ANNULOSA INSECTA. 



63 



called stigmata (Gr. stigma, a mark), placed on both 

 sides of the segments of the thorax and abdomen. 

 There are seldom more than nine pairs 

 of stigmata. The membranous walls of 

 the tracheae are kept distended by an 

 elastic, chitinous thread which is coiled 

 up within them in a spiral form. The 

 blood in its passage through the tissues 

 is purified by the oxygen of the air con- 

 veyed by the tracheae. 



74. Nervous System and Senses. Tho 

 nervous system in the Insecta bears a 

 general resemblance to the form usually 

 found in the Annulosa. It consists of a 

 chain of ganglia, connected by a double ner- 

 vous cord, occupying the ventral position of 

 the body, and crossed in front by the gullet. 

 The ganglia placed above the oesophagus, 

 called the cephalic (Gr. cephale, the head) 

 ganglia, or " brain," furnish nerves to the 



eyes and antennae. The eranglia below m Fl - 25. SPIRAL 

 , , ,, . THREAD OF THE 



tne oesophagus supply nerves to the legs TRACHEA OF AN 

 and wings. INSECT. 



75. The eyes are usually compound, consisting often of 

 many thousand hexagonal lenses. " They are formed by 

 a large number of eye tubes closely pressed together, so 

 that each of them has become hexagonal." (Haughton.) 

 Each of these lenses is supplied with a nervous filament. 

 There are usually simple eyes in addition to the compound 

 eyes; and, in a few cases, these alone are present. These 

 simple eyes consist of single hexagonal lenses. The only 

 other organs of sense are the antennae, which have 

 already been described. 



76. Development. Insects are oviparous animals, and 

 have the sexes distinct. The animal emerging from the 

 egg has generally a very different form from that which 

 it finally assumes. The changes which they undergo are 

 tc-vmcd metamorphosis (Gr. meta, change; morphe, 



