198 THE INTEGUMENTAL SKELETON OF THE IMAGO. 



The adhesive fluid which covers the plantar surface of the 

 pads is secreted by a pair of simple saccular glands. These 

 occupy the greater part of the cavity of the four last tarsal 

 joints. The secretion exudes through the hollow papillae and 

 hairs, often so rapidly when a fly is captured and held between 

 the finger and thumb by the thorax, that it forms small glisten- 

 ing drops on the pads, and may be collected on a slip of 

 glass, when it immediately becomes solid. The tarsal secretion 

 solidifies equally well under water, in which the coagulum is 

 quite insoluble. 



e. The Wings and Mechanism of Flight. (Plate X.) 



It is well known that the wings of insects generally present 

 three well-marked areas or regions the costal, median, and 

 posterior or anal areas. 



These areas are easily distinguished in the wings of the 

 Diptera ; the costal area is bounded behind by the great 

 subcostal nervure, and in front by the margin of the wing. 

 The median area is also called the disc or discal area, a term 

 which I prefer to median. The disc is bounded behind by the 

 median nervure. The posterior area I shall term the ' patagium '; 

 it is folded when the wing is flexed, and is supported by the 



Bibliography : 



90. BORELLUS, J. A., 'De motu animalium,' 410, 2 vols. Lugd. Bat., 1710. 



91. PETTIGREW, J. BELL, ' On the Mechanical Appliances by which 



Flight is attained in the Animal Kingdom.' Linn. Soc. Trins., 

 Lond. (1867), 1868, vol. xxvi. 



92. MAREY, E. J., ' Animal Mechanism : a Treatise on Terrestrial and 

 Aerial Locomotion,' 1873 ! 2r >d edit., 1874. Internal. Sci. Sen, vol. xi., 

 Paris and London. 



93. LENDENFELD, R. VON, ' Der Flug der Libellen ; ein Beitrag zur 

 Anatomie und Physiologie der Flugorgane der Insecten.' 7 plates 

 and figures. Sitzungb. der K. Akad. Math. Naturwissen. CL, Wien. 

 Bd. Ixxxiii., pp. 289-376, 1881. 



91 ADOLPH, E., ' Die Dipterenflugel, ihr Schema und ihre Ableitung.' 

 Nov. Act. C.L.C. Acad., Bd. xlvii., 1885. 



This paper is entirely systematic. 



95. AMANS, P. C., ' Comparaisons des Organes du Vol dans le Serie 

 Animal.' Ann. Sc. Nat. Zool., ser. vi., torn, xix., 1885. 



This paper is of great length, but is inaccurate and does not 

 advance our knowledge of the subject. 



