42 THE VOYAGE OF H.M.S. CHALLENGER. 



Abdomen : Sixth ventral segment nearly as long as the other five, the disk rather 

 flatly depressed posteriorly. 



Genital Segments : $ First below nearly as long as all the abdominal segments taken 

 together, nearly as long as broad, hind margin straight, posterior angles rounded. Second 

 below only visible at the sides ; the right horn reaching nearly to the apex of the next 

 segment (below) ; the left horn bent just above the base, and directed outwards and a 

 little upwards (PL III. fig. 22). The horns (PI. III. fig. 23) have thickened margins; the 

 outer margin and the surface generally are covered with slight tubercles tipped with hairs ; 

 the apical margin is smooth ; the inner is fringed at the base with long hairs, and from 

 near the base to the apex is armed with tubercle-like teeth, arranged in several series on 

 the thickened part, and pointing downwards. The hairs on the surface also point down- 

 wards. The posterior angles of this segment are prolonged into a broad tubercle-like 

 process. The tliird segment above (PL I. fig. 1 g, and I ga) has the lateral angles some- 

 what produced.^ The third segment below has apparently thickened margins. 



$ . (PL I. fig. 1, ¥ .) Similar in appearance to the male, 1)ut differs in the following 

 particulars : — • 



Form more widely oval ; femora of front legs and first joint of antennae more distinctly 

 steel-blue. Front femora less stout. Hind margins of ventral segments and apex of third 

 genital segment indistinctly brownish-red. Below, the first genital segment about covers 

 the second. 



Halobates imlllerstorffi has been found from January to April, from June to August, 

 and in October, and the larvae from January to April, so that it probably occurs through- 

 out the whole year. 



The only note that I can find regarding the life history is in Dr. Cuthbert CoUing- 

 wood's Eambles of a Naturalist, p. 358. The specimens referred to were given by 

 him to Mr. J. W. Douglas, and by Mr. Douglas to me, and appertain to this species. " A 

 notable circumstance occurred in the Indian Ocean, in lat. 25° S., just south of the Mauri- 

 tius. For several days in succession the net produced Halobates, glass-crabs, Velellce, 

 and the beautiful oceanic shell Janthina, of a ri«h deep violet colour." A footnote 

 is added as foUows : — " The occurrence of this singular Hemipterous insect at sea is 

 at least very remarkable. There appear to be several species, of which I met with two, 

 one on the coast of China, and the other some 500 miles from the land, in the South 

 Indian Ocean. That they are veritable marine insects I think cannot admit of a doubt, 

 though how they exist in the open ocean is a mystery. They are of a deep bluish-black, 

 with six legs, the two hindermost furnished with a delicate brush on the inner side of the 

 tarsus. The abdomen is remarkably undeveloped. Although taken occasionally in the 



^ For the form of this segment in the present and succeetling species, the student is referred to the figures, which 

 will give a much better idea of the shape than any description could do. 



