1877.] J. Wood-Mason — On Organs of Fliglit in Insecis. 55 



*' When an insect quits the e^^ it has no wings nor the slightest rudi- 

 ments of such, these making their first appearance at one of the earlier 

 changes of skin as slight prolongations of the posterior angles of the dorsal 

 arcs of the two hindermost divisions of the thorax, the mesothorax and the 

 metathorax. These prolongations are so many duplicatures or flattened 

 evolutions of the integument, the chitinous membrane that covers them 

 above and below and on the edges being in direct continuity with that which 

 covers the insect's body, — being, in fact, part of it, — and the intermediate 

 cellular layer which produces this chitinous membrane being similarly conti- 

 nuous with that which underlies the skin of the rest of the insect's body. 

 They increase in size slightly at each successive moult, soon acquiring a de- 

 finite triangular form and the principal nervure dividing the wing into its 

 two principal areas ; but relatively to the future wings they are small and 

 insignificant even at the last moult, at which the organs of flight are sudden- 

 ly developed to their fullest extent. If a wing-rudiment be examined just 

 prior to a moult, it is found that its external chitinous covering has sejDarated 

 off so as to be easily detachable from a new wing-rudiment that has formed 

 beneath it ; and that the new wing-rudiment itself lies quite flat within its 

 sheath, as the portion of the chitinous external layer which covers it may be 

 called after its detachment. The new wing-rudiments are found to lie simi- 

 larly flat within their sheaths at every change of skin down to and including 

 the last but one, into the interval between which and the last it is that 

 the growth of the wings from small and insignificant rudiments to their 

 full extent is compressed. The penultimate change of skin accomplished, 

 new wing-rudiments are produced in due course from the cellular layer, 

 and, at the time when their sheaths first become detachable from them, 

 they, like all their predecessors, lie extended quite flat within these sheaths ; 

 but the detachment of these is no sooner accomplished than they commence 

 to grow with great rapidity. The first outward and visible signs of the 

 growth that now ensues are the thickening of the prolongations (which 

 up to this time were thin plates with thin and sharp edges closely 

 embracing the insect's body, but which now gradually become biconvex 

 masses with thick and blunt edges standing out from it) and the 

 gradual obliteration of the principal nervure. The walls of the sheaths 

 soon become distended to such a high degree of tenuity and consequent 

 transparency under the enormous pressure put upon them from within by 

 the rapidly growing wings, that it is possible to see, even without dissection, 

 the manner in which these are forced to arrange themselves in so limited 

 a space : it can be seen that the wings have thrown themselves into a 

 multiplicity of closely-packed transverse folds representing increments of 

 growth in length and that these again have disposed themselves, in groups, 

 in wavy (longitudinal) folds representing growth in breadth; so that 



