352 General Biology 



Lice (Fig. 228), and other so-called vermin (all of these belong to 

 the insect group), are not only injurious to higher forms of life by their 

 acts, but are dangerous carriers of disease. 



The common house fly carries dirt and filth from the garbage can 

 and manure pile to the food it lights upon, as well as to the baby's 

 drinking bottle. In the filth thus deposited, there are hundreds of tiny 

 eggs and seeds, which require only the necessary moisture and heat 

 of the interior of the human or animal body to begin developing. This 

 is the common way in which typhoid fever is carried. One can hardly 

 get this disease, unless some excreted matter from a typhoid patient has 

 been taken into the intestinal tract. 



An excellent way to demonstrate the fact that insects' eggs are 

 on our foodstuff is this : Take any fruit, such as bananas, apples, cher- 

 ries, or grapes, and place the fruit in a bottle plugged with cotton, so 

 that air, but nothing else, may pass in. In a short time, various forms 

 of animal life will be found therein. As these forms of life hatched 

 from eggs, the eggs must have been on the fruit before it was placed 

 in the bottle. It is of value to note that even after one has washed the 

 fruit well, such hatching will almost always occur. This shows how 

 thoroughly insects fasten their eggs either on or into the surface struc- 

 tures of fruits. 



When different kinds of crops are planted, different kinds of insects 

 will thrive, and those alone will survive which have a sufficient food 

 supply. Those not feeding on the new plants, either leave for more 

 satisfactory fields or die. If it is remembered that the flesh of a duck, 

 which feeds on fish, tastes quite different from that of one not so fed, 

 it will be seen that the food of an animal makes a great chemical differ- 

 ence in the body tissues. It can then be understood how different dis- 

 eases may come forth when parasites change their food and environment. 

 If the food it eats makes a chemical difference in the flesh of an animal, 

 it also means that, if a new chemical substance in a parasite is poisonous 

 to man, then the same parasite, when feeding on one food, may not 

 be poisonous and not cause disease, whereas when feeding on another 

 type of food, such chemical poison may cause disease. Then there is 

 the interesting fact that many diseases of birds will not affect a frog 

 normally when such disease germs are injected, but, if the frog is placed 

 in an incubator where its blood is kept at the same temperature as that 

 of the bird from which the disease is taken, the disease will develop. 

 This illustrates how different temperatures change the susceptibility of 

 different organisms to different diseases. 



The animals commonly called grasshoppers are of varying types 

 (Fig. 229). The true grasshopper is long-horned; that is, it has two 

 antennae as long or longer than its entire body. The family to which 

 these belong is known as Locustidae, while the short-horned grasshop- 

 pers belong to the family Acridiidae. 



