HEPTAGENIA LONGICAUDA. 305 
defined, the posterior wings being of course much smaller than 
the anterior, but bearing some similitude to the latter in the 
character of neuration; the neuration of the anterior wing is 
somewhat complex, the longitudinal and transverse nervures being 
rather numerous, the latter especially so in the costal area, and the 
interneural veinlets of the terminal margin being continuous with 
either the transverse or longitudinal nervures. ‘The legs are thin, 
the forelegs of the male having extended considerably in length ; 
the tarsi have five joints, the second, third, and fourth gradually 
decreasing in length, the fourth joint being the shortest, and the 
fifth terminating in a single claw with an oval pulvillus or pad. The 
antennz are somewhat shorter than before, but the caudal setz have 
undergone a sudden elongation, so that in some species they are 
several times their former length. This is notably the case in 
Heptagenia longicauda, as its specific title indicates. 
I observed the transformations of the tail in one specimen. It 
was a male, having the three-jointed abdominal claspers common 
to the sex in this genus. The length of its body was rz millimetres. 
The mature nymph had the central seta of the tail 9 mm. in 
length, and the external sete 8 mm., each of the latter consisting 
of 71 segments. In the subimago, these sete expanded to 13 mm., 
with the same number of segments as before. In the imago, how- 
ever, the sete had undergone a sudden extension to about 27 mm. ; 
but upon observing that one of them possessed only 60 and the 
other 64 segments, I looked for the lost segments in the last caudal 
moult, and there found an opacity in the terminal segments indica- 
tive of the presence of the missing extremities. Adding a pro- 
portionate amount for the inclosed segments, when expanded, to 
the previously ascertained length of the seta, I found that each 
seta, when perfect, would be about 32 mm. long, or four times its 
length in the nymph; and that its rapid extension was doubtless 
due to the sudden relief from compression which the segments 
had undergone during growth in the two previous states, the seg- 
ments being then probably in a state of intussusception, to use a 
medical term with an appropriate signification. This specimen 
had considerable difficulty in extricating itself from its subimago 
skin, owing to an accident which it met with in leaving the water. 
Observing the nymph to be rather restless, and to be making fre- 
quent essays to the surface of the water, I placed it in a small 
glass trough, about half full of water, to watch the process of 
moulting. I had not long to wait, for the insect set about it as 
soon as it had familiarised itself with its new environment ; but, 
in escaping from the trough, one wing-case was caught by the top 
of the wet glass, and dragged along it for a short distance, flatten- 
ing its tip. This tip, belonging to one of the anterior wings, the 
imago was unable to extricate when engaged in its final moult ; so 

