2/4 PTILOTA : IMAGO EXTERNAL ANATOMY. 



comotion ; these are either wings, or their representatives, 

 or legs. 



1 . Wings, or the Organs of Aerial Progression. 



These organs, unlike the wings of birds, consist simply of 

 a double membrane, of a very slender and generally trans- 

 parent consistence, inclosing numerous nervures or veins 

 of a firmer substance. These nervures are a kind of solid 

 tubes, inclosing the tracheae, or aeriferous vessels in their in- 

 terior, of which I have already spoken in the account of 

 the escape of the perfect insect from the pupa skin. Some 

 appear, indeed, to be real veins. 



These organs undergo very great modifications of form 

 and structure in the different orders of insects ; their num- 

 ber is also liable to corresponding variations; I say cor- 

 responding, because, as one pair of wings is sometimes so 

 completely modified as to be no longer serviceable as an 

 organ of flight, the number of these wings is necessarily 

 reduced from four to two. These modifications occur either 

 in the anterior or posterior pair of wings ; thus in the Cole- 

 optera, the fore-wings, although ample, are transformed into 

 a pair of scales, serving for the defence of the wings, and 

 unserviceable as instruments of flight. The same occurs in 

 a greater degree in the Strepsiptera. IntheHemiptera(Hetero- 

 ptera), the four wings are of a leathery structure at the base, 

 but are membranous at the tip; whilst in the homopterous He- 

 miptera and Orthoptera, the upper wings are of a membranous 

 nature, but much thicker than the lower wings throughout. 

 In the Hymenoptera, Neuroptera, and Lepidoptera, all the 

 wings are equally membranous. In the Diptera the fore- 

 wings are alone to be found as organs of flight, the posterior 

 wings being reduced to a pair of slender knobbed filaments. 

 There are many cases in which the wings are totally "ant inir. 

 as in the glow-worm, many Orthoptera and Hemiptera, &c. ; 

 but these are to be accounted only as casual exceptions. 



