ANNULOSA: INSECTA. 213 



does not differ essentially from the above in any of the Insects, 

 but the alimentary canal is, generally speaking, considerably 

 lengthened in the herbivorous species. 



There is no definite and regular course of the circulation in 

 the Insects. The propulsive organ of the circulation is a long 

 contractile cavity, situated in the back and termed the ' dorsal 

 vessel.' This is composed of a number of sacs, opening into 

 one another by valvular apertures, which allow of a current in 

 one direction only, viz. towards the head. The blood is col- 

 lected from the irregular venous sinuses which are formed by 

 the lacunas and interstices between the tissues, and enters the 

 dorsal vessel from behind ; it is then driven forwards, and is 

 expelled at the anterior extremity of the body. 



Respiration is effected by means of ' tracheas,' or branched 

 tubes, which commence at the surface of the body by lateral 

 apertures, called * stigmata,' or ' spiracles,' and ramify through 

 every part of the animal. In structure the trachea are mem- 

 branous, but their walls are strengthened by a chitinous fila- 

 ment, which is rolled up into a continuous spiral coil.^ The 

 wings, also, whilst acting as locomotive organs, doubtless 

 subserve respiration. 



The nervous system in Insects, though often concentrated 

 into special masses, consists essentially of a chain of ganglia, 

 placed ventrally, and united together by a series of double 

 cords or commissures. The cephalic, or ' pree-cesophageal,' gan- 

 glia are of large size, and distribute filaments to the eyes and 

 antennas. The post-oesophageal ganglia are united to the pre- 

 ceding by cords which form a collar round the gullet, and they 

 supply the nerves to the mouth, whilst the next three ganglia 

 furnish the nerves to the legs and wings. 



The organs of sense are the eyes and antennas. The eyes 

 in Insects are usually * compound,' and are composed of a 

 number of hexagonal lenses, united together, and each supplied 

 with a separate nervous filament. Besides these, simple eyes 

 ' ocelli,' or ' stemmata ' are sometimes present, or, in rare 

 cases, may be the sole organs of vision. In structure these 

 resemble the single elements of the compound eyes. The an- 

 tennas are movable, jointed filaments, attached usually close 

 to the eyes, and varying much in shape in different Insects. 

 They doubtless discharge the functions of tactile organs, but 

 are probably the organ of other more recondite senses in 

 addition. 



The sexes in Insects are in different individuals, and most 

 are oviparous. Generally speaking, the young insect is very 

 different in external characters from the adult, and it requires 

 to pass through a series of changes, which constitute the ' meta- 



