ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 237 



head with the antennae as appendages. (4) Of the fourth, fifth, and 

 sixth segments, the sixth has a feebly developed main body, which fuses 

 with those of the second and third ; this is probably also the case with 

 the fifth, while the fourth is wanting (as in many Diptera) or fuses 

 with other segments. The appendages, however, of these reduced 

 segments are important respectively, as (sixth segment) labium, 

 (fifth) maxillae, (fourth) mandibles. (5) The third segment is repre- 

 sented by the labrum, and appears to have no appendage beloncrinc 

 to it. (6) The mandibles, maxilla, and labium are homologous. 



In Musca, of the three chief external divisions of the head, 

 epicranium, pars hasilaris, epistomum, the first is by far the largest. 

 It may be divided into, (1) frons, (2) vertex, (3) occiput, and (4) 

 gense. 



(1) The fwns reaches from the point of insertion of the antennae 

 between the compound eyes to the triangular plate of the vertex, 

 which bears the simple eye. Inferiorly the frons has a membranous 

 process, the frontal sac. The vertex (2) rises up vertically behind 

 the frons, and extends as far as the fold which surrounds the occipital 

 pit. The occiput (3) is divided into two halves by the vertex, and 

 lies behind and to the inner side of the compound eyes. (4) The gence 

 lie in front of the compound eye and in front of and below the frone. 

 The pars hasilaris extends from the occipital cavity behind to the 

 insertion of the proboscis in front. The epistomum forms the anterior 

 wall of the head. It commences at the point of insertion of the 

 antennte, is bounded laterally by the genfe, and passes directly into 

 the wall of the proboscis below. Behind the head the neck is formed 

 by a thin chitinous wall or collar surrounding the occipital cavity. 

 These parts are in many cases but imperfectly distinguished from one 

 another. 



The internal skeleton of the head consists of (1) two bands running 

 across the cavity ; they connect the pars hasilaris and the occiput, and 

 are perhaps fused processes of these parts; of (2, 3, and 4) similar 

 bands between the side of the occiput and the vertex, between the 

 frons and the genae, and a single semicircular one between frons and 

 epistomum respectively. There are further (5) the septum of the 

 occipital cavity, which consists of two feebly united, hollow lateral 

 processes, between which the brain lies ; and (6) the diaphragm of the 

 eyes (sclerotic of Leydig), which consists of two rings, an outer one 

 connecting the eye with the head, and one to the inner side of this, 

 surrounding the point where the optic ganglion breaks uj) into 

 branches. This arrangement of parts occurs in all Diptera, with 

 trifling modifications. 



The antennce of Musca are situated on the lower edge of the frons 

 and lie in two depressions on the epistomum. At the base of its 

 third segment is a long-fringed, two-jointed process, to which the name 

 plumula is applied ; some hairs which resemble it, but are not fringed, 

 occur in the second antennal segment. The proboscis is studied in con- 

 nection with the less specialized forms of Dipteran mouths, viz. those 

 of the Tabanid forms, Hcematopota and Chrysops. In these insects 

 there is an elongated grooved labrum, and a similar, but more pointed, 



Ser. 2.— Vol. 1. S 



