EXTERNAL SKELETON. 



as the ordinary Crustacea^ this hardness is obtained 

 by the deposition of calcareous matter^ principally 

 carbonate of lime (or 

 chalk) ^ in the skin ; but 

 in the rest it is pro- 

 duced simply by a 

 thickening of the skin 

 in those particular 

 parts, leaving the inter- 

 vening portions soft and 

 flexible. Thus the body 

 of an articulated ani- 

 mal may be regarded 

 as a jointed tube ; and 

 as the skin of the limbs 

 and other appendages 

 is also hardened in the 

 same or even a greater 

 proportion, the whole 

 skin of the creature 

 forms a sort of case 

 within which all the 

 soft parts are securely 

 sheltered. To this firm 

 skin are attached the 

 muscles, by which the 

 movements of the dif- 

 ferentparts are efiected; 

 and as these act upon 

 their points of sup- 

 port in exactly the same way that the muscles of 

 man do upon the bones of his skeleton, the hardened 

 covering of these animals has been denominated their 

 skeleton ; but with this difierence, that whilst in the 



The external skeleton of an Insect, 



a. The head, with the eyes, the bases of the 

 antennas, and the palpi, b. The pro- 

 thorax, with the first pair of legs. c. The 

 mesothorax, with the second pair of legs, 

 and the bases of the first pair of wings, 

 d. The metathorax, with the third pair of 

 legs, and the bases of the second pair of 

 wings, e. The femur or thigh. /. The 

 tibia or shank, g. The tarsus, h. The 

 abdomen, i. The ovipositor. 



