112 
INTRODUCTION TO 
instrument of motion. From this circumstance, a 
gradation can he traced, as to the share the upper 
wings take in flight from the Coleoptera to the He- 
miptera, and from these to the Ortlioptera. The in- 
sects possessing the greatest powers of flight, must, 
therefore, be sought among those in which the upper 
wings are wholly membranous ; and we accordingly 
find them among the Neuroptera, Hyinenoptera, and 
Lepidoptera. Every one who has paid the slightest 
attention to the subject, must have noticed how much 
more vigorous and sustained is the flight of a dragon- 
fly, a bee, or a butterfly, than that of a beetle, a 
grasshopper, or a plant-bug. The under wings are 
in every instance membranous, and are exclusively 
organs of flight. 
A wing of the latter sort, whether it belong to the 
superior or inferior pair, has the external appearance 
of a firm, dry, membrane, usually transparent, and 
traversed by numerous salient horny ribs. Although 
the membrane appears simple, it consists, in reality, 
of two membranous leaves, closely applied to each 
other, and enveloping the ribs just spoken of. This 
can be made to appear very distinctly when the insect 
has just emerged from the pupa and immersed in 
spirits of wine, as the fluid can be introduced between 
the still flaccid membranes, and thus distends them 
like a bag. Even this membrane, which appears of 
the finest and most glossy surface to the naked eye, 
is found under the microscope to be clothed more or 
less densely with hairs ; and in some gnats* these 
* See the immensety magnified figure of a gnat in Swam- 
merdam’s Book of Nature, Plate XXXVI. 
