352 THE TRACHEA J. SYSTEM OJ' THE IMAGO. 



proper, but communicate with them through the medium of a 

 pouch or sac, the spiracular sac or vestibule. The vestibule is 

 separated from the tracheae proper by u valve, the internal 

 spiracular valve ; and the spiracular opening is also closed by 

 an external valve. Sometimes the external valve is situated at 

 the inner extremity of a funnel-shaped or tubular depression of 

 the integument, which forms the atrium of the vestibule. 



The Atrium is frequently lined by numerous seta, and the 

 ducts of wax glands (PI. XXII., Fig. 8) open into it and on 

 the surfaces of the external valves. Both sets of valves open 

 inwards, and the vestibule is surrounded by loops of muscle 

 fibre, the rhythmic contractions of which pump the air from 

 the vestibule into the trachea; proper ; hence the vestibule 

 performs the function of an air heart (see physiology of 

 respiration). 



The Tracheae proper are the longitudinal trunks and the 

 arborescent tracheae. The longitudinal trunks are a pair of 

 parallel vessels which extend from the anterior to the posterior 

 extremity of the insect. The}' are connected with each other 

 by transverse unbranched commissures, and with the spiracles 

 by short lateral branches, which, in those larvae with a closed 

 tracheal system, are represented by solid strings of cells 

 attached to the integument at the spots where the spiracles are 

 ultimately developed. 



156. HURST, C. H., 'On the Life-history and Development of a Gnat 

 (Culex).' Trans, of the Manchester Microscopical Society, 1890. 



The following papers deal with the function of respiration in insects : 



157. RK<;XAUI.T AND KKISET. ' Chemische Untersuchungen iiber die 

 Respiration der Thiere aus verchiedenen Klassen (pp. 129-179 and 

 2 57-3 21 )- ' Annalen der Chemie und Pharmacia,' Bd. Ixiii., 1850. 

 Heidelberg (Wohler und Liebig) : only a small section of this paper 

 is devoted to the respiration of insects (pp. 298-300). 



158. I'I.YKON, I., ' Sur ['Atmosphere interne des Insectes comparce a 

 celle dcs Feuilles.' Comptes Rendus, Tom. cii., p. 1339. 1886. 



159. I)K\vn/, H., ' Kinige Beobachtungen, betreffend das geschlossene 

 Tiucheensystem bei Insecten larven.' Xool. An/eig., Bd. xiii., 1890 



'PP- 5 2 5-53' > 



160. CKII i mis, A. I'.., -The Physiology of the Invenebrata.' Svo., Lon- 

 don, 1892. 



