THE NERVOUS SYSTEM. 63 



thoracic, whilst in the larva all are united in a single complex 

 nerve-mass, in which no trace of the primitive separation of the 

 post-oral ganglia remains perceptible externally. 



The anterior pair of cephalic centres is always pre-oral, and 

 is developed from the pro-cephalic lobes. In the perfect insect 

 it consists of a central pair of hemispherical ganglia, having 

 a more or less convoluted surface united by a commissure. 

 These support two pairs of pedunculated bodies — the corpora 

 fungiformia — as well as two pairs of sensory ganglia, the optic 

 and olfactory (antennal) lobes. These structures are usually 

 known as the ' brain,' but the term is more conveniently 

 applied to the whole of the cephalic ganglia whenever they are 

 united into a single cephalic centre, and when this is the case 

 the commissures which unite the supra- and infra-oesophageal 

 centres are called crura, and are not usually visible externally. 



The first post-oral centre, or the infra-oesophageal ganglion, 

 usually consists of three primitive ganglia united ; these belong 

 to the segmented portion of the head, the mandibular, maxillary 

 and labial somites. In the Diptera the first pair are apparently 

 wanting in both the larva and imago. 



In the Hymenoptera and Coleoptera, three pairs of nerves 

 are given off from these ganglia ; in the Diptera and Muscidse 

 there are only two pairs, whilst the Lepidoptera have only a 

 single pair (Brandt [29]). 



A small nerve arises, in most insects, from the commissure 

 between the supra- and infra-oesophageal ganglia, on each side. 

 These are usually known as the ' labral ' nerves, but I prefer the 

 term ' pharyngeal ' nerves, as they supply the pharynx as well 

 as the labrum — in the Diptera, at least. 



The pharyngeal nerves give off a pair of recurrent branches 

 which join the frontal ganglion of the splanchnic system 

 (PL II., Fig. 2). These are the recurrent nerves; sometimes 

 they have a separate origin distinct from that of the pharyngeal 

 nerves. They are also described as arising from a special 

 ganglion in the commissure — the ganglion of the commissure. 



Each of the other segmental centres, those of the thorax and 

 abdomen, except the last, usually consists of a single pair of 



