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Réaumur. Mémoires pour servir à l’histoire des insectes, v, 1740, p. 318, Pl. 28, Figs. 4, 7, 8, 9, 10, 11 l.

Kirby and Spence. Intr. to entomology, iii, 1828, p. 457.

De Geer. ii, 1778; v, 26, Fig. 11, M.

Kirby and Spence. Pl. xii, Fig. 2 K.

Latreille. Organisation extérieure des insectes, p. 184. (Quoted from Kirby and Spence.)

Savigny. Mémoires sur les animaux sans vertèbres. Partie I^re, 1816, p. 12.

Walter, Alfred. Beiträge zur Morphologie der Schmetterlinge. Erster Theil. Zur Morphologie des Schmetterlingsmundtheile. (Jena. Zeits., xviii, 1885, p. 752.)

Cheshire, F. R. Bees and bee-keeping, i, London, 1886, p. 93.

Joseph, Gustav. Zur Morphologie des Geschmacksorganes bei Inseckten. (Amtlicher Bericht der 50 Versammlung deutscher Naturforscher u. Artzte in München. 1877, pp. 227, 228.)

Dimmock, George. The anatomy of the mouth-parts and of the sucking apparatus of some Diptera, 1881. (Also in Psyche, iii, pp. 231–241, Pl. 1, 1882.)

Packard, A. S. On the epipharynx of the Panorpidæ. (Psyche, 1889, v, pp. 159–164.)

—— Notes on the epipharynx and the epipharyngeal organs of taste in mandibulate insects. (Psyche, v, pp. 193–199, 222–228, 1889.)

Attachment of the head to the trunk.—The head is either firmly supported by the broad prothoracic segment in Orthoptera, many beetles, etc., into which it is more or less retracted, or it is free and attached by a slender neck, easily turning on the trunk, as in dragon-flies, flies, etc. In some insects there are several chitinous plates, situated on an island in the membrane on the under side of the neck; these are the “cervical sclerites” of Sharp, occurring “in Hymenoptera, in many Coleoptera, and in Blattidæ.”

The basal or gular region of the head.—At the hinder part of the head is the opening (occipital foramen) into the trunk. The cheek (gena) is the side of the head, and to its inner wall is attached the mandibular muscle; it thus forms the region behind the eye and over the base of the mandibles. In the Termitidæ, where the head is broad and flat, it forms a distinct piece on the under side of the head bounding the gulo-mental region (Fig. 28). In the Neuroptera (Corydalus, Fig. 29, and Mantispa, Fig. 30) it is less definitely outlined.

Fig. 29.—Head of Corydalus cornutus, ♂: A, from above. B, from beneath. C, from the side. a. cly, clypeus anterior; p. cly, clypeus posterior; lbr, labrum; md, mandible; mx, base of first maxilla; mp, its palpus; m, mentum; sm, submentum; plpr, palpifer; lig, fused second maxillæ; ant, antenna; occ, occiput.

Fig. 28.—Head of Termopsis angusticollis, seen from beneath, showing the gena and gula: m, mentum; sm, submentum; labr, under side of the labrum; x, hypopharyngeal chitinous support.

All the gulo-mental region of the head appears to represent the base of the second maxillæ, and the question hence arises whether the submentum is not the homologue of the cardines of the first maxillæ fused, and the mentum that of the stipites of the latter also fused together. If this should prove to be the case, the homologies between the two pairs of maxillæ will be still closer than before supposed. Where the gula is differentiated, this represents the basal piece of the second maxillæ. In Figs. 28, 29, 30, and 31, these three pieces are clearly shown to belong to the second maxillary segment. It is evident that these pieces or sclerites belong to the second maxillary or labial segment of the head, as does the occiput, which may represent the tergo-pleural portion of the segment. Miall and Denny also regarded the submentum as the basal piece of the second maxillæ.

Fig. 30.—Head of Mantispa brunnea, under side: e, eye; other lettering as in Fig. 29.

Fig. 31.—Head of Limnephilus pudicus, under side: e, eye; l, ligula; p, palpifer; lp, labial palpi.

The occiput (Fig. 29, B, C), as stated beyond, is very rarely present as a separate piece; in the adult insect we have only observed it in Corydalus. The occipital region may be designated as that part of the head adjoining and containing the occipital foramen. Newport considers the occiput as that portion of the base of the head “which is articulated with the anterior margin of the prothorax. It is perforated by a large foramen, through which the organs of the head are connected with those of the trunk. It is very distinct in Hydroüs and most Coleoptera, and in some, the Staphylinidæ, Carabidæ, and Silphidæ is constricted and extended backwards so as to form a complete neck.” (See also p. 51.)

