APPENDAGES TO THE SKELETON OF INSECTS. 307 



it represents a transparent sacculus, in which the ribs or nervures of 

 the wing are enclosed*. This structure, however, is only to be displayed 

 while the wings, after being withdrawn from the pupa- case, are still 

 soft and moist ; for they soon become so intimately united with the 

 horny framework upon which they are extended, that they seem to 

 form a single membranous expansion. 



(803.) The ribs, or nervures, whereby the two plates of the wing are 

 thus supported, are slender hollow tubes, filled with a soft parenchyma : 

 in the interior of some, Burmeister detected an air-vessel (recognizable 

 by the texture of its walls) and a minute nervous filament. 



(804.) We have still, in order to complete our description of the ex- 

 ternal anatomy of an insect, to describe certain appendages which not 

 unfrequently clothe the exterior of the skeleton, and exhibit great 

 diversity of appearance in different tribes. These may be divided into 

 spines, hairs, and scales ; and, however much they may appear to be di- 

 stinct structures, all these are essentially very nearly related to each other. 



(805.) The spines are horny processes developed from the epidermis ; 

 and sometimes, especially in the Coleopterous order, as in some lamel- 

 licorn Beetles, exhibit considerable dimensions. These spines are some- 

 times bifurcated or branched ; but, whatever their shape or size, they 

 never grow from bulbs implanted in the cutis, but are mere prolonga- 

 tions of the exterior layer of the integument. 



(806.) The hairs appear to resemble those of quadrupeds in their 

 mode of growth, inasmuch as they are secreted from roots imbedded in 

 the substance of the cutis or true skin : they are fine horny cylinders, 

 and frequently are found to be branched and divided like the feathers 

 of birds ; but the manner of their formation will be more conveniently 

 discussed hereafter. 



(807.) The wings of the Lepidoptera are covered with minute flat 

 scales of various shapes, and not unfrequently tinted with the most 

 beautiful colours ; such scales, nevertheless, are in reality only flattened 

 hairs, into which, indeed, they frequently degenerate by insensible trans- 

 itions ; and, moreover, they grow from bulbs of precisely similar con- 

 struction. The variety of colours exhibited by the scales of a Butterfly 

 depends upon a film of pigment interposed between the two plates of 

 transparent epidermic matter forming each ; but the gorgeous hues de- 

 rived from this source must not be confounded with the iridescent tints 

 for which they are not unfrequently remarkable, as these have a very 

 different origin : the surface of every scale, that with the changing light 

 reflects evanescent prismatic colours, is seen, when examined under a 

 microscope, to be marked with regular parallel striaB of exquisite minute- 

 ness ; and such a surface, even when grossly imitated by human art, 

 has been found to give rise to the brilliant appearances exhibited by 

 polarized light. 



* Heusinger, System der Histologie, 2. Heft. Burmeister, op. cit. p. 224. 



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