INSECTS AND ARACHNIDA. 



scissors. In order to mount them, they should be floated upon the slide, 

 on which they should then be laid-out in the position best adapted for 

 displaying them. If they are to be mounted in Canada balsam, they 

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

 the usual way; but their natural appearance is best preserved by mount- 

 ing 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 stom- 

 ach 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 downward) only to such a degree as to allow the balsam to 

 spread and the cover to be pressed-down. The spiracles are easily dis- 

 sected-out by means of a pointed knife or a pair of fine scissors; they 

 should be mounted in glycerine-jelly when their texture is soft, and in 

 balsam when the integument is hard and horny. 



638. 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 prolon- 

 gations 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 is completion ( 633). This is the simple 

 structure presented to us in the Wings of Neuroptera (Dragon-flies, etc.), 

 Hymenoptera (Bees and Wasps), Diptera (two-winged-Flies, and also of 

 many Homoptera (Cicadas 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 Neuroptera; since it is the dis- 

 tinguishing 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 margin of the 

 wings, and the areolae 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 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 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 those of the same side are connected 

 together, so as to constitute in flight but one large wing; this consists of 

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