376 



INSECTA. 



The development of the cephalic region in the Mwcidae is more 

 complicated, and still, in spite of the descriptions of Weismann 

 (No. 129), Van Rees (No. 121), and Iyowalevsky (No. 112), difficult 

 to understand. In this connection we must recall the fact that the 

 cephalic region, in the larva of the Muscidae, occurs in an extremely- 

 reduced condition, the head-region being represented only by the 

 most anterior and smallest of the twelve segments of which the 

 body of the conical larva is composed. Its small size is partly to be 

 ascribed to the fact that a considerable portion of the head is here 

 present in an invaginated condition. For, as has been shown by the 

 researches of Weismann, the anterior part of the head, the mandibles, 

 and the whole of the region surrounding the mouth are invaginated 

 in the last embryonic stages, and in the fully-formed maggot are 

 represented by that depression (Fig. 185, p) in which the hook- 

 apparatus, characteristic of the larvae of the Muscidae, develops. 

 This invaginated part of the head into the base of which the 

 oesophagus now opens, has been named, not very happily, the 

 oesophageal bulb or pharynx, and it must for the present be held 

 that the cavity thus named does not belong to the alimentary canal. 

 It is an invaginated section of the head, and the formation of the 

 imaginal head consists for the greater part merely in the evagination 

 of this region. 



The first rudiments of the most important parts of the head (the 

 eyes, the antennae, and the frontal region) are found in the youngest 

 larvae as a pair of cell-masses lying in the thorax, closely applied to 

 the halves of the brain (and therefore called by Weismann brain- 

 ap>pendages). These are, probably from their first origin, connected 

 anteriorly with the pharynx, and might be described as the imaginal 

 discs of the head. In the later stages these assume the form of a 

 pair of long sacs expanding posteriorly (Fig. 185 A and B, h), and 

 may no doubt, according to their origin, be regarded as outgrowths 

 of the larval pharynx (see above). Epithelial thickenings soon 

 appear in the walls of these sac-like "brain-appendages," and in 

 these can be recognised the rudiments of definite parts of the head. 

 A disc-Jike thickening appears in the posterior widened portion of 

 each of the appendages, this represents the rudiment of the com- 

 pound eye, and is consequently called the optic disc (as). On the 

 basal surface of the optic disc there is a cellular expansion connected 

 with the supra-oesophageal ganglion by a nerve. This nerve becomes 

 the optic nerve of the adult, while the optic ganglion becomes more 

 distinctly separated from the brain. In the anterior, more cylindrical 



