ANNULOSA: INSECTA. 2S3 



hian vessels truly perform the functions of a liver — as their 

 position would appear to prove — then the kidneys -will be 

 represented by a series of csecal tubes which are only occa- 

 sionally present, and which open into the rectum, close to the 

 cloaca. There are no absorbent vessels, and the products of 

 digestion simply transude through the walls of the alimentary 

 canal into the sinuses or irregular cavities which exist between 

 the abdominal organs. The apparatus of digestion 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 (ordinarily 

 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 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 trachese 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 trachese 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 loco- 

 motive organs, doubtless subserve respiration, the nervures 

 being hollow tubes enclosing trachea. 



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-oesophageal" 

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

 and antennae. The post-cesophageal 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. In larvas, 

 thirteen pairs of ganglia, one to each segment, can be recog- 



