124 THE INTEGUMENTAL SKELETON OF THE IMAGO. 



the heads are being prepared. It commences between the 

 lateral part of the occipital ring and the pars basilaris, and 

 extends forwards in the tentorium towards the upper edge of 

 the epistome. It is more fully developed in the Syrphidae and 

 in Volucella, in which Kunckel d'Herculais figures and describes 

 it [25, Plate XII., Fig. i, c, and p. 88] as arising from the 

 junction of the pars basilaris with the occipital ring and ex- 

 tending forwards to the anterior and lower margin of the 

 compound eye, its upper edge giving off a curved process 

 to the upper margin of the epistome. In the true flies the 

 whole is such a slender rod, and so easily detached from both 

 the occipital ring and the epistome, that I have generally 

 failed to find it, and have never actually traced it to the 

 epistome. It is mentioned by Men^bier [50]. 



The Immature Head Capsule. — The head of the imago, when it 

 first emerges from the pupa case (Fig. 25), has the form of a 

 cone with a convex base ; the antenna; are placed on its under 

 surface, and the proboscis, which is not capable of retraction, 

 lies on the breast between the legs, like the rostrum of an 

 hemipterous insect. The vertex is the most anterior part ; 

 the major axis of the head is nearly horizontal (Fig. 25, / 

 and 2), and makes an acute angle with the minor axis. At 

 first the frontal sac (/s), or posterior cephalocele, is capable of 

 enormous e.xtension, so that it frequently appears larger than 

 the rest of the head. The posterior surface of the head capsule 

 is convex instead of concave, and the whole, if we e.xcept the 

 greatly distended frontal sac, is much smaller than the head 

 of the adult insect. 



The frontal sac is distended by the contraction of the thorax 

 and abdomen, which drives the blood of the insect into it; 

 by this means the operculum of the pupa case is separated, 

 allowing the insect to escape. 



During the subsequent evolution of the head the anterior 

 part of the frontal sac is withdrawn into the head capsule, 

 leaving a curved fissure, the lunula, between the antennal 

 rostrum and the forehead. This fissure is permanent, and 

 communicates with the cavity of the withdrawn frontal sac. 



