THE CIRCULATORY SYSTEM 



29 



or to the body cavity again (Tabanus, Dipt.; Vespa, Apis, Hym.). 

 Accessory pumping organs may irrigate the antennae (Periplaneta, 

 Hymenoptera, &c), the legs (Hemiptera), and the wings (Diptera). 



pn n 



fig. 6. — A, insect with fully developed circulatory system: schematic; 

 B, transverse section of thorax of the same; C, transverse section of 

 abdomen. Arrows indicate course of circulation. (Based largely on 

 Brocher) 



a, aorta; apo, accessory pulsatile organ of antenna; d, dorsal diaphragm with aliform 

 muscles; h, heart; n, nerve cord; o, ostia; pc, pericardial sinus; pn, perineural sinus; po, 

 meso- and metathoracic pulsatile organs; s, septa dividing appendages; v, ventral dia- 

 phragm; vs, visceral sinus 



But in many insects these structures are wanting, and the circulation 

 through the appendages is then effected (i) partly by their own 

 muscular movements, (ii) partly by the pressure changes in the ab- 

 domen brought about by the respiratory movements {Aeschna larva, 

 Ephemeroptera larvae) - in which case the pulse-beats in the leg 

 synchronize not with the heart-beat but with the respiratory con- 

 tractions, and (iii) partly owing to the fact that the efferent stream (in 

 the posterior chamber of the limb) communicates with the perineural 



