372 



is the invariable threefold convexity of the free outline of 

 the head when that segment is looked at from the required 

 point of view (more or less obliquely from behind). In the 

 other primary group of Heteronyces this threefold convexity 

 of the free outline of the head (which I call in the following 

 pages the "trilobed outline") is altogether wanting. In the 

 majority of its species the whole labrum is much below the 

 level of the clypeus, so that the whole free outline of the head 

 is clypeus (as in Scitala and most other Seriroid genera), while 

 in some species the labrum (the head being regarded ob- 

 liquely from behind) is visible in a deep emargination of the 

 clypeus where its outline appears as a concave curve ; in some 

 species the ends of the labrum only are visible projecting 

 from the outline as two more or less conspicuous processes, 

 and in some the clypeus has a deep angular cleft in which 

 the labrum is not any part of the outline from any point of 

 view that looks obliquely at the head from behind; but 

 always the free outline of the head has no appearance of 

 threefold convexity. When the Heteronyces are thus 

 divided into primary groups there are scarcely any species 

 at all about whose location there can be the slightest doubt. 

 The only instances known to me are those of a few small 

 species in each group in which from a certain point of view 

 the outline is in a sense trilobed, but the middle lobe (the 

 labrum) from that point of view presents a truncate apex. 

 In two or three of these species the trilobed appearance is 

 caused merely by the labrum being projected so far forward 

 without overtopping the front margin of the clypeus that it 

 comes into view. These species belong to the first group. 

 In the other species referred to the labrum overtops the 

 front margin of the clypeus, and the truncate appearance 

 of its apex when viewed obliquely from behind is due to 

 an exceptional structure of the labrum itself, and conse- 

 quently these insects stand in the second group. It may 

 here be noted that the first group contains no species in 

 which the labrum overtops the front margin of the clypeus. 

 For the sake of brevity in tabulating I call the lobes of the 

 trilobed outline simply ^'middle lobe" and "lateral lobes," 

 omitting the words "of the trilobed outline," and also in 

 comparing the width of the lobes omit the words 'in width" 

 (e.g., "middle lobe more than i lateral lobes," meaning that 

 it is more than J as wide as each lateral lobe). 



For the formation of secondary groups I take the num- 

 ber of joints in the antennae — a character which does not 

 call for any further remark. Finally, I divide each of the 

 aggregates resulting from the use of the two characters 

 already mentioned into two groups, according as the claws of 