Fig. 32.—Interior and upper and under surface of the head of Hydroüs piceus: d, clypeus; e, labrum; g, maxilla; h, its palpus; i, labium; k, labial palpus; p, sutura epicranii; q, cotyloid cavity; r, torulus; s, v, laminæ squamosa; t, laminaæ posteriores; u, tentorium; w, laminæ orbitales; x, os transversum; y, articulating cavity for the mandible; z, os hypopharyngeum.—After Newport.

The tentorium.—The walls of the head are supported or braced within by two beams or endosternites passing inwards, and forming a solid chitinous process or loop which extends in the cockroach downwards and forwards from the lower edge of the occipital foramen. “In front it gives off two long crura or props, which pass to the ginglymus, and are reflected thence upon the inner surface of the clypeus, ascending as high as the antennary socket, round which they form a kind of rim.” (Miall and Denny.) The œsophagus passes upwards between its anterior crura, the long flexor of the mandible lies on each side of the central plate; the supraœsophageal ganglion rests on the plate above, and the subœsophageal ganglion lies below it, the nerve cords which unite the two passing through the circular aperture. (Miall and Denny.) In Coleoptera (Hydroüs) it protects the nervous cord which passes under it. (Newport, Fig. 32, u.)

Fig. 33.—Posterior view of head of Anabrus; t, tentorium. Joutel del.

In Anabrus the tentorium is V-shaped, the two arms originating on each side of the base of the clypeus next to the base of each mandible the origin being indicated by two small foramina partly concealed externally and passing inwards and backwards and uniting just before reaching the posterior edge of the large occipital foramen (Fig. 33).

Palmén regards the tentorium as representing a pair of tracheæ (with the cephalic spiracles) which have become modified for supports or for muscular attachment, since he finds that in Ephemera the tentorium breaks across the middle during exuviation, each half being drawn out of the head like the chitinous lining of a tracheal tube. This view is supported by Wheeler, who has shown that the tentorium of Doryphora originates from five pairs of invaginations of the longitudinal commissures, and which are anterior to those of the second maxillary segment. “These invaginations grow inwards as slender tubes, which anastomose in some places. Their lumina are ultimately filled with chitin.” (Jour. Morph., iii, p. 368.)

This view has also been held by Carrière and Cholodkowsky, but Heymons concludes from his embryological studies on Forficula and Blattidæ (1895) that it is unfounded. That this is probably the case is proved by the fact that the apodemes of the thoracic region are evidently not modified tracheæ, since the stigmata and tracheæ are present.

Number of segments in the head.—While it is taken for granted by many entomologists that the head of insects represents a single segment, despite the circumstance that it bears four pairs of appendages, the more careful, philosophical observers have recognized the fact that it is composed of more than a single segment. Burmeister recognized only two segments in the head; Carus and Audouin recognized three; Macleay and Newman four; Straus-Durckheim even so many as seven. Huxley supposed that there are five segments bearing appendages, remarking, “if the eyes be taken to represent the appendages of another somite, the insect head will contain six somites.” (Manual of Anat. Invert. Animals, p. 398.)

These discordant views were based on the examination of the head in adult insects; but if we confine ourselves to the imago alone, it is impossible to arrive at a solution of the problem.

Newport took a step in the right direction when he wrote: “It is only by comparing the distinctly indicated parts of the head in the perfect insect with similar ones in the larva that we can hope to ascertain the exact number of segments of which it is composed.” He then states that in the head of Hydroüs piceus are the remains of four segments, though still in the next paragraph, when speaking of the head as a whole, he considers it as the first segment, “while,” he adds, “the aggregation of segments of which it is composed we shall designate individually subsegments.”

That the head of insects is composed of four segments was shown on embryological grounds by the writer (1871) and afterwards by Graber (1879). The antennæ and mouth-parts are outgrowths budding out from the four primitive segments of the head; the antennæ grow out from the under side of the procephalic lobes, and these should therefore receive the name of antennal lobes. In like manner the mandibles and first and second maxillæ arise respectively from the three succeeding segments.

Fig. 34.—Embryo of Anurida maritima: tc. ap, minute temporary appendage of the tritocerebral segment, the premandibular appendage; at, antenna; md, mandible; mx¹, first maxilla; mx², second maxilla; p¹–p³, thoracic; ap¹, ap², abdominal appendages; an, anus—After Wheeler.

