154 
Psyche 
[June 
supracoxal lobe, homologous with the supracoxal lobe of the meso- 
thoracic epimeron which was described in a previous paper (Par- 
sons 1974). 
The boundary between the thorax and abdomen is less clearly 
marked than that between the mesothorax and metathorax. The 
low third phragma (Fig. 21, 3PH) is anteriorly continuous with 
the metathoracic postnotum (PN III) and possesses a short pos- 
terior wall, the posttergite (PT; sensu Snodgrass 1935). The 
phragma represents the first abdominal antecosta, while the post- 
tergite represents, according to Snodgrass, the anterior part of the 
first abdominal segment. Thus the membrane which attaches to 
the posttergite is the functional thoracico-abdominal membrane 
(TAM) rather than the morphological one (Parsons 1970). 
The lateral part of the phragma is continuous with a low internal 
ridge (Fig. 21, IR) which runs along the posteroventral edge of the 
thoracico-abdominal sclerite. The ridge, which is here interpreted 
as the thoracico-abdominal boundary, becomes indistinct pos- 
teromedially. It can be traced as far as a process (MP), of variable 
size and shape, from which one or more dorsoventral muscles 
arise. The muscles attach dorsally on the first abdominal tergite, 
suggesting that the process is a part of the abdomen which is fused 
with the posteriormost part of the metathoracic epimeron. The 
process may be separated from the rest of the abdomen by the 
functional thoracico-abdominal membrane (Fig. 21), as in Ambry- 
sus (Parsons 1970), Belostoma (Parsons 1972a) and Diaprepocoris 
(Fig. 22), or it may be fused with the abdomen, as in Notonecta 
(Parsons 1971), Micronecta (Fig. 23) and Hesperocorixa (Fig. 24). 
When fused with the abdomen its crest is medially continuous 
with the second abdominal antecosta (Figs. 23 and 24, AC2). 
The first abdominal segment, unlike the more posterior ones, 
is incomplete laterally. Its tergite (Fig. 14A, Tl) is separated from 
its sternal portion by the functional thoracico-abdominal mem- 
brane, which usually contains the first abdominal scolopophorous 
organ dorsolaterally (Figs. 14A and 21, SO 1). The lateral portion 
of the reduced first abdominal sternite is represented by at least 
part of the process MP (Fig. 21). The medial portion of the sternite 
may be entirely membranous and incorporated into the metacoxal 
membrane (Fig. 21), as in Belostoma and Ambry sus, or may be 
represented by a narrow sclerotized region anterior to the crest 
of the second abdominal antecosta, as in Notonecta (Parsons 1971), 
