10 MR. J. LUBBOCK ON THE DEVELOPMENT OF 
Fourth State (fig. 4). 
The fourth state, like the third, lasts only two or three days, at the end of which time 
the insect has attained a length of about 35 ths. 
The antennæ (fig. 21) are 24;ths in length, and the segments are seventeen in number, 
the increase both in length and number being again due to the third segment, the rest of 
the organ having remained almost stationary. 
Owing to the fact that the split, through which the insect emerges, runs along the top 
of the head, it is difficult to study the development of the eyes; but I noticed that on 
each side of the upper part of the head there was a group of the curious “ rings " which 
I have already mentioned. 
The tarsi still have three or four supporting hairs, the number being the same on all 
the legs. 
The gills are more developed than before; the five intermediate pairs stand out at 
right angles with the body, while the first and last are still rudimentary, the first, how- 
ever, being the more advanced of the two. The five intermediate gills begin to vibrate 
soon after the change of skin, but the motion is not so continuous as it subsequently 
becomes. "The trachez in them, though small, are easily visible, and communicate with 
the great longitudinal trunks. The circulation of the blood is also apparent, and the 
beating of the heart can be distinctly seen. In form the gills resemble a sharp pear; 
they are about z}5th in length; the front margin is, for half its length, strengthened by 
having the border somewhat thickened; the margin is more or less waved; and each 
gill, excepting the first and last, which are still rudimentary, has a small hair in the 
middle of the free edge, as well as sometimes one or two elsewhere. 
The caudal appendages are about 335;ths in length, with twenty-eight segments. 
The basal part now consists of ten segments, instead of six: my impression is that the 
four basal have divided. 
. The terminal segments are almost unaltered. The circles of spines are largest round 
the second, fourth, sixth, and eighth segments. In addition to the two or three hairs 
near the apex of each segment, the ninth has on the inner side, somewhat nearer to the 
apex than the base, a long hair, which is the commencement of the swimming-fringe. 
The knob between the two tails is again larger, but still consists of only one segment. 
Fifth State (fig. 5). 
The fifth state, like the third and fourth, lasts for two or three days, beginning when 
the insect is about =82,ths in length. 
The antenne are gy jths to 4%;ths in length, and consist of twenty segments. The ten 
apical ones are almost unaltered: the arrangement of the hairs is as before, and, what 
Is very remarkable, these segments have scarcely increased at all in size; indeed, their 
length is little greater than in the smallest specimen examined. Some, however, of the . 
segments are much less strongly marked, and, indeed, but for their homologies with the 
same parts in earlier stages, I should have considered that there were only five seg- 
ments in this portion, as practically the first four on the one hand and the second three 
on the other have coalesced. 
