Natural History 507 



eyes project on each side of the head as convex, immovable struc- 

 tures. There is only one pair, though each eye may be partially 

 divided, as in some of the aquatic Whirligig Beetles in which 

 half of the eye is directed up to keep a look-out for danger from 

 above, while the other half is scanning the water below in search 

 of prey. The compound eye consists of a great many similar 

 parts each a complete organ of vision but requiring the sur- 

 rounding elements to form the whole image. Each of the many 

 elements of the eye makes a small image, so that the whole image 

 is a mosaic of separate contributions, which combine in a unified 

 visual impression conveyed to the brain. For the amorous insect 

 does not see 1,000 desired mates, one through each of its eye- 

 elements. The question is not an easy one, but it should be 

 noticed that, in some cases, e.g., fireflies, the eye-elements no 

 longer act separately, but a single combined image is thrown on 

 *he back of the eye. (See figures facing pages 307-308.) 



The antennae are appendages set in sockets on the crown 

 of the head, and consist of a series of joints, varying from one 

 or two to a large number, and of many different shapes. They 

 are of the greatest importance to the insect as organs of touch, 

 by means of sensory bristles connected with underlying nerve- 

 fibres, and also in connection with the sense of smell. Of hear- 

 ing, in insects, very little is securely known. Further, the head 

 carries three pairs of mouth-appendages (homologous with legs) , 

 which are variously transformed for different modes of feeding, 

 biting, or sucking. It is very interesting to find that the same 

 three parts are changed in scores of different ways. 



Insects' Legs 



The legs, which are borne on the three rings or segments of 

 the thorax region, show many different peculiarities to suit differ- 

 ent habits. The front pair is considerably lengthened in certain 

 beetles that climb about the bark of trees; in the Mole Cricket 

 they are converted into burrowing implements, the terminal 



