998 INSECTS AND AKACHNIDA 



by mounting them in fluid (weak spirit or Goadby's solution), using 

 a shallow cell to prevent pressure. The finer ramifications of the 

 tracheal system may generally be seen particularly well in the mem- 

 branous wall of the stomach or intestine ; and this, having been laid out 

 and dried upon the glass, may be mounted in balsam so as to keep the 

 tracheae full of air (whereby they are much better displayed), if care 

 be taken to use balsam that has been previously thickened, to drop 

 this on the object without liquefying it more than is absolutely 

 necessary, and to heat the slide and the cover (the heat may be 

 advantageously applied directly to the cover after it has been put 

 on by turning over the slide so that its upper face shall look down- 

 ward) only to such a degree as to allow the balsam to spread and 

 the cover to be pressed down. The spiracles are easily dissected out 

 by means of a pointed knife or a pair of fine scissors ; they should 

 be mounted in glycerin jelly when their texture is soft, and in 

 balsam when the integument is hard and horny. 



Wings, These organs are essentially composed of an extension 

 of the external membranous layer of the integument over a frame- 

 work formed by prolongations of the inner horny layer, within 

 which prolongations tracheae are nearly always to be found, whilst 

 they also include channels through which blood circulates during 

 the growth of the wing and for a short time after its completion. 

 This is the simple structure presented to us in the wings of Neuro- 

 ptera (dragon-flies, &c.), Hymenoptera (bees and wasps), Diptera 

 (two- winged flies), and also of many Homoptera (Cicadce and Aphides) ; 

 and the principal interest of these wings as microscopic objects lies 

 in the distribution of their ' veins ' or ' nervures ' (for by both names 

 are the ramifications of their skeleton known) and in certain points 

 of accessory structure. The venation of the wings is most beautiful 

 in the smaller ISTeuroptera, since it is the distinguishing feature of 

 this order that the veins, after subdividing, reunite again, so as to 

 form a close network ; whilst in the Hymenoptera and Diptera such 

 reunions are rare, especially towards the margins of the wings, and 

 the areolse are much larger. Although the membrane of which 

 these wings are composed appears perfectly homogeneous when 

 viewed by transmitted light, even with a high magnifying power, 

 yet when viewed by light reflected obliquely from their surfaces 

 an appearance of cellular areolation is often discernible ; this is well 

 seen in the common fly, in which each of these areolae has a hair in 

 its centre. In order to make this observation, as well as to bring 

 out the very beautiful iridescent hues which the wings of many 

 minute insects (as the Aphides') exhibit when thus view r ed, it is con- 

 venient to hold the wing in the stage-forceps for the sake of giving 

 it every variety of inclination ; and when that position has been 

 found which best displays its most interesting features, it should be 

 set up as nearly as possible in the same. For this purpose it should 

 be mounted on an opaque slide, but instead of being laid down 

 upon its surface the wing should be raised a little above it, its 

 * stalk ' being held in the proper position by a little cone of soft wax, 

 in the apex of which it may be imbedded. The wings of most 

 Hymenoptera are remarkable for the peculiar apparatus by which 



