THE HEAD AND ITS APPENDAGES
a. The head
Fig. 24.—Presumed larva of Nemoptera (Necrophilus arenarius), Pyramids of Egypt.—After Roux, from Sharp.
While the head is originally composed of probably not less than six segments, these are in the adult insect fused together into a capsule or hard chitinous box, the epicranium, with no distinct traces of the primitive segments. The head contains the brain and accessory ganglia, the mouth or buccal cavity, also the air-sacs in many winged forms, and gives support to the external organs of sense, the antennæ, and to the buccal appendages, the larger part of the interior being filled with the muscles moving these structures. The solid walls of the head serve as a lever or support for the attachment of these muscles, especially those of the mandibles. Thus there is a correlation between the large size of the mandibles of the soldier white ants and ants, the head being correspondingly large to accommodate the great mandibular muscles. The other extreme is seen in the larva of Necrophilus (Fig. 24), with its long slender neck and diminutive head.
The clypeus.—This is that part of the head situated in front of the epicranium, and anterior to the eyes, forming the roof of the posterior part of the mouth, and is, as embryology shows, probably a tergal sclerite. It varies greatly in shape and size in the different orders of insects. It is often divided into two parts, the clypeus posterior and clypeus anterior, or which may be designated as the post- and ante-clypeus (Figs. 29, B).
The labrum.—The “upper lip” or labrum is an unpaired flap-like piece hinged to the front edge of the clypeus, and may be seen to move up and down when the insect moves its mandibles. It forms the roof of the anterior part of the mouth (Figs. 69, 74), and its inner side is lined with a soft membrane, usually provided with hairs and sense-papillæ or cups, forming the epipharynx.
The labrum is more or less deeply bilobed, especially in caterpillars and in adult Staphylinidæ, and has been thought by some writers (Kowalevsky, Carrière, and also Chatin) to represent a pair of appendages, but Heymons (1895) refutes this view, stating as his reason that the labrum arises between the two halves of the nervous system (protocerebrum), while all the true appendages arise on each side of the nervous system. (See also Fig. 34.)
Fig. 25.—Front view of the head of C. spretus: E, epicranium; C, clypeus; L, labrum; O, o, ocelli; e, eye; a, antenna; md, mandible; mx, portion of maxilla uncovered by the labrum; p, maxillary palpus; p′, labial palpus.
In the fleas (Siphonaptera) both the clypeus and labrum are wanting.
While it apparently forms an anterior specialized portion of the procephalic lobes, Viallanes regarded it as belonging to the third, or his tritocerebral, segment, since the labral nerves arise from the tritocerebral ganglia. But since in all the early as well as late stages of embryonic life it appears to be situated in front of the mouth, it would seem to belong to the first segment.
In the embryo of Blatta it first appears as a thick crescentic fold being slightly divided anterior to the mouth, and in Doryphora it appears as a heart-shaped or deeply bilobed prominence situated in front of the mouth (Wheeler).
The epipharynx and labrum-epipharynx.—The epipharynx is the under surface or pharyngeal lining of the clypeus and labrum, forming the membranous roof of the mouth. As it contains the organs of taste and has been generally overlooked by entomologists, we may dwell at some length on its structure in different orders.
Réaumur was, so far as we have been able to ascertain, the first author to describe and figure the epipharynx, which he observed in the honey bee and bumble bee, and called la langue, remarking that it closes the opening into the œsophagus, and that it is applied against the palate. According to Kirby and Spence, De Geer described the epipharynx of the wasp; and Latreille referred to it, calling it the sous labre.
The name epipharynx was bestowed upon this organ by Savigny, who thus speaks of that of the bees: “Ce pharynx est, à la vérité, non seulement caché par la lèvre supérieure, mais encore exactement recouvert par un organe particulier que Réaumur a déjà décrit. C’est une sorte d’appendice membraneux qui est reçu entre les deux branches des mâchoires. Cette partie ayant pour base le bord supérieur du pharynx, peut prendre le nom d’épipharynx ou d’épiglosse.”
He also describes that of Diptera. What Walter has lately proved to be the epipharynx of Lepidoptera was regarded by Savigny and all subsequent writers as the labrum.
