292 



THK MICROSCOPE. 



Each hexagob forms the slightly horny case of an eye. Their 

 margins of separation are often thickly set with hair, as in the bee ; 

 in other instances naked, as in the dragon-fly, house-fly, Ac. The 

 number of these lenses has been calculated by various authors, and 

 their multitude cannot fail to excite astonishment. Hooke counted 

 7000 in the eye of a house-fly ; Leeuwenhoek more than 12,000 in that 

 of a dragon-fly ; and Geofii-y cites a calculation, according to which 

 there are 34,650 of such facets in the eye of a butterfly."* 



The trunk is situated between the head and the abdomen ; the legs 

 and wings are inserted into it. The thorax is the upper part of the 

 trunk ; the sides and back of which ar« usu- 

 ally armed with points or hairs. The abdomen 

 forms the posterior part of the body, and is 

 generally made up of rings or segments, by 

 means of which the insect can lengthen or 

 shorten itself Running along the sides of 

 the abdomen are the spiracles, or breathing 

 apertures, fig. 129, communicating directly 

 with the internal respiratory organs. Pure 

 air being thus freely admitted to every part, 

 and the circulating fluids kept exposed to the 

 vivifying influence of the atmosphere, the ne- 

 cessity for more complicated and cumbersome 

 breathing organs is at once obviated ; and thus 

 the whole body is at the same time rendered 

 lighter. They are usually nine or ten in 

 number, and consist of a horny ring, gene- 

 rally of an oval form. The air-tubes are ex- 

 quisitely composed of two thin membranes, 

 between which a .delioate elastic thread, or spiral fibre, is interposed, 

 forming a cylindrical pipe, and keeping the tube always in a dis- 

 tended condition ; thus wonderfully preserving the sides from collapse 

 or pressure in their passage through the air, which would occasion 

 sufibcation. No. 4, Plate IX. represents the beautiful mechanism 

 of a portion of the trachese of the silk- worm moth. Fig. ^3^ is a , 

 small portion of a trachese, highly magnified, and showing the peculiar 

 arrangement of the spiral tubes, giving elasticity and strength to 

 the air-tubes of the ITydrophilus. 



The legs of insects are extremely curious and interesting, each leg 

 consisting of several horny cylinders, connected by joints and liga- 

 * John GoAam, Esq., Microscopical Journal, 1853. 



fig. 129. 

 Breatlii;ig-aperture or spi- 

 racle of Silkworm. The 

 circle encloses the object 

 of the natui'al size. 



