NO. 1687. TIIPJ THORAX OF l\tiEd\S—i^NODaRASS. 527 



occurrence but are not constant nor do they present the same rehition 

 to the ventral ridges in the different orders. From these facts it fol- 

 loAvs that the notal divisions are not necessarily homologous wher- 

 ever the}'^ occur, though they may be so within limited series, and that 

 (])ey are simply secondary adaptations to some common demand upon 

 the notum. 



The above statements and conclusion can be verified by a study of 

 I he species illustrated on the plates. Since the subdivisions of the 

 notum are best developed in the highest orders these will be described 

 first. The ordinary names of prescutum (psc)^ scutum {set), and 

 scutcllimh (scI) will be used to designate the notal regions, but the 

 reader must bear in mind that they are not used in the different 

 orders in a homologous sense, and that the abbreviations on tlie fig?ires 

 do not designate parts necessarily homologous. The " postscutel- 

 lum " will be called the pseudonotum (P^^). 



The notal divisions are probably as well shown in the mesothorax 

 of a Tipulid fly as in any other in.sect. In Ilolorusia (jrandis (174, 

 175) the prescutum {psc) is a large plate with its posterior margin 

 produced posteriorly in a large V-shaped angle, having no relation 

 to the anterior notal ridge. The lateral posterior angles are pro- 

 duced into two small lobes (175, u) lying opposite the anterior angles 

 of the wing bases. The scutum {set) is a wide plate carrying the 

 anterior notal wing processes {ANP). The scutellum {scJ) consists 

 of a median elevated shield and of a depressed area on each side. 

 The posterior wing processes belong to the latter, though they are sep- 

 arated from it by a tongue of membrane. The ventral V ridge is 

 present just as in the diagram (fig. 2), but it marks only the apex {v) 

 of the scutellum, its lateral parts not showing on the surface. Thus 

 the three divisions of the Tipulid notum are but slightly influenced by 

 the ventral ridges. The pseudonotum {P^) is w^ell developed, con- 

 sisting of a median and two lateral plates, the latter articulated 

 with the epimera (174, Epm). In a Tabanid (179, 180) the third 

 division of the notum {sd) is distinct but the first {psc) and the 

 second {set) are not separated mesially. The lateral angles {u) of 

 the prescutum, as in Holorusla, lie opposite the wing bases. 



In the Hymenoptera similar divisions of the notum occur, as is 

 well shown in the drawing (160) of Parasiohla, a Tenthredinid. 

 An examination of the ventral surface reveals the V ridge present 

 but situated entirely behind the suture between the scutum and 

 scutellum. These two plates, furthermore, are easily separable along 

 this suture and, hence, the latter can in no way be compared with 

 the dividing line between the scutum and scutellum of the Orthop- 

 tera. In the example given (ICO) the prescutum (psc) is perfectly 

 exposed, but it is more commonly hidden in the Hymenopteran meso- 

 thorax by the pronotum (169, iVJ, which is attached to and overlaps 