The latest account of the function of this organ is that by Cheshire, who states that the tube made by the maxillæ and labial palpi cannot act as a suction pipe, because it is open above. “This opening is closed by the front extension of the epipharynx, which closes down to the maxillæ, fitting exactly into the space they leave uncovered, and thus the tube is completed from their termination to the œsophagus.”
Fig. 26.—Epipharynx of Phaneroptera angustifolia: cl, clypeus; lbr. e, labrum-epipharynx; t c, taste cups, both on the clypeal and on the labral regions.
Fig. 27.—Epipharynx of Hadenœcus subterraneus, cave cricket.
It is singular that this organ is not mentioned in Burmeister’s Manual of entomology, in Lacordaire’s Introduction à l’entomologie, or by Newport in his admirable article Insecta in Todd’s Cyclopedia of anatomy. Neither has Straus-Durckheim referred to or figured it in his great work on the anatomy of Melolontha vulgaris.
In their excellent work on the cockroach, Miall and Denny state that “The epipharynx, which is a prominent part in Coleoptera and Diptera, is not recognizable in Orthoptera” (p. 45). We have, however, found it to be always present in this order (Figs. 26, 27).
We are not aware that any modern writers have described or referred to the epipharynx of the mandibulate orders of insects. Although Dr. G. Joseph speaks of finding taste-organs on the palate of almost every order of insects, especially plant-feeding forms, we are unable to find any specific references, his detailed observations being apparently unpublished.
The epipharynx is so intimately associated with the elongated labium of certain Diptera, that, with Dr. Dimmock, we may refer to the double organ as the labrum-epipharynx; and where, as in the lepidopterous Micropteryx semipurpurella, described and figured by Walter, and the Panorpidæ (Panorpa and Boreus), the labrum seems pieced out with a thin, pale membranous fold which appears to be an extension of the epipharynx, building up the dorsal end of the labrum, this term is a convenient one to use.
In the lower orders of truly mandibulate insects, from the Thysanura to the Coleoptera, excluding those which suck in liquid food, such as the Diptera, Lepidoptera, and Hymenoptera, and the Mecoptera (Panorpidæ) with their elongated head and feeble, small mandibles, the epipharynx forms a simple membranous palatal lining of the clypeus and labrum. In such insects there is no soft projecting or pendant portion, fitted to close the throat or to complete a partially tubular arrangement of the first and second maxillæ.
In all the mandibulate insects, then, the epipharynx forms simply the under surface or pharyngeal lining of the clypeus and labrum, the surface being uniformly moderately convex, and corresponding in extent to that of the clypeus and labrum, posteriorly merging into the palatal wall of the pharynx; the armature of peculiar gathering-hairs sometimes spreading over its base, being continuous with those lining the mouth and beginning of the œsophagus. The suture separating the labrum from the clypeus does not involve the epipharynx, though since certain gustatory fields lie under the front edge of the clypeus, as well as labrum, one may in describing them refer to certain fields or groups of cups or pits as occupying a labral or clypeal region or position.
The lack of traces of a suture in the epipharynx corresponding to the labral suture above, suggests that the labrum does not represent a pair of coalesced appendages, and that it, with the clypeus, simply forms the solid cuticular roof of the mouth.
The only soft structures seen between the epipharynx and labrum, besides the nerves of special sense, are the elevator muscles of the labrum, and two tracheæ, one on each side.
The structure and armature of the epipharyngeal surface even besides the taste-pits, taste-cups and rods, is very varied, the setæ assuming very different shapes. There seem to be two primary forms of setæ, (1) the normal forms which arise from a definite cell; and (2) soft, flattened, often hooked hairs which are cylindrical towards the end, but arise from a broad triangular base, without any cell-wall. These are like the “gathering hairs” of Cheshire, situated on the bees’ and wasps’ tongue; they also line the walls of the pharynx and extend toward the œsophagus. They are also similar to the “hooked hairs” of Will. The first kind, or normal setæ, are either simply defensive, often guarding the sense-cups or sensory fields on which the sense-cups are situated, or they have a nerve extending to them and are simply tactile in function.
The surface of the epipharynx, then, appears to be highly sensitive, and to afford the principal seat of the gustatory organs, which are described under the head of organs of taste.