604: EXTERNAL ANATOMY OF INSECTS. 
broad and concave surface to the air, for then they are 
usually nearly vertical, they may assist in some measure 
as sails, and help them in flying traversely and before 
the wind ?. 
i. Tegmina>. By this name the learned Illiger has 
distinguished the upper organs of flight of the Orthoptera 
and Homopterous Hemiptera‘. They may be consi- 
dered under the same heads nearly as elytra. 
1. Substance. Tegmina differ very materially from 
elytra in their substance, being generally more or less 
diaphanous, though in Blatta Petiveriana the dark parts 
are as opaque as elytra, and those of the Mantes that 
resemble dry leaves are only semidiaphanous. These 
organs are also of a less dense substance than elytra, 
something between coriaceous and membranous, which 
I shall express by the term pergameneous, as somewhat 
resembling parchment or vellum. Another circum- 
stance relative to this head also distinguishes them,— 
they are not lined with membrane. In some instances, 
as in B. Petiveriana just named, they approach nearly 
to the substance of elytra, and in B.viridis, some Mantes, 
and Cicada, &c., they are little different from wings in 
their substance; but this does not diminish their right 
to be considered as ¢egmina, since their structure is al- 
together the same. 
2. Articulation with the trunk. I observed above that 
aM. Chabrier says that the are described by the wings of Melo- 
lontha vulgaris to that of the elytra, is as 200 to less than 50. Sur le 
Vol des Ins. c. i. 440. 
> Pirate X. Fic. 2. and XXVIII. Fic. 18—20. 
© Magas. 1806. Terminologie der Insekt. 18. 1675. 
