_Functions of the Antennæ and Mouth-parts._ We must now shortly consider the functions of the parts just described. The antennæ have long been regarded as sense-organs, and even the casual observer can hardly fail to remark that they are habitually used by the Insect to gain information concerning its immediate surroundings. Long antennæ, such as those of the Cockroach, are certainly organs of touch, but it has been much disputed whether they may not also be the seat of some special sense, and if so, what that sense may be. Several authors have found reason to suppose that in the Insect-antenna resides the sense of hearing, but no evidence worth the name is forthcoming in favour of this view. Much better support can be found for the belief that the antenna is an olfactory organ,[52] and some experiments which seem conclusive on this point will be cited in a later chapter. [52] Rosenthal, Ueb. d. Geruchsinn der Insekten. Arch. f. Phys. Reil u. Autenrieth, Bd. X. (1811). Hauser, Zeits. f. wiss. Zool., Bd. XXXIV. (1880). In the Cockroach the mandibles and maxillæ are the only important instruments of mastication. The labium is indirectly concerned as completing the mouth behind and supporting the lingua, which is possibly of importance in the ordinary operations of feeding. Plateau[53] has carefully described the mode of mastication as observed in a Carabus, and his account seems to hold good of biting Insects in general. The mandibles and maxillæ act, as he tells us, alternately, one set closing as the others part. The maxillæ actually push the morsel into the buccal cavity. When the mandibles separate, the head is slightly advanced, so that the whole action has some superficial resemblance to that of a grazing quadruped. [53] Mém. Acad. Roy. de Belgique, Tom. XLI. (1874). Prof. Plateau’s writings will often be referred to in these pages. We owe to him the most important researches into the physiology of Invertebrates which have appeared for many years. The palps of the maxillæ and labium have been variously regarded as sensory and masticatory instruments. Not a few authors believe that they are useful in both ways. The question has lately been investigated experimentally by Plateau,[54] who finds that removal of both maxillary and labial palps does not interfere either with mastication or the choice of food. He observes that in the various Coleoptera and Orthoptera submitted to experiment the palps are passive while food is being passed into the mouth. [54] Exp. sur le Rôle des Palpes chez les Arthropodes Maxillés. Pt. I. Bull. Soc. Zool. de France, Tom. X. (1885).