NARRATIVE OF THE CRUISE. 
513 
Halobatodes) the only insects with an organisation adapted to a truly pelagic mode of 
life. The head, in addition to the antennae (fig. 179, a), bears three tubercles on either 
side, surmounted by a single hair, but of unknown function. The mandibles are pointed 
and serrated, and serve to puncture the creatures upon which they feed, whilst they suck 
out the juices through a kind of tubular proboscis formed by the united maxillae. The 
thorax forms by far the largest part of the body ; the first segment is transverse and 
collar-like, the second and third elongated and fused, and produced behind and below into 
the acetabula with which the hinder legs are articulated (fig. 179, $ , $ ). The abdomen 
is larger in the male than in the female ; it consists of six ring-like segments followed by 
three others specially modified (fig. 179, g,g.a). The abdomen of the female consists of 
Fig. 179. —Halobates wiillerstorffi, Frauenfeld. 
6 , Male, upper side ; ? , outline of the body of the female ; a , antenna ; ft, front tarsus ; fta, front tarsus of the larva : 
m.t, middle tibia and tarsus ; g , genital segments of the male, from above ; g.a, the same in profile. 
the same number of segments and carries an ovipositor made up of four valves ; in both 
sexes the ventral surface of the first segment bears a curious tubercle whose extremity is 
pierced by a transverse perforation. The fore-legs are short, and for the greater part' of 
their length lie well in advance of the body, without being raptorial they are fitted for 
grasping ; and the second joint of the tarsus is furnished with two curved sharp-pointed 
claws, as well as a thin ribbon-like process (fig. 179 , ft). The two hinder legs are long, 
and by their means the little creatures scud over the surface of the water. 
The tarsus of the middle legs (fig. 179, m.t) has a fringe of long hairs, which probably 
serves to aid the animal in swimming or to prevent its being so readily driven by the 
wind. 
