liEY. A. E EATON ON RECENT EPIIEMERID^ OR MAYFLIES, 15 



2-joiuted, the termiual joint claw-shapod. The legs, as a whole, are modified to suit the 

 habits of tlie nymph: the hinder pairs of burrowing nymphs are often short and weak, 

 whilst their fore legs are strong (e. g. Polymitarcijs, PI. XXVIII.) ; the fore tibite and 

 femora of many such nymphs also are often strongly bearded with long stiff hair. In 

 Prosopisioma the corresponding armature is a row of minute pectinate spinules. Dif- 

 ferent nymphs have different manners of disposing their legs Avhilst swimming, according 

 to their kindred : those which swim laboriously and slowly keep theirs at work as if 

 they were running, without much effect. Rcptagenia and its allies employ their 

 flattened femora to some advantage; others, such as Cloeoii and its associates, trail 

 their legs at length through the water, darting swiftly about, propelled solely by their 

 caudal setaj ; but the legs of Prosopistoma (which is just as nimble as the nymphs last 

 referred to) are folded up closely beneath the thorax, which is grooved for their recep- 

 tion. The legs of dead specimens in fluid are often set out in the postures appropriate 

 to them during natation. 



Abdomen sessile, differing considerably in length relatively to the head and thorax, 

 according as the anterior segments are or are not abbreviated in comparison witli the 

 hinder ones ; in cross-section it is either sul)circular, or arched above and flattened 

 beneath. The hinder lateral angles of some of the segments are in many genera pro- 

 longed backwards into acute teeth of diverse dimensions. Erom the last segment just 

 Ijelo'iV the tergum issue two or three many-jointed caudal setae of various lengths in pro- 

 portion to the body ; sometimes they are much longer than it {Heptagenia &c.), at 

 others unequal to it, while in a Palingenia from Ceylon and a few other genera they are 

 many times shorter than it. The median seta is in some instances the shortest or even 

 abortive, and in many more is eventually deciduous at the jienultimate moult through 

 atrophy. Throughout more or less of their extent, on one or on both sides, their hair is 

 usually lengthened so as to feather them and render them fit to serve as organs of 

 propulsion. Tiie outer tails can be moved at will towards or away from the median 

 tail ; and the amount of their natural divergence from it during repose is an item of 

 importance in classification. In Prosopisioma the sette can be simultaneously retracted 

 into the abdomen so as to be entirely hidden. 



Tracheal branchiae are movable, membranaceous, or filamcntose appendages to the 

 integuments, enclosing branching tracheae, which are deciduous witli the epidermis, and 

 are media for the oxygenation of the systemic air distributed throughout the body. It 

 is usual to assert that the insect employs them as fins ; but however rapidly it may 

 agitate them to and fro, they do not seem to increase its rate of progression. They are 

 principally outgrowths of some or all of the first seven abdominal segments, and arise 

 from only one region of the same segment at a time ; but the point of origin need not be 

 the same in consecutive segments. In the large majority of genera their places of 

 attachment are latero-dorsal, and then either well in advance of the posterior angle of 

 the segment {Paliiif/etiia, Lcp)top)hlebia, &:e.), or at the apex of the angle, or else at the 

 hinder border of the segment, within a sinus at the base of the lateral tooth-like pro- 

 longation of the same. The first abdominal pair in Olkjoneuria and RhUhroc/ena is 

 latero-ventral instead of dorsal ; and in the former genus, as well as in Jolia, one pair is 



