No. 1774. THORAX OF HYMENOPTERA—SNODGRASS. 69 



The pronotum, as already stated, appears to belong to the front of 

 the mesothorax rather than to the prothorax. Its posterior lateral 

 angles are more or less produced toward the bases of the wings, often 

 forming a distinct lobe (1, N^, w) on each side partially overlapping 

 the first spiracle (ISp). In other families it usually completely 

 covers and conceals the spiracle. 



The propectus is always very loosely connected with the pronotum 

 by membrane, and its lateral parts reach forward in the walls of the 

 neck to the base of the head. In Tremex, as already described, the 

 propleurum consists of the episternal plates alone (1, 2, Eps^), but 

 many other forms show at least a trace of an epimerum. In a species 

 of Arge (12) both pleural plates {Eps^ and Epm^) are well developed 

 and are separated by a distinct pleural suture {PS), just as in any other 

 segment. Lygsenematus erichsonii (18) also possesses a comparatively 

 large proepimerum (Epin^). The posterior angle of the epimerum is 

 produced internally as a large epimeral arm (12, EpmA), but when the 

 epimerum is absent this arm appears to arise from the episternum 

 (2). This internal process is apparently not the homologue of the 

 pleural arm of other segments (9, PA), since it does not arise from 

 between the pleural plates. 



The prosternum (2, 12, S^) is a small plate lying between the front 

 coxse {Cx^) and behind the ventral parts of the episterna (Eps^). 

 It carries two internal apodemes constituting the anterior or pro- 

 sternal furca (2, Fu). In Tremex columha a small plate (2, d) lies 

 between the prosternum and the coxa on each side, and in Arge 

 there is a smaller one on the side (12,6) between the coxa, the sternum, 

 and the episternum. 



Many entomologists regard the prothoracic plates that the writer 

 calls the episterna as the prosternum. Most of them, however, 

 apparently do not observe that the true prothoracic sternum lies 

 behind these plates and between the bases of the coxse. Berlese 

 (1906) defines the lateral part of each pleural plate as the "episternum" 

 and the ventral part as the " mesosternite " of the mesosternum. 

 The true prosternum he calls the "metasternite." In studying 

 Scolia ruffrons he finds two other parts, which are illustrated as well 

 by Scolia duhia (fig. 17), the first being a median subdivision (A and 

 B, n) of the episternum (Eps), which he calls the "prosternite," 

 the second a median sclerite (l) in the ventral wall of the neck, which 

 he calls the "acrosternite." Thus Berlese is able to establish in 

 the Ilymenopteran prothorax the four consecutive parts which he 

 thinks are the primitive elements of every sternum. The only con- 

 trary argument to this homology is that appearances are too strongly 

 against it. The lateral ridge (fig. 17 A, j) simply marks the line 

 where the edge of the pronotum laps over the episternum, while 



