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[Vol. 95 
derivation of the abdominal structure of specimens of Hydrovatini 
and Methlini, comparisons were made with members of other 
hydroporine and nonhydroporine genera listed in Table 1. 
Relationships between taxa were determined by phylogenetic 
techniques (essentially Hennigian) that are outlined in Wiley (1980), 
and Nelson and Platnick (1981). However, phylogenies in the analy- 
sis herein were generated and/or analyzed in part with computer 
programs developed by Dr. D. Swofford (PAUP, Phylogenetic 
Analysis Using Parsimony, version 2.4). 
Results 
Abdominal structure in members of Hydrovatini and Methlini 
Three basic abdominal morphotypes (1-3) are recognized. These 
three types form a spectrum from the unmodified posteriorly 
rounded, lightly sclerotized terminal tergum found on members of 
L. difformis (Figs. 1 A-B) (type 1), to intermediate modifications as 
found in specimens of H. pustulatus Melsheimer (type 2) (Figs. 
1C-D), to the most derived condition in methlines (type 3) (Figs. 
2A-D). 
Morphotype 1. (Fig. 1A). In all examined specimens of Can- 
thyporus Zimmermann, Deronectes Sharp, Hydroporus Clairville, 
Laccornis Gozis, Oreodytes Seidlitz and all nonhydroporines, the 
posterior edge of the eighth tergum is broadly and evenly convex in 
dorsal view. The eighth tergite is folded ventrally inward as a ventral 
flap or fold; this folded portion is hereafter referred to as the ventral 
fold (Fig. IB). The ventral fold extends anteriorly for about 25 per 
cent of the length of the last segment. The outwardly visible poste- 
rior edge of tergum-8 then, is actually the point at which the tergum 
folds underneath. This character state is clearly evident in many of 
the abdominal/ genitalic illustrations in Burmeister (1976, 1980). 
Morphotype 2. (Figs. 1C-D). On males and females of Hydro- 
vatus, only the apical tergum is distinctly modified; it is narrow, 
acutely pointed, and it is differentiated into a dorsal and ventral 
lamina (Fig. 1C). Neither lamina is distinctly sclerotized, but the 
ventral lamina is more membranous than the dorsal lamina (Fig. 
ID). Dissections were difficult but it appeared that the dorsoapical 
portion of the ventral lamina possessed a slight inward fold and no 
ventral fold was evident on the dorsal lamina. 
