
56 THE NORTHERN MICROSCOPIST. 
on a oon GS ae 
to undergo a rapid elongation after the last moult, until in many 
cases they are from three to six times their previous length. In 
one English species, however, according to Pictet’s measurements, 
the female has tails nearly three times the length of the body. In 
the nymph and subimago they are usually shorter than the body. 

Fig. 6. 
The drawing represents an unarmed nymph, not quite arrived at 
maturity, and about one-tenth of an inch in length. The thorax is 
large, the head somewhat hammer-shaped, with two compound eyes 
and three ocelli, the femora compressed, the wing cases consider- 
ably developed, and the central caudal filament somewhat shorter 
than the other two. The shape of its limbs indicates its digging — 
propensities. It is furnished with six pairs of external abdom- 
inal branchize, consisting of single plates, which are supplied with 
branches of the tracheal system for the aération of the blood. — 
The first pair have their origin in the antero-lateral portion of the 
first abdominal segment, near its junction with the metathorax. 
