102 NATURAL HISTORY OF WASPS. 
air. In the wasp this cohesion is further delayed; 
the fluid secretion requires a longer exposure to 
make it harden; the separate threads meanwhile 
melting into each other, so as to form a membrane 
rather than a web. 
From the earliest period, as we shall see further 
on, the secretion of the salivary glands is more free 
in the wasp than in the honey-bee, their cocoon is 
thicker and stronger in all its details, increasingly so 
to the cap, the crown of larval labours. In the 
honey-bee this is reversed, and after the larval period 
her physiological energies are turned in another 
direction; she now makes wax instead of silk, the 
secretion of the salivary glands, never very abundant, 
is henceforth quite secondary to that of the abdo- 
minal fat masses, and loses all power of independent 
cohesion whatever. 
Besides the salivary glands the thorax contains 
portions of the general circulatory, respiratory, and 
nervous systems, and the cesophagus on its way to 
the stomach. All these will be most conveniently 
examined in connexion with the abdomen, where, if 
not their most important, at least their most pro- 
minent portions, such as would be most likely to 
catch the eye of a general observer, are displayed. 
If exception be made to this arrangement in any 
particular it should be in reference to the respiratory 
apparatus, which is so highly developed in the thorax. 
Large air. cells fill the spaces between the muscles, 
trachee ramify among their fibres with a minuteness 
not seen elsewhere, and it is the thoracic spiracles 
which are adapted in wasps to be the chief organs of 
