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The occiput

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.

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.)