CHLOEON (EPHEMERA) DIMIDIATUM. 77 
had divided; the sixth and seventh, which were now the eighth and ninth, showed, 
indeed, traces of commencing subdivision, but they could not yet be considered as consti- 
tuting joints. 
Sixteenth State (fig. 15). 
The rudimentary wings cover one-third of the first abdominal segment. 
The anterior and second gills are figured in figs. 28 & 24; the former have the 
secondary lobe slightly longer and narrower than before. As is shown by the figures, its 
form is quite different'from that of the corresponding part of the other branchis. 
The development of the caudal appendages has proceeded as before, so that the most 
strongly marked joints are now the third, fifth, seventh, ninth, thirteenth, seventeenth, 
twenty-first, twenty-fifth, twenty-ninth, thirty-third, thirty-seventh, and forty-first. The 
thirty-third and six following segments are darkened, and the fringe extends to the 
fifty-fifth. 
In some specimens, however, the forty-fifth, forty-ninth, and fifty-third segments also 
were more strongly marked than the intermediate ones. 
These, indeed, seemed to me (and this was especially the case near the base) to be less 
distinct than before ; while of the newly developing ones it was difficult to draw any line 
between true segments and joints which were merely indicated. 
Seventeenth State (fig. 16). 
The wing-cases cover more than half of the first abdominal segment. 
The secondary lobes of the anterior gills reach almost to the summit of the primary 
lobes; their form is, however, almost the same as before. 
The divisions into groups of segments, beginning as before with those lying between 
the ninth and thirteenth, extends now to the sixty-first, where also the fringe terminates. 
The darkened part extends from the thirty-eighth to the forty-fifth, inclusive, but the first 
four are very slightly affected. 
Thus I have endeavoured, as far at least as my observations at present reach, to 
describe the different stages through which these larvee pass, in their progress from birth 
to maturity. There is, however, one other point, in which these changes remind us 
rather of growth than of metamorphosis, which is, that the development of the different 
organs does not seem in all cases to progress with equal rapidity. ‘There are, of course, 
many differences which are merely the result of injuries; but, on the other hand, there 
are many which cannot, I think, be so accounted for. 
One specimen which had the posterior angles of the mesothorax more developed 
than those of the metathorax, without, however, covering quite half of that segment, 
would be considered, if we took these organs as our test, as belonging to the tenth or 
perhaps the eleventh state. It was {ths in length. The lateral tail had the most 
conspicuous circles round the third, fifth, seventh, eleventh, fifteenth, and seventeenth 
segments, and the fringe reached to the twenty-eighth. 
The middle tail in this specimen closely resembled the lateral ones, at least in the 
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