574 INSECTS AND ARACHNIDA. 



allowed to dry upon the slide, and should then be treated in the 

 usual way; but their natural appearance is best preserved 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 

 membranous 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 downwards) 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 fluid, when their texture is soft; and in balsam, 

 when the integument is hard and horny. 



395. Wings. These organs are essentially composed of an 

 extension of the external membranous layer of the integument, 

 over a framework formed by prolongations of the inner horny 

 layer; within which prolongations, tracheae are nearly always to 

 be found, whilst they also contain 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 Neuroptera (dragon-flies, &c.), Hymenoptera 

 (bees and wasps), Diptera (two-winged flies), and also of many 

 Homoptera (cicadse and aphides) ; and the principal interest of 

 these wings as microscopic objects, lies in the distribution of 

 their "veins" or "nerves" (for by both names are the ramifica- 

 tions of their skeleton known), and in certain points of acces- 

 sory structure. The venation of the wings is most beautiful in 

 the smaller Neuroptera; 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 Dip- 

 tera such reunions are rare, especially towards the margin of 

 the wings, and the areolse are much larger. Although the mem- 

 brane 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 areola- 

 tion is often discernible; this is well seen in the common Fly, in 

 which each of these areolse 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 viewed, it is convenient 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 



