PREPARATION OF TRACHE.E. WINGS. 677 



Dissecting-trough (§ 134), and removing the whole Visceral mass, 

 taking care to leave as many as possible of the branches which 

 will be seen proceeding to this from the two great longitudinal 

 Trachea?, to whose position these branches will serve as a guide. 

 Mr. Quekett recommends the following as the most simple method 

 of obtaining a perfect system of Tracheal tubes from a Larva: — a 

 small opening having been made in its body, this is to be placed in 

 strong Acetic acid, which will soften or decompose all the viscera ; 

 and the tracheae may then be well-washed with the Syringe, and 

 removed from the body with the greatest facility, by cutting away 

 the connections of the main tubes with the spiracles by means of 

 fine pointed 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 mounting them in fluid 

 (weak Spirit or Groadby'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 abso- 

 lutely 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 or Medium when their texture is 

 soft, and in Balsam when the integument is hard and horny. 



528. Wings. — These organs are essentially composed of an ex- 

 tension 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 include channels through which Blood circulates 

 during the growth of the Wing and for a short time after its 

 completion (§ 523). This is the simple structure presented to 

 us in the Wings of Neuroptera (Dragon-flies, &c), Hymtnoptera 

 (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 distinguishing feature of this Order 



