352 THE TRACHEAL SYSTEM OF 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 trachee proper by a 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 sete, and the 
ducts of wax glands (Pl. 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 trachee proper; hence the vestibule 
performs the function of an air heart (see physiology of 
respiration). 
The Tracheew proper are the longitudinal trunks and the 
arborescent tracheze. The longitudinal trunks are a pair of 
parallel vessels which extend from the anterior to the posterior 
extremity of the insect. They are connected with each other 
by transverse unbranched commissures, and with the spiracles 
by short lateral branches, which, in those larve 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, REGNAULT AND REISET, ‘Chemische Untersuchungen iiber die 
Respiration der Thiere aus verchiedenen Klassen (pp. 129-179 and 
257-321). ‘Annalen der Chemie und Pharmacie,’ 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, PryRon, I., ‘Sur l’Atmosphére interne des Insectes comparée 4 
celle des Feuilles.’ Comptes Rendus, Tom. cii., p. 1339. 1886. 
159, Dewitz, H., ‘Einige Beobachtungen, betreffend das geschlossene 
Tracheensystem bei Insecten larven.’ Zool. Anzeig., Bd. xiii., 1890 
(pp. 525-531). 
160. GrirriTHs, A. B., ‘The Physiology of the Invertebrata.’ 8vo., Lon- 
don, 1892. 
