132 OUR HOUSEHOLD INSECTS 



covering by the deposition within its substance of mineral 

 salts, especially carbonate of lime ; but that is not the 

 case with insects, the hard skin of which owes its strength 

 and power of resistance to the animal substance chitin, 

 which has a chemical composition represented by the 

 formula C 15 H 26 N Q 10 . No greater contrast could be 

 imagined to the vertebrate leg, with its well-nigh solid 

 rods of bone up the centre, and its soft muscles wrapped 

 round the outside of these, than this arthropterous 

 (jointed-footed) limb with its hard tubular envelope, to 

 the insid^ of which its soft muscular apparatus is fitted. 

 At the point where the leg is attached to the body, the 

 skin is soft and flexible, whereby alone any motion with 

 regard to the body becomes possible. 

 At this point the limb may be easily 

 detached with a pair of scissors, and 

 we will suppose one of the hind pair 

 to have been amputated in this way. 

 Four main divisions to the limb now 

 become apparent (Fig. 39), and in 

 order thoroughly to understand the 

 process of locomotion, it will be 

 necessary to consider these divisions 

 in some detail. First, there is a 

 stout triangular portion, by one edge 



FIG. 39. Left Hind Leg of ,, 1-1,1 T i n j j. 0.1 



cockroach, a, coxa; or which the limb is attached to the 

 murTttrb&V^'tar: body ; it is capable chiefly of a back- 

 ward and forward movement, as will 

 become manifest by working a leg about while it is still 

 attached to the body. This triangular basal joint is 

 called the coxa, and in the cockroach is chiefly re- 

 markable for its large size and for the great extent to 

 which it projects from the body. Following the coxa 

 is a longer narrower piece, flattened at the sides, and 



