132 THE ANIMAL KINGDOM. 



secreting a glutinous substance, which enables the fly to 

 attach itself firmly to glass or ceiling, whilst, those two 

 strong hooks are used to detach it from the surface to which 

 it clings. 



The wing is next to be observed. It is the most impor- 

 tant to the naturalist. Those small winglets at the base 

 are called the alula ; they distinguish some families, and 

 generally cover and protect two small organs, the halteres, 

 supposed to be the seat of smell, described at page 147. 

 The wing itself consists of a double membrane, more or less 

 transparent, attached to nervures or veins, which are hollow 

 tubes containing spiral air-vessels, communicating with the 

 spiracles or lungs in the trunk. This construction is won- 

 derful for lightness and for strength ; the larger and heavier 

 the body is, the more of these strengthening veins the wing 

 has. And as by breathing only these vessels are filled 

 with air, or some subtle fluid, the very act of flying may be 

 but the palpitating of a joyous little heart. We know 

 that the tracheae are filled with air, and that the dorsal vessel 

 is in truth the heart, sending forth streams of the life-blood 

 throughout the body and into the wings, as may be 

 distinctly seen in the transparent veins of a newly-hatched 



fly- 



But to learn the names of the nervures or veins of a fly, 

 a most useful lesson, no wing is better than this simple 

 one of the Dung-fly. That strong vein bordered with hairs 

 on the foreside is the costal vein ; it runs round the tip of 

 the wing, and ends where it meets the cubital vein. The 

 sub-costal is a pale short vein nearest to the fore-border, 

 and ending at one third of its length. 



The mediastinal is the next and a stouter vein, ending at 

 beyond half the length of the wing. 



'The radial forks at its base, and the farthest branch is 

 the cubital vein, joined to the prabrachial vein by a clouded 

 transverse vein. 



The upper transverse vein, which unites the pr&braclii-al 

 to the pobracJdal, is called the discal transverse. 



The membrane itself is exquisitely dotted with fine hairs, 

 and fringed all round with longer ones. 



