THE INTEGUMENT OF THE HEAD. 39 



(esophagus as the axis around which they are arranged, it will be 

 readily seen that the more anterior is horizontal and surrounds the 

 base of the proboscis ; its anterior extremity forming the boundary 

 of the oral cleft, whilst the posterior is vertical, and is arranged 

 around the cephalo-thoracic opening. The fourth segment bears 

 the antennae, and the fifth the ocelli ; the great eyes also seem to 

 " 3long to this segment, although externally they appear between 

 tne two ; hence I shall call the fourth the antennal, and the 

 fifth the optic segment, naming their dorsal plates respectively, 

 from the positions they occupy, the facial and occipito- frontal 

 plates. 



Each of the plates which form the head will need a separate description, 

 and as the dorsal plate of the fifth segment, the occipito frontal, is the 

 centre around which the others are all arranged, it presents the fittest 

 starting point : it is colored in figs. 1 and 2, Plate II. It commences at 

 the superior and lateral margins of the occipital foramen, forms the 

 central portion of the occipital region, and then bends forwards and down- 

 wards, forming the forehead, and divides into two lateral processes, which 

 bound the lower margins of the great eyes and terminate by becoming 

 intimately blended with the cheeks. The line of division is indicated 

 only by a more or less perceptible ridge, but it is well marked in several 

 of the Syrphidae. I do not think, however, that the blending of the two 

 segments in this region would afford the slightest argument against their 

 distinctness, as it is in the dorsal region that we first find the segments 

 become blended with each other, and as already stated, the two correspond 

 ing segments 'in the larva ere united into one above. It is true that the 

 anterior edge of this plate is only loosely attached to the dorsal plate of 

 the fourth segment ( the facial plate ) by a membrane, but then this 

 membrane, as will hereafter appear, serves a special purpose, and the 

 union of the dorsal plate of the fifth with the lateral plates of the fourth 

 segment, gives the chief strength to the anterior portion of the head. 

 On each side of the occipital foramen the occipito-frontal plate is hollowed 

 out into a cnp-shaped depression to receive the apex of the conical condyle, 

 which steadies the head, and which consists of the lateral plate of this 

 segment. 



Below the occipital foramen is the basilar plate, the ventral or inferior 

 portion of the fourth segment ; it bears a pair of internal processes, which 

 support the cephalic nerve-centre and the main tracheae of the head. 



The lateral plates of the fourth segment are very large, as they compose 

 the greater part of the occipital region, and bend forwards to form the 

 cheeks, where they bear a number of strong hairs. 



