CLASS INSECTS. 413 



is made in the fabrication of ink and in the preparation of 

 dark dyes. The small fissure produced by the auger of the 

 insect gives rise to an effusion of vegetable juices, and this 

 produces an excrescence, in the centre of which may be 

 found the eggs of the cynips. 



518. Insects have the organs of sense highly developed ; 

 they possess sight, smell, as well as tact, taste, and hearing ; 

 but hitherto the organs of olfaction and audition have not 

 been discovered : but little is known of the organ of taste. 

 The structure of the eyes differs very much from that of the 

 higher order of animals. In general, the organ which at 

 first sight appears to be a single eye, is in reality formed by 

 the agglomeration of a number of small eyes, having each a 

 cornea, vitreous humour of a conical form, -a layer of colour- 

 ing matter, and a nervous filament. In the may-bug, for 

 example, we find nine thousand such in a single compound 

 eye, and there are insects which have more than twenty-five 

 thousand. All these small cornese are hexagonal, and unite 

 together so as to form a kind of common cornea, whose sur- 

 face presents a vast number of facettes. Moreover, each of 

 these small constituent parts of these multiple or compound 

 organs is quite distinct from those around it, and forms 

 with them a bundle of tubes, each terminating in a nervous 

 filament, proceeding from the terminal enlargement of the 

 same optic nerve. Almost all insects have a pair of these 

 compound eyes ; but they are sometimes replaced by simple 

 eyes, and at other times both sorts are present. The simple 

 eyes, also called stemmata, have the greatest analogy with 

 the structure of each of the elements of the compound eyes. 

 In general, these simple eyes form a group of three on the 

 summit of the head. Nothing precise is known respecting 

 the manner in which these eyes act on the light, nor of the 

 mechanism of vision in insects. 



519. Several insects have the power of producing 

 sounds, which they do by friction of certain parts of their 

 body on each other, or they may depend on the movements 

 impressed on the special instruments for the contraction of 

 the muscles. The monotonous and deafening sounds of the 

 balm cricket, or chirping grasshopper, result from alternate 

 tension and relaxation of an elastic membrane, disposed like 

 the skin -of a drum, over the base of the abdomen ; in crickets, 

 the sounds are caused by certain parts of the wings, which, 

 rubbing against each other, vibrate intensely, and the struc- 



