THE INSECTS 247 



pairs) is definite in the insects; in the crustaceans the number 

 sometimes varies (as in the Entomostraca), but is always more than 

 three pairs. The exoskeleton, composed wholly of chitin in the 

 insects, is usually strengthened with lime in the crustaceans. 

 Both groups have compound eyes, but those of the Crustacea are 

 staked and movable. The other sense organs do not differ greatly. 

 The most marketl differences are physiological. The crustaceans 

 take in oxygen from the water by means of gills, while the insects 

 are air breathers, using for this purpose air tubes called trachece. 



The young of both insects and crustaceans usually undergo 

 several changes in form before the adult stage is reached. They 

 are thus said to pass through a metamorphosis. Both insects and 

 crustaceans, because of their exoskeleton, must molt in order to 

 increase in bulk. 



Classification of Abthbopoda 

 Phylum AnTHnoPODA 



Class, Crustacea. Arthropods with limy and chitinou3 exoskeleton, rarely more 

 than 20 body segments, usually breathing by gills, and having two pairs of 

 antennae. 



Subclass I. Enlomoslraca. Crustacea with a variable number of segments, 

 chiefly small forms with simple appendages. Some degenerate or parasitic. 

 Examples: barnacles, water flea (Daphnia), and copepod (Cyclops). 



Subclass II. Malacostraca. Usually large Crustacea having nineteen pairs 

 of appendages. Examples : American lobster {Homarus A mericanwi) , crab 

 (Cancer), and shrimp (Palcemonetes) . 

 Class, Hexapoda (insects). Arthropoda having chitinous exoskeleton, breathing 

 by air tubes (Irachew), and having three distinct body regions. 



Order, 4p(era (without wings). Several wingless forms. Examples: springtails. 



Order, Orthoptera (straight wings). Example: Rocky Mountain locust. 



Order, Lepidoptera (scale wings). Examples : cabbage butterfly, cecropia moth. 



Order, Dipiera (two wings). Examples: fly, mosquito. 



Order, Hemiptera (half wing) . Examples : all true bugs, plant lice, and cicada. 



Order, Neuroptera (nerve wings) . Examples : May fly , dragon fly. 



Order, Coleoptera (shield wings) . Examples : beetles. 



Order, Hymenoptera (membrane wings). Examples: bees, wasps, ants. 

 Class, Arachnida. Arthropoda with head and thorax fused. Six pairs of appen- 

 dages. No antcnnie. Breathing by liotli lung sacs Cspidors) or trachea^. Ex- 

 amples : spiders and scorpions. 

 Class, Myriapoda. Arthropoda, having long bodies with man.\' sennients ; one 

 or two pairs of appendages to racli segment. Breathing by means of traclieae. 

 Example : centipede. 



An exercise for field work with a simple key for iilcntification of orders will bo 

 found in the Labratory Manual, I'rob. X.XX, 



