FORM, GROWTH, AND CHANGE 9 



the head consists of a number of sclerites firmly joined together, 

 so as to build up a firm, hollow capsule. On top is the crown 

 (or epicranium, Fig. 9 ep), in front, the forehead (or frons) 

 above, and below the face (or clypeus), to which is hinged the 

 upper lip (or labrum, Fig. 9 la), bounding the mouth in front ; 

 at the sides are the cheeks (or genae, ge) , and the conspicuous 

 oval eyes. 



Examined with a strong magnifier the surface of each eye 

 is seen to be made up of hundreds of six-sided areas, resembling 

 the cells of a honey-comb ; these are the corneal facets, each 

 of which has beneath it a series of nervous structures, forming 

 an element (or ommatidium] of a compound eye. Compound 

 eyes, closely similar to those of most insects, are found in 

 shrimps, crabs and other members of the Crustacea ; such 

 creatures have their compound eyes at the tip of stalks which 

 may be regarded as shortened limbs. Therefore it is possible 

 that these organs really represent a pair of appendages ; from 

 the manner in which they develop in the embryo before hatch- 

 ing, we know that they belong to a head-segment situated in 

 front of the one that bears the feelers. 



The feelers (or antennae) are elongate, jointed appendages 

 which, when examined with a strong lens are seen to be clothed 

 somewhat densely with structures that may be described as 

 fine " hairs " resembling as they do in appearance the hairs 

 of the human head for example. But while the hairs of a 

 back-boned creature are modified parts of the skin, the " hairs '' 

 or " bristles " of an insect or a spider are modified parts of the 

 cuticle, each hair jointed on to the surrounding area by a piece 

 of flexible membrane (see Fig. 2, s). An insect hair or similar 

 structure has been formed by a special cell of the underlying 

 skin ; if this cell be continued as a nerve-fibre leading to the 

 central nervous system, the hair is " sensory " in function, so 

 called because through it the cell in the skin can be stimulated 

 so as to transmit a nerve-impulse inwards, which may be pre- 

 sumed to give rise to something in the insect's consciousness 

 resembling sensation in our own. The hairs on an insect's 

 feeler are to a large extent sensory, some being regarded as 

 concerned with touch and others with a sense akin to 

 smell. 



Very different from the slender, flexible feelers are the strong 



