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 burgh. 



ARACHNIDA 



The Arachnida are characterized by an absence of antennae and by four 

 pairs of legs in the adult. Many people erroneously call them insects. 

 Although there are many hundreds of species of Arachnida in the United 

 States, little general information is available about many of them. In addi- 

 tion to the spiders, the group contains the mites and ticks, the scorpions, and 

 the harvestmen or daddy-long-legs. 



Sometimes also included with the Arachnida are the microscopic, aquatic 

 creatures called Tardigrada or Water Bears. Their common name refers to 

 the odd profile they sometimes present under the microscope. Actually they 

 have cylindrical, unsegmented bodies and four pairs of short legs, each end- 

 ing in several hooks or claws. They have no respiratory, circulatory or ex- 

 cretory organs and are considered either the most primitive or the most de- 

 generate of the Arthropoda. 



The mites are the most variable in structure of the Arachnida and many 

 are peculiarly adapted to a semiparasitic or parasitic life upon other animals. 

 The larger forms, usually called ticks, frequently carry and transmit protozoan 

 diseases from one host to another. Most of the mites differ from the rest of the 

 Arachnida in that they may have larval stages in v^'hich only six legs are 

 present, a fact that often misleads the amateur taxonomist. One family of 

 mites lives in the hair follicles of mammals and brings on the condition known 

 as mange. The red "spider", which is often a serious pest in greenhouses, 

 and the cheese mite are other examples of this group. The "jigger" or harvest 

 mite is seldom seen but its effects on the human skin are only too evident. One 

 family of mites lives in fresh water. 



The scorpions rate a position at the other end of the group, if we con- 



202 



