644 



ZOOLOGY 



SECT. 



degree of concentration of the ventral ganglionic chain (Fig. 523), 

 and in some of the Diptera this reaches such an extreme that all 

 the ventral ganglia, with the exception of the sub-oesophageal, are 

 united into one continuous elongated mass. The Insects, like the 

 higher Crustacea, possess a visceral or sympathetic nervous system, 

 connected with the oesophageal connectives, and passing backwards 

 on the oesophagus and crop. 



The most highly-developed organs of special sense are the 

 large compound eyes. The surface of the compound eye is 

 marked out, as in the case of the Crayfish, into a great number of 

 minute hexagonal facets, each of which represents one of the 



Flu. 523 — Nervous systems of four species of Diptera to illustrate various degrees of concentration. 

 A, non -concentrated nervous systems of Chironomus plumosus with three thoracic, and 

 six abdominal ganglia ; B, nervous system of Empis stercorea with two thoracic and five 

 abdominal ganglia ; C, nervous system of Tabanus bovinus, with one thoracic ganglion 

 and with the abdominal ganglia closely approximated ; O, nervous system of Sarcopliaga 

 carnaria, with all the ganglia of the ventral chain united together with the exception 

 of the sub-oesophageal. (From Lang's Comparative Anatomy.) 



elements (ommatidia) of the eye. Of these there may be as 

 many as 28,000 (Dragon-fly). When the eye is examined in 

 section, each ommatidium is found to consist of a cornea-lens — the 

 outer surface of which forms the facet, a crystalline cone, and a 

 rhabdome. The crystalline cone is not always developed, its place 

 being taken in the eyes of some Insects by four crystal cells. The 

 rhabdome is an elongated rod. Beneath the rhabdomes is a 

 fenestrated membrane, beneath which, again, is a dense plexus of 

 nerve-fibres. Nerve-fibres pass through the fenestrated membrane 

 and terminate in a delicate sheath which incloses each rhabdome, 

 the sheath, together with the nerves that end in it, constituting 

 the retinula. Pigment surrounds the crystalline cones and retinulse. 



