THE INSECTS 



245 



part of the food tube. They may live as larvae from one summer to as 

 long as two years in the water. They then crawl out on a stick, molt by 

 splitting the skin down the back, and come out as adulis. 



A nearly related form is the damsel fly. This may be distinguished 

 from the dragon fly by the fact that when at rest the wings are carried 

 close to the abdomen, while in the dragon fly they are held in a horizontal 

 position. 



May Flies. Another near relative of the dragon fly is the May fly. 

 These insects in the adult stage have lost the power to take food. Most 

 of their life is passed in the larval stage in the water. The adults some- 

 times live only a few hours, just long enough to mate and deposit their 

 eggs. 



The Order Hymenoptera 



We have already learned something of the structure of the bee, an 

 example of this order. Other relatives are the wasps, ichneumons (wasp- 

 like insects with long ovipositors) , and the ants. The structural characters 

 of this group are the presence of two pairs of membranous wings, the mouth 

 parts being fitted for biting and lapping. All undergo a complete meta- 

 morphosis, the young being helpless wingless creatures somewhat like the 

 maggots of the fly. Of this group we shall learn more later. 



The orders of insects mentioned above are only some examples of this 

 very large group. In all of the above forms we have seen certain like- 

 nesses and certain differences in structure, but all of the above have had 

 three body divisions, three pairs of legs, and have possessed in the adult 

 state air tubes called tracheae. These are the principal characters by 

 which we may identify the insects. 



Spiders and Myriapods. Spiders are not true insects, although they 

 are nearly allied to them. 

 The body shows the same 

 division as do the higher 

 crustaceans, cephalothorax 

 and abdomen ; four pairs of 

 walking legs mark another 

 difference. These animals 

 usually have four pairs of 

 simple eyes and breathe by 

 means of lunglike sacs in the 

 abdomen, the openings of 

 which can sometimes be seen 

 just behind the most pos- 

 terior pair of legs. Another 

 organ possessed by the 

 spider, which insects do not 

 have (except in a larval 



Tarantula on its back : p, poison fang ; s, spin- 

 neret. Reduced from photograph by Davi- 



