THE I NSECTS— ARTHROPODS 215 



Insects have compound eyes. All but a very few primitive insects 

 have this method of vision, which was described in the discussion of the 

 crayfish. Some species have larger compound eyes, with a correspond- 

 ingly greater number of individual facets than others, and accuracy of 

 vision seems to depend on the number of facets. Since the eyes tend to 

 curve around the head, there are some facets pointing in almost every 

 direction so that it is quite difficult to approach an insect without being 

 seen by some of these structures. About the only way a person could 

 approach a fly without being seen is from below and to the rear and it 

 would be a little difficult to get in that position. 



Insect Metamorphosis 



Through observation of larger animals we think it natural that the 

 newly born or newly hatched offspring should rather closely resemble 

 their parents, except for size, and that they should gradually grow into 

 adult size. This is by no means true among insects, however ; many 

 larval insects would be classed in a different phylum if their life history 

 were not known. There is no marvel of nature more astounding than 

 the transformation of a green worm-like caterpillar into a beautiful 

 butterfly. 



Insect metamorphosis may broadly be divided into three major types. 

 The first that we will consider is complete metamorphosis. Life starts 

 with the egg which hatches into a larva, this being usually worm-like in 

 nature. Common names of larvae indicate their similarity to worms — 

 bag worms, grub worms, silk worms, measuring worms, and fuzzy 

 worms are all insect larvae. Insect larvae have a voracious appetite and 

 eat nearly all the time. They are usually able to do this because their 

 mothers have laid the eggs near a plentiful supply of food. Because of 

 their rather gluttonous feeding habits they grow rapidly ; a fly maggot 

 will double its size in a few hours and increase its size several hundred 

 times in a few days. A single tomato worm may eat half of a large 

 tomato in a day and increase its size accordingly. 



Finally, the larva seems to have eaten its fill and goes into a quiescent 

 stage called the pupa. It may spin threads of silk around itself to form 

 a cocoon in which it spends its pupal state, or its outer body covering 

 may simply harden to form a pupa case. The metamorphosis into the 

 adult condition takes place within this covering and a fully developed 

 adult emerges as the fourth and final stage in the development of an 

 insect with complete metamorphosis. The adult is as large as it will 

 ever be when it emerges ; big flies do not grow from little flies ; they are 



