294 INSECTS AND OTHER SMALL ANIMALS 



The trichinas, small microscopic animals, are the most important. 

 Hogs become infected by eating some animal, possibly a rat, that has 

 the trichinas in its muscles. When taken into the intestines of the 

 hog, they increase to great numbers, and then work their way into the 

 flesh, where they become surrounded by a cyst. Man may, in turn, 

 become infected by eating raw or partly cooked pork. The only way 

 to avoid this danger is always to have pork thoroughly cooked before 

 it is eaten. 



INSECTS (Insectd) 



Characteristics. Many people improperly include 

 in this class all small animals, speaking of the coral 

 insect and also spiders as belonging to this class. 

 There are more different species of insects than of any 

 other class in the animal kingdom, but there should 

 be included under this term only such as have certain 

 definite characteristics. In general it may be said that 

 insects have their bodies divided into three distinct 

 parts : the head, the thorax, and the abdomen. The 

 head, which is the first segment, is furnished with 

 a pair of jointed organs of variable length, called an- 

 tenna ; the thorax has three segments, to each of 

 which is attached a pair of jointed legs, six in all; the 

 abdomen has usually nine segments. Many insects 

 have two pairs of wings, some have only one pair, while 

 still others have none. 



Metamorphosis. All insects go through certain 

 changes of form from the egg to the fully developed 

 insect. Such a change is called a metamorphosis. 

 A complete metamorphosis involves four distinct states : 

 first, the egg; second, the larva, which is the cater- 

 pillar, grub, or maggot stage; third, the pupa, or 

 chrysalis stage, in which the larva is inclosed in a cell 

 of silken material, or a protecting case, or often is naked ; 



