420 NORTH AMERICAN INDEX FOSSILS. 



labium often carry jointed appendages or palpi. The mouth parts 

 may be greatly modified in the various orders for biting, for suck- 

 ing, lapping, etc. The head further carries the compound eyes 

 and may bear simple ocelli. 



The thorax consists of three segments, prothorax, mesothorax, 

 and matathorax, each with a pair of legs, and the last two typi- 

 cally with a pair of wings each. The last pair of wings may be 

 rudimentary or wanting (Diptera). Each thoracic segment con- 

 sists of an upper region or tergum (notum or dorsum) ; of a 

 lateral region or pleuron on each side, and a ventral region or 

 sternum {tergite, pleurite and sternite). Each region is further 

 designated according to the segment of the thorax to which it 

 belongs : pronotum, propleiiron, prosterniim, mesonotum, meso- 

 pleuron, etc. 



The pleuron is further divisible into an anterior and a posterior 

 portion (episternum and epimeron). The episternum rests on 

 the sternum. The mesonotum and metanotum are each divisible 

 into an anterior portion or scutum^ which extends across the back, 

 and a posterior part or scutellum, often smaller and shield-like. A 

 prccscutum and a postscutcUuin often occur before and behind 

 these two divisions, but they are usually very small and may be 

 obsolete. In the prothorax, these are not differentiated. The 

 wings are inserted between tergum and pleuron and the legs 

 between pleuron and sternum. 



The legs consist of five joints each; named from the body out- 

 ward these are: (i) coxa, or basal segment; (2) trochanter; (3) 

 femur; (4) tibia and (5) tarsus, or foot. The last consists gen- 

 erally of five members, of which the outer may end in a claw. The 

 coxa connects the trochanter with the episternum while an addi- 

 tional joint, the trochantin, exists in some cases, and this may 

 connect the trochanter and epimeron. 



The wings are among the most important organs from a syste- 

 matic point of view, and they are generally the best preserved 

 part of the animal. They consist typically of a thin expanded 

 membrane, including a network of zving-vcins (nervures) and 

 ribs, the arrangement of which is of the highest systematic im- 

 portance. These veins or nervures are hollow tubes, more or less 

 branching and anastomosing and contain tracheae or air-tubes and 



