INSECTS AND THEIR NEAR RELATIVES 3 



gills, organs which differ essentially in structure from true gills, as de- 

 scribed later. The Crustacea also differ from other Arthropoda in having 

 two pairs of antennae. Rudiments of two pairs of antennae have been 

 observed in the embryos of many other arthropods; but in these cases 

 one or the other of the two pairs of antennae fails to develop. 



The examples of crustaceans named above are the more conspicuous 

 members of the class; but many other smaller 

 forms abound both in the sea and in fresh water. 

 Some of the more minute fresh-water forms are 

 almost sure to occur in any fresh-water aquarium. 



In Figure (5) are represented three 



of these enlarged. The minute 



crustaceans form an important b, Y Cydop7i?, T Daphn?a. a,Cypris; 



element in the food of fishes. 



Some crustaceans live in damp places on land, and are 



often found by collectors of insects; those most often ob- 

 Fig. 6. — a Sow- served are the sow-bugs (Oniscoida), which frequently 

 bug " occur about water-soaked wood. ~ Figure (6) repre- 



sents one of these. 



Class ARACHNIDA 

 Scorpions, Harvestmen, Spiders, Mites, and others 



The members of this class are air-breathing arthropods , in which the 

 head and thorax are usually grown together, forming a cephalothorax, which 

 has four pairs of legs, and which apparently has no antennas. The reproduc- 

 tive organs open near the base of the abdomen. 



The Arachnida abound wherever insects occur, and are often mistaken 

 for insects. But they can be easily distinguished by the characters given 

 above, even in those cases where an exception occurs to some one of 

 them. The more important of the exceptions are the following: in one 

 order, the Solpugida, the head is distinct from the thorax; as a rule the 

 young of mites have only six legs, but a fourth pair is added during 

 growth; and in the gall-mites there are only four legs. 



The Arachnida arc air-breathing; but it is believed that they have 

 been evolved from aquatic progenitors. Two forms of respiratory organs 

 exist in this class: first, book-lungs; and second, tubular tracheae. Some 

 members of it possess only one of these types; but the greater number 

 of spiders possess both. 



The Arachnida lack true jaws. Therefore they do not masticate their 

 prey and swallow the parts. They crush their victims and suck out the 

 juices. The first pair of organs on the cephalothorax constitutes the 

 principal crushing organs. They are modified antennae and are known as 

 chelicerae. Other appendages, however, may aid in the crushing process. 

 Each one of the second pair of appendages just behind the chelicerae 

 is leglike in form. These are known as the pedipalps. Following the 

 pedipalps are the four pairs of legs. Thus the cephalothorax of the 

 Arachnida bears six pairs of appendages. The Arachnida have only 

 simple eyes. 



The more familiar forms of this class are the scorpions, harvestmen, 

 spiders, and mites. 



