THE STUDY OF INSECTS 



Fig. 3. — An Itch-mite: a, from 

 below; b, from above. 



ary modification of form, the result of adaptation to special modes of 

 life. Thus the segmentation of the body may be obscured, as in spiders 



and in mites (Fig. 3); or the jointed appen- 

 dages may be absent, as in the larvae of flies, 

 of bees, and of many other insects. In all of 

 these cases, however, a careful study of the 

 structure of the animal, or of its complete life- 

 history, or of other animals that are evidently 

 closely allied to it removes any doubt regarding 

 its being an arthropod. 



The phylum Arthropoda is the largest of 

 the phyla of the animal kingdom, including many more known species 

 than all the other phyla taken together. This vast assemblage of ani- 

 mals includes forms differing widely in structure, all agreeing, however, 

 in the possession of the essential characteristics of the Arthropoda. Several 

 distinct types of arthropods are recognized ; and those of each type are 

 grouped together as a class. 



The number of distinct classes that should be recognized, and the 

 relation of these classes to each other are matters regarding which there 

 are still differences of opinion; we must have much more knowledge 

 than we now possess before we can speak with any degree of certainty 

 regarding them. Some authorities recognize thirteen classes of arthro- 

 pods; but since most of them contain forms which are rarely seen by 

 most students only the more common representatives will be discussed. 

 These are distributed among the following classes: Crustacea, Arachnida, 

 Dipjopoda, Chilopoda, and Hexapoda. 



Class CRUSTACEA 



The Crustaceans 



The members of this class are aquatic arthropods, which breathe by true 

 gills! They have two pairs of antenna 

 and at least five pairs of legs. The 

 position of the openings of the reproduc- 

 tive organs varies greatly; but as a ride 

 they are situated far forward. 



The most familiar examples of the 

 Crustacea are the cray-fishes, the 

 lobsters, the shrimps, and the crabs. 

 Cray-fishes (Fig. 4) abound in our 

 brooks, and are often improperly 

 called crabs. The lobsters, the 

 shrimps, and the true crabs live in 

 salt water. 



The Crustacea are distinguished 

 from nearly all other arthropods by 

 their mode of respiration, being the 

 only ones that breathe by true gills. 

 Many insects live in water and are 

 furnished with gill-like organs; but 

 these are either tracheal gills or blood- 



FlG. 4. — A Cray-fish. 



