m.s., one of the middle ventral segments, magnified six times; mx′, 2d maxilla.
It now seems most probable that in the first maxillæ we have the primary form of buccal appendage of insects, the appendage being composed of three basal pieces with three variously modified distal lobes or divisions; and that the mandibles and second maxillæ are modifications of this type.
How wonderfully the maxillæ of the Lepidoptera are modified, and the peculiar shapes assumed in the Diptera, Hymenoptera, and other groups, will be stated in the accounts of those orders, but it is well to recall the fact that in the most primitive and generalized moth, Eriocephala, the lacinia is well developed (Fig. 64).
As Newport remarks, the office of the maxillæ in the mandibulate insects is of a twofold kind; since they are adapted not only for seizing and retaining the food in the mouth, but also as accessory jaws, since they aid the mandibles in comminuting it before it is passed on to the pharynx and swallowed. Hence, as the food varies so much in nature and situation, it will be readily seen that the maxillæ, especially their distal parts, vary correspondingly. Thus far no close observations on the exact use of the first and second maxillæ have been published.
The palpi also are not only organs of touch, but in some cases act as hands and also bear minute sense-organs, the function of which is unknown, but would appear to be usually that of smell.
Fig. 63.—Selandria larva, common on Carya porcina, with details of mouth-parts: leg, leg; mx, maxilla; gal, galea; lac, lacinia.
The second maxillæ.—The “under-lip” or labium of insects is formed by the fusion at the basal portion of what in the embryo are separate appendages, and which arise in the same manner as the first maxillæ. They are invariably solidly united, no cases of partial or incomplete fusion being known. The so-called labium is situated in front of the gula or gular region, and is bounded on each side by the gena, or cheek. As already observed, the second maxillæ appear to be the appendages of the last or occipital segment of the head.
