62 Appendages of the Thorax. 
APPENDAGES OF THE THORAX. 
The organs of flight are the most noticeable appendages 
of the thorax. The wings are usually four, though the 
Diptera have but two, and some insects—as the worker- 
ants—have none. The front or primary wings (Fig. 4, 
A) are usually larger than the secondary or hind wings 
(Fig. 4, 8), and thus the mesothoracic or middle ring of 
the thorax, to which they are attached, is usually larger 
than the metathorax or third ring. The wings consist of 
a broad frame-work of veins (Fig. 4), covered by a thin, 
tough membrane. The main ribs or veins are variable in 
Fic. 15. 
Muscles of Thorax, after Wolff. 
£ Muscles to raise front wing. D Muscles to lower front wing. 
A Muscles of hind wing. 
number, while towards the extremity of the wing are more 
or less cross-veins, dividing this portion of the wings into 
more or less cells. In the higher groups these cells are 
few, and quite important in classifying. Especially useful 
in grouping bees into their families and genera, are the 
cells in the second row, from the front or costal edge of 
the primary wings, called the sub-costal cells, Thus in the 
genus Apis there are three such cells (Fig. 4, A, 6, 7, 8), 
while in the Melipona there are only two. The ribs or 
veins consist of a tube within a tube, the inner one form- 
ing an air tube, the outer one carrying blood. On the costal 
