ARACHNIDA 337 



ORDER VIII. HYMENOPTERA 



This Order includes the ants, bees, and wasps. A few 

 examples have been found in the Solenhofen Limestone 

 and the Purbeck Beds ; a larger number are met with in 

 the Oligocene amber, and in the Miocene of Oeningen and 

 Colorado. 



CLASS V. ARACHNIDA 



Scorpions, spiders, and mites are common forms of the 

 Arachnida. In the members of this Class the anterior 

 segments of the body are fused together, forming a 

 prosoma or cephalothorax which is covered by a carapace. 

 This region usually bears six pairs of appendages, of which 

 one pair is in front of the mouth. Antennae are absent, 

 and no pair of appendages is modified to serve exclusively 

 as jaws. The first and second pairs, known as chelicerce 

 and pedipalpi, serve partly as jaws ; the four remaining 

 pairs are long limbs, placed near the mouth, and used for 

 locomotion and to some extent as jaws. The abdomen 

 may or may not be segmented; in some groups it is 

 divided into an anterior and a posterior region (mesosoma 

 and metasoma), each of which consists typically of six 

 segments. The first segment of the mesosoma bears the 

 genital pore. The metasoma bears no appendages, and 

 those on the mesosoma are never in the form of loco- 

 motory limbs, but are connected with respiration ; in the 

 primitive aquatic arachnids they are plate-like and bear 

 lamellar gills ; in the terrestrial forms the gills are re- 

 placed by lung-books or by tracheae. 



The Arachnida are divided into two sub-classes : — 

 (1) Merostomata, (2) Euarachnida. 



w. p. 22 



