238 MANUAL OF ZOOLOGY. 



eight), opening into one another by valvular apertures, which 

 allow of a current in one direction only viz., towards the 

 head. The blood is collected from the irregular venous 

 sinuses which are formed by the lacunae and interstices be- 

 tween the tissues, and enters the dorsal vessel from behind ; 

 it is then driven forwards, and is expelled at the anterior ex- 

 tremity of the body. 



Respiration is effected by means of " tracheae," 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 tracheae are mem- 

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

 ment, which is rolled up into a continuous spiral coil. In the 

 aquatic larvae of many insects, and in one adult insect, 

 branches of the tracheae are sent to temporary outgrowths 

 which are termed " tracheal gills," and in which the blood is 

 oxygenated. In all, however, except the single insect above 

 mentioned, these temporary external appendages fall off when 

 maturity is attained. The wings, also, whilst acting as locomo- 

 tive 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 " prae-cesophageal " 

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

 and antennae. The post-oesophageal ganglia are united to the 

 preceding 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 antennae. 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 often present, or, in rare 

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

 resemble the single elements of the compound eyes. In a few 

 cases the eyes are placed at the extremities of stalks or ped- 

 uncles, but in no case are these peduncles movably articulated 

 to the head, as is the case in the Podophthalmous Crustaceans. 

 The antennae are movable, jointed filaments, attached usually 

 close to the eyes, and varying much in shape in different In- 

 sects. They doubtless discharge the functions of tactile prgans, 

 but are probably the organ of other more recondite senses in 

 addition. 



The sexes in Insects are in different individuals, and most 



