XI 



PHYLUM ARTHROPODA 



620 



pairs of thoracic and as many as eight of abdominal ganglia in 

 the adult insect ; but in many cases there is a greater or less degree 

 of concentration of the ventral ganglionic chain (Fig. 534), 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 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 



FIG. 534. Nervous systems of four species of Diptera to illustrate various degrees of concen- 

 tration. 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 ; D, nervous 

 system of Sarcophaga carnaria, with all the ganglia of the ventral chain united 

 together with the exception of the sub-cesophageal. (From Lang's Comparative Anatomy.) 



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

 hexagonal facets, each of which indicates one of the 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 omma- 

 tidium 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 afenestrated 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 encloses each rhabdome, the sheath, together 



