GENERAL EXTERNAL STRUCTURE OF INSECTS. 



19 



"ventral," and "lateral/' The diitinous parts of the dorsum con- 

 stitute the tergum; of the venter, the sternum; and of the latus, the 

 pleurum. 



The tergum of the wing-bearing segments usually consists of 

 two plates — a front one or true notum, (fig. 4, N) carrying 

 the wings, and a jiosterior one, which the writer has termed the 

 postnotum or pseudonotum (PN), having no connection with the 

 wings. The first is often more or less distinctly marked into three 

 transverse parts called the j)re.seutum (Psc), scntum {Set), and scu- 

 tellum {Sd). In such cases the exposed part of the postnotum is 

 called the postso/feUion (PscI). From either the anterior or the pos- 

 terior margin of the tergum, or from 

 both, a thin transverse plate projects 

 downward into the interior of the 

 thorax for the attachment of muscles. 

 These plates are the phragmas {Aph 

 and PpK). The notum supports the 

 wing on each side by two small lobes, 

 the anterior and posterior notal loing 

 processes {AA^P and PNP). Behind 

 the latter is the attachment of the 

 axillary cord (AxC) or basal ligament 

 of the Aving. A large V-shaped ridge 

 on the under surface of the notum hav- 

 ing its apex forward is the " entodor- 

 sum." (A better name would be 

 entotergum.) 



The pleurum consists principally of 

 two plates, the epistermim (fig. 4, Ejjs) 

 and the epimerum, {Epm) lying before 

 and behind a vertical groove, the pleural sutvre (PS), which extends 

 from the pleural coxal process (CxP) below to the pleural iviiig 

 process (WP) above. The pleural suture marks the position of a 

 heavy internal ridge, the pleural ridge or entopleunnn. The epi- 

 merum is connected with the postnotum {PN) behind the base of the 

 wing. These parts occur in almost all insects. In some of the lower 

 ones another plate is present in front of the episternum Avhich may 

 be called the preepistemum. {Peps)."^ Lying along the upper edge of 



Fig. 4. 



/ SI PsI 



TnC 

 -Diagram of generalized 



thoracic segment, left side. 



« Objection may be made to the use of the term " preepistemum " on the 

 ground that it combines a Latin prefix with a word compounded of Greek ele- 

 ments. The same may be urged against " prephragma," " postphragma," " pre- 

 paraptera," and •' postparaptera," words introduced by the present writer in a 

 former paper on the thorax (Proc. U. S. Nat. Mus., XXXVI, 1909, pp. 511-595). 

 However, we are barred from maliing up equivalent terms with the Greek pre- 

 fixes pro and meta because these are used to designate the first and the third 