While the postoral segments and their appendages are readily seen to be four in number, the question arises as to whether the eyes represent the appendages of one or more preoral segments. In this case embryology thus far has not afforded clear, indubitable evidence. We are therefore obliged to rely on the number of neuromeres, or primitive ganglia. In the postoral region of the head, as also in the trunk, a pair of neuromeres correspond to each segment. (See also under Nervous System, and under Embryology.) We therefore turn to the primitive number of neuromeres constituting the procephalic lobes or brain.

From the researches of Patten, Viallanes, and of Wheeler, especially of Viallanes, it appears that the brain or supraœsophageal ganglion is divided into three primitive segments. (See Nervous System, Brain.) The antennæ are innervated from the middle division or deutocerebrum. Hence the ocular segment, i.e. that bearing the compound and simple eyes, is supposed to represent the first segment of the head. This, however, does not involve the conclusion that the eyes are the homologues of the limbs, however it may be in the Crustacea.

The second head-segment is the antennal, the antennæ being the first pair of true jointed appendages.

The third segment of the head is very obscurely indicated, and the facts in proof of its existence are scanty and need farther elucidation.

Viallanes’ tritocerebral lobes or division of the brain is situated in a segment found by Wheeler to be intercalated between the antennal and mandibular segments. He also detected in Anurida maritima, the rudiments of a pair of appendages, smaller than those next to it, and which soon disappear (Fig. 34, tc. ap). He calls this segment the intercalary.^[12] Heymons (1895) designates it as the “Vorkiefersegment,” and it may thus be termed the premandibular segment.

Fig. 35.—Head of embryo of honey bee: B, a little later stage than A. pr.m, premandibular segment; cl, clypeus; ant, antenna; md, mandible; mx, first maxilla; mx′, second maxilla; sp, spiracle.—After Bütschli.

As early as 1870 Bütschli observed in the embryo of the honey bee the rudiments of what appeared to be a pair of appendages between the antennæ and mandibles, but, judging by his figures, nearer to and more like the mandibles than the rudimentary antennæ (Fig. 35); they seemed to him “almost like a pair of inner antennæ.”

“I find,” he says, “in no other insects any indication of this peculiar appendage, which at the time of its greatest development attains a larger size than the antennæ, and which, afterwards becoming less distinct, forms by fusion with that on the other side a sort of larval lower lip. That this appendage does not belong to the category of segmental appendages is indicated by the site of its origin on the upper side of the primitive band.” (Zeitschr. wissen. Zool., xx, p. 538.)

Grassi has also observed it in Apis, and regards it as the germ of a first, but deciduous, pair of jaws. In the embryo of Hylotoma Graber (Figs. 134, 135) found what he calls three pairs of “preantennal projections,” one of which he thinks corresponds to the “inner antennæ” of Bütschli. This subject needs further investigation.

It thus appears that the procephalic lobes of the embryo of insects, with the rudiments of the antennæ, constitute the primitive head, and perhaps correspond to the annelidan head, while gradually the antennal appendages were in the phylogenetic development of the class fused with the two segments of the primary head. That the second maxillary segment, the occiput, was the last to be added, and at first somewhat corresponded in position to the poison-fangs of centipedes (Chilopods), is shown by our observations on the embryology of Æschna (Fig. 36).

Fig. 36.—Æschna nearly ready to hatch: 4, labium, between T and e the occipital tergite; 5–7, legs.

Fig. 37.—Head of embryo Nematus, showing the labial segment: occ, forming the occiput; cl, clypeus; lb, labrum; md, mandible; mdm, muscle of same; mx, maxilla; mx′, second maxilla (labium); oe, œsophagus.

The mandibular segment appears to form a large part of the post-antennal region of the epicranium on account of the great mandibular muscle which arises from so large an area of the anterior region of the head (Fig. 37).

Judging from the embryo of Nematus (Fig. 37), the first maxillary segment is tergally aborted, there being no tergo-pleural portion left.^[13]

The second maxillary segment tergally appears to be represented by the occipital region of the head.

