144 
Psyche 
[June 
teromedially. The lateral lobes (LL) subdivide each pleurite into 
dorsal and ventral portions. The dorsal portions (D) of the pleurites 
are vertical and concealed by the forewings; the ventral portions 
(V) are more horizontal and are exposed to the water. The pos- 
terior lobes (LP) of both pleurites are continuous with the lateral 
ones. The posterior lobe of the mesothoracic epimeron overlaps 
the metathoracic episternum, whose posterior lobe overlaps the 
metacoxa (CX). 
The posterolateral corner of the mesothoracic epimeron bears 
a knob-like process (Fig. 14A, K) which fits into a depression on 
the costal margin of the forewing, anchoring it against the edge of 
the lateral epimeral lobe. The subalar air space between the fore- 
wing and the body is enlarged lateral to the vertical, dorsal portions 
of the mesothoracic epimeron and metathoracic episternum, which 
are indented on the body. 
The mesothoracic postalar bridge projects laterally into the 
subalar space forming a collar-like postalar projection (Fig. 14 A, 
PP II). A double-walled lateral ridge (LR) of the mesothoracic 
epimeron runs from the postalar projection to the wing-anchoring 
knob. The postalar projection encircles the membrane of a meso- 
thoracic scolopophorous organ (SO II) either partially, as in Fig. 
14A, or completely. The function of this paired sense organ, and 
of its homologues on the metathorax (SO III) and first abdominal 
segment (SO 1) has been debated. Larsen (1957) believed them 
to be equilibrium perceptors. More recently, however, experi- 
mental evidence has indicated that the mesothoracic scolopo- 
phorous organs of Notonecta obliqua (Notonectidae; Arntz 1972) 
and Corixa punctata (Corixinae; Prager 1973) and the mesothoracic 
and metathoracic organs of Nepa cinerea (Nepidae; Arntz 1975) 
are sensitive to sound. 
The boundary between the mesothorax and metathorax curves 
sharply anteriorly into the mesothoracic region in the lateral part 
of the body (Fig. 14A, heavy broken line). In some Hydrocorisae 
this lateral intersegmental boundary forms a prominent, flap-like 
invagination which grows anteriorly during postecdysial develop- 
ment (Parsons 1974). A lateral intersegmental air space (heavy 
stippling), which is posteriorly continuous with the subalar air 
space, is formed between the invaginated boundary and the lateral 
ridge of the mesothoracic epimeron. It is ventromedially con- 
tinuous with a much smaller ventral intersegmental air space 
which lies dorsal to the posterior epimeral lobe. 
