164 INA'ERTEBRATE ANIMALS. 



air-tubes or trachese (fig. Ill), which commence at the surface by so 

 many apertures or spiracles, and branch repeatedly as they proceed 

 inwards through the tissues. They have the same structure as in 

 the Arachnida, consisting of membranous tubes strengthened by 



Fig. 111. — Ideal transverse section of an Insect. Ti Dorsal vessel; i Intestine; n 

 Ventral nerve-cord ; i t Stigmata leading into the branched tracheal tubes ; w w 

 wings; a Coxa of one leg ; b Trochanter; c Feniur ; (Z Tibia ; e Tarsus. (After 

 Packard.) 



means of a spirally coiled filament of chitine. The tracheae are 

 prolonged into the wings, so that these structures likewise assist in 

 respiration. 



The nervous system in insects, though sometimes somewhat modi- 

 fied, has essentially the regular Annulose form of a ventral chain of 

 ganglia, traversed in front by the gullet. The organs of sense are 

 the eyes and antennte. The eyes are usually " compound," and are 

 composed of numerous six-sided lenses, united together, and each 

 supplied by a separate nervous filament. As many as eight thousand 

 of these lenses liave been counted in one of the eyes of the common 

 Cockchafer, and this number is sometimes greatly exceeded. Besides 

 these compound eyes there are sometimes " simple " eyes, identical 

 in structure with the single lenses of the compound eyes ; and in 

 rare cases these are the only organs of vision. The feelers or 

 antennte, with which all insects are furnished, are jointed filaments 

 attached close to the eyes, and assuming very different shapes in 