All the gular region, including the submentum and mentum, probably represents the base of the labium or second maxillæ.^[14] The so-called “occiput” forms the base of the head of Corydalus, a neuropterous insect, which, however, is more distinct in the larva. In most other adult insects the occiput is either obsolete or fused with the hinder part of the epicranium. We have traced the history of this piece (sclerite) in the embryo of Æschna, a dragon-fly, and have found that it represents the tergal portion of the sixth or labial segment. In our memoir on the development of this dragon-fly, Pl. 2, Fig. 9, the head of the embryo is seen to be divided into two regions, the anterior, formed of the antennal, mandibular, and first maxillary segments, and the posterior, formed of the sixth or labial segment. This postoral segment at first appears to be one of the thoracic segments, but is afterwards added to the head, though not until after birth, as it is still separate in the freshly hatched nymph (Fig. 4; see also Kolbe, p. 132, Fig. 59, sq. 5). A. Brandt’s figure of Calopteryx virgo (Pl. 2, Fig. 19) represents an embryo of a stage similar to ours, in which the postoral or sixth (labial) segment is quite separate from the rest of the head. The accompanying figure, copied from our memoir, also shows in a saw-fly larva (Nematus ventricosus) the relations of the labial or sixth segment to the rest of the head. The suture between the labial segment and the preoral part of the head disappears in adult life. From this sketch it would seem that the back part of the head, i.e. of the epicranium, may be made up in part of the tergite or pleurites of the mandibular segment, since the mandibular muscles are inserted on the roof of the head behind the eyes. It is this labial segment which in Corydalus evidently forms the occiput, and of which in most other insects there is no trace in larval or adult life, unless we except certain Orthoptera (Locusta), and the larva of the Dyticidæ.

The following table is designed to show the number and succession of the segments of the head, with their respective segments.

Tabular View of the Segments, Pieces (Sclerites), and Appendages of the Head
Name of Segment	Pieces or Regions of the Head-capsule	Appendages, etc.
Preoral, in early embryo.	1. Ocellar (Protocerebral).	Epicranium, anterior region with the clypeus labrum, and epipharynx.	Compound and simple eyes (Ocelli).
Postoral, in early embryo.	2. Antennal (Deutocerebral).	Epicranium, including the antennal sockets.	Antennæ.
3. Premandibular, or intercalary (Tritocerebral).	Wanting in postembryonic life, except in Campodea.	Premandibular appendages (in Campodea).
4. Mandibular.	Epicranium behind the antennæ, genæ.	Mandibles.
5. 1st Maxillary.	Epicranium, hinder edge? Tentorium.	1st Maxillæ.
6. 2d Maxillary, or labial.	Occiput.	2d Maxillæ or Labium. Post-gula, gula, submentum, mentum, hypopharynx (lingua, ligula), paraglossæ, spinneret.

Fig. 38.—Larva (a) of a chalcid, about to pupate, with the head, including the eyes and three ocelli, in the prothoracic segment: b, c, pupa.

The composition of the head in the Hymenoptera.—Ratzeburg stated in 1832 that the head in the adult Hymenoptera (Cynips, Hemiteles, and Formica) does not correspond to that of the larva, but is derived from the head and the first thoracic segment of the larva. Westwood and also Goureau made less complete but similar observations, though Westwood afterwards changed his opinion, and the same view was maintained by Reinhard. Our own observations (as seen in Fig. 38) led us to suppose that this was a mistaken view; that the larval head, being too small to contain that of the semipupa, was simply pushed forward, as in caterpillars. Bugnion, however, reaffirms it in such a detailed way that we reproduce his account. He maintains that the views of Ratzeburg are exact and easy to verify in the chalcid genus Encyrtus, except, however, that which concerns the ventral part and the posterior border of the prothoracic segment.

As the time of transformation approaches, the head of the larva, he says, is depressed and soon concealed under the edge of the prothoracic segment; the latter elongates, becomes thicker and more convex, and within can be seen the two oculo-cephalic imaginal buds. The head of the perfect insect is derived not only from the head of the larva, but also from the portion of the prothoracic segment which is occupied by the buds, i.e. almost its entire dorsolateral face. But the hinder and ventral part of this segment (which contains the imaginal buds of the first pair of legs) takes no part in the formation of the head; these parts, according to Bugnion, towards the end of the larval period detaching themselves so as to become fused with the thorax and constitute the pronotum and the prosternum.

Fig. 39.—Anterior half of larva of Encyrtus, ventral face, showing the upper (wing) and lower (leg) thoracic imaginal buds: b, mouth; ch, chitinous arch; gl, silk gland; g, brain; n, nervous cord; a¹, bud of fore, a², bud of hind, wing; p¹–p³, buds of legs; st¹–st³, stigmata.

Fig. 40.—Anterior part of Encyrtus larva, 1.2 mm. in length; dorsal face; the cellular masses beginning to form the buds of the wings, eyes, and antennæ: o, eye bud; e, stomach.