ARTHROPODA. 199 



Latin word Insect. The hard covering which is so 

 remarkable a feature of the class is composed of 

 chitiu. This substance may be obtained by boiling 

 insect wing-cases, etc., in dilute caustic soda, and 

 afterwards washing successively with water, dilute 

 acid, and boiling alcohol and ether. The coat of 

 insects consists of this substance alone ; that of Crus- 

 tacea is additionally hardened by calcareous matter. 

 Most of the hard structures of invertebrates either 

 consist of chitin or have a basis of chitin, and this 

 is true of internal hard structures as vi'ell as of ex- 

 ternal ones. Chitin is indigestible by animal digestive 

 juices; therefore, although whole orders of birds and 

 mammalia live on insects, they are apt to disagree 

 with animals not hereditarily accustomed to them ; 

 and the domestic cat, when slie unluckily makes the 

 blunder of classing herself with the Insectivora by 

 eating the cockroaches that infest town kitchens, 

 usually wastes and pines, and eventually succumbs. 



It has been already mentioned that insects breathe 

 by Tracliese. These are hollow tubes with walls 

 thickened by a spiral ring of chitin, which penetrate to 

 all the organs of the insect's body, carrying a supply 

 of air, which gets into them through holes called 

 Stigmata, which are situated in a row down the 

 bides of the body. Hence it is that land insects are 

 drowned without being completely immersed, and 

 while the head is not covered with water. The neces- 

 sity is not, as with us, to keep the head above water 

 but the sides, which is impossible as soon as the 



