16 CLASS INSECTA. 



are composed of two membranes applied one over the other, 

 and traversed in various directions by nervures more or less 

 numerous, which are so many tracheal tubes, and form 

 sometimes a net-work, sometimes simple veins. A celebrated 

 naturalist, the elder Jurine, has made an advantageous use, for 

 the purposes of classification, of the disposition and crossing 

 of these nervures. The dragon-flies, bees, wasps, butterflies, 

 &c., have four wings; but those of the butterflies are co- 

 vered with small scales, which, at first view, resemble dust, 

 and give them the colours with which they are adorned. 

 They may be easily removed with the finger, and the portion 

 of the Aving which has lost them is transparent. Through 

 the microscope we can see that these scales, of figures very 

 various, are implanted by means of a pedicle, and disposed in 

 a graduated series, like tiles upon a roof. In front of the 

 upper wings of these insects, are two species of epaulettes 

 {pterygodes), which are prolonged behind along a portion of 

 the back, on which they are attached. In certain insects the 

 wings remain straight, or fold back upon themselves. In 

 others they are doubled or folded longitudinally, like a fan. 

 Sometimes they are horizontal, sometimes inclined. In many 

 they cross on the back, being separate elsewhere.* The in- 

 sects with two wings, of the order of diptera, have under 

 the wings two small moveable threads, terminated like a 

 club or bat, and which according to the most common-f- 



* The insect is supposed to be in a state of rest. The rapidity of the 

 vibrations of these organs appears to me to be one of the principal causes 

 of the humming noise made by divers animals of this class. The explica- 

 tions which have been given of it are far from satisfactory. 



t Appendages, in my opinion, of the tracheae of the first abdominal 

 segment, and corresponding to that space, pierced by a small hole adjacent 

 to the anterior side of an aperture with a membranous and internal dia- 

 phragm, which is seen on each side at the same segment, in many crickets, 

 or crydia. — See my Memoir on the Articulated Appendages of Insects, 

 in the Collection of Memoirs of the Museum of Nat. Hist. 



