TEN SPECIES OF ARRENURI BELONGING TO THE 
SUBGENUS MEGALURUS THON. 
BY RUTH MARSHALL. 
The work upon the genus Arrenurus Duges has been carried 
on from] the spring of 1893 up to the present time, with many 
interruptions. Collections have been made at several places 
in Wisconsin and one in Massachusetts. This paper embodies 
the results of the studies upon the species of the subgenus Mega- 
lurus Thon, most of the work having been centered on this 
group. 
The Arrenuri form one of the most easily recognizable genera 
of the Hydrachnidae. They form' a sharply defined, highly 
organized and well differentiated group, very rich in species. 
They are characterized by the possessession of a very hard, 
transparent, porous integument, an area of which is separated 
on the back by a circular furrow from the rest to form the 
so-called “dorsal shield”; and by great secxual diamorphism. 
The females have a more or less rounded form and closely 
resemble each other; while the ntales have a peculiar 1 prolonga¬ 
tion of the posterior end of the body, presenting numberless 
modifications in the different species and offering the chief spe¬ 
cific characters. 
Then (1900) mjakes three subgenera of the Arrenuri. Petio- 
lurus embraces species in which the posterior appendage is 
short, or altogether wanting and represented by humps on the 
dorsal side of the body; and in which there is a peculiar and 
characteristic structure, the “petiole,” a small oblong chitinous 
structure at the extreme posterior end of the body. Arrenurus 
pustidator Muller is an illustration. Subgenus Micrurus has 
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