102 ELEMENTARY STUDIES IN INSECT LIFE 



CHAPTER IX 

 THE WEALTH OF INSECT LIFE ORDERS 



INSECTS, numerically considered, comprise four- 

 fifths of the animal life of the globe. There are now 

 about 250,000 species to which names have been given. 

 This number, it is estimated, is about one-tenth of the 

 existing forms. If we look at a number of these we 

 find great differences in appearance and structure. If 

 an examination be made of those which at first glance 

 appear to be similar, marked distinctions arise. For 

 example, we frequently strike at a fly biting the back 

 of our hand, thinking it to be the common house-fly, 

 while in reality the common house-fly has not the nec- 

 essary mouth-parts to enable it to pierce the cuticle. 

 Again, we might find two insects apparently widely 

 separated by color, or structure, distinctions due to 

 the character in the sexes of the same kind of insect. 

 Then if we change the food and surroundings of an 

 insect, we soon find a subsequent generation of this in- 

 sect changing in form and appearance. The sheep tick, 

 a wingless, almost grub-like insect, was once a two- 

 winged fly, but on account of its parasitic habits it has 

 lost its wings through disuse. Other forms, such as 

 the viceroy butterfly (Fig. 48, 6), have changed their 

 outward appearance. If we remove an insect to a new 

 country w r e find changes arising, due largely to changed 

 climatic conditions. 



And so in classification, we find it difficult at all 



