4 INJURIOUS AND USEFUL INSECTS 



usually in many thin layers, which cohere to form a cuticle 

 of whatever thickness may be requisite. Beneath the cuticle 

 the cells of the epidermis often remain dormant for a long 

 time, though they are always ready to resume their activity, 

 and produce fresh chitin, when the occasion arises. In 

 this way a new cuticle is formed from time to time to 

 allow for the growth of the body. If enlargement of the 

 surface is called for, the new cuticle is thrown into many 

 folds or wrinkles, which become effaced when the old 

 skin is cast. When all is ready the old cuticle cracks, the 

 animal withdraws its limbs from their sheaths, and creeps 

 out, often in a very soft and flabby condition, but sometimes 



ready for active exertion the 

 moment it becomes free. 



The chitinous cuticle not 

 only serves for protection but 

 also for the attachment of the 

 muscles of the trunk and 

 limbs; it is, therefore, not 

 only armour but skeleton as 

 well. In vertebrate animals 

 the skeleton is internal, and 

 the muscles clothe the bones ; 

 but in an Arthropod the 

 skeleton is hollow and exter- 

 nal, and the muscles spring 

 from its inner surface. 



The arrangement of the chief 

 joinlf; X"v^rebtTe jcin?,^ht"k:ro' Organs of the body differs in 

 clothed with muscles. B, Arthropod ArthroDods and Vertebrates. 



joint, the skeleton enclosing the muscles, t tt \ i_ ^ .i i • j 



In Vertebrates the bram and 

 spinal cord lie along the back; that is, they are dorsal in 

 position. The digestive tube runs through the middle of 

 the body, and the heart is ventral, lying on that side which 

 is turned towards the ground. But in an insect or any 

 other Arthropod, the heart is dorsal, and the nerve-cord 

 ventral ; in fact, the vertebrate arrangement is precisely 

 reversed. One part of the nervous system, however, called 

 the brain, is dorsal in an Arthropod, lying above the gullet. 

 A pair 'of nervous threads, lying one on either side of the 

 gullet, connect the brain with the central nerve-cord. In 



