ANNULOSA: INSECTA. 



339 



(ordinarily eight), opening into one another by valvular aper- 

 tures, which allow of a current in one direction only viz., to- 

 wards the head. The blood is collected from the irregular 



Fig. 178. Ideal transverse section of an Insect, h Dorsal vessel ; i Intestine ; n _Ven- 

 tral Nerve-cord ; 1 1 Stigmata, leading into the branched tracheal tubes ;iv iv Wings ; 

 a Coxa of one leg ; b Trochanter ; c Femur ; d Tibia ; e Tarsus. (After Packard.) 



venous sinuses which are formed by the* lacunae and interstices 

 between the tissues, and enters the dorsal vessel from behind, 

 and by lateral valvular openings ; it is then driven forwards, 

 and is expelled at the anterior extremity of the body. The 

 blood of the Insecta is corpusculated, and usually colourless. 

 Whilst the general belief is that there is no regular system of 

 blood-vessels (arteries and veins), and that the blood simply 

 circulates through the interstices of the tissues, some observers 

 affirm the partial existence of true vessels, and others maintain 

 that the blood circulates in the spaces between the tracheae 

 and their enveloping sheaths, which thus become converted 

 into blood-vessels. 



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 or two adult Insects 



