42 Rev. A. HE. Eaton’s Monograph 
not being exhausted by one union. But in other genera, 
the coitus once consummated, the eggs are deposited in 
one mass altogether. 
The egg laid in the water, after some time develops 
into a nymph, which at first has only the two outer 
caudal sete, and respires through the integument at 
large. But, subsequently, when the nymph increases in 
bulk, special breathing organs grow out from the pos- 
tero-lateral region of some of the abdominal segments, 
as well as from the hinder segments of the thorax. 
These are usually lamellar in form, and nearly always 
external. The thoracic out-growths persist as the wings ; 
those belonging to the abdominal segments are deci- 
duous with the integument, and they are not reproduced 
after the insect emerges from the water as the sub-imago. * 
A cloaca at the end of the intestine is a supplementary 
breathing organ. From the tenth segment, between the 
two setee first formed, a third seta grows, which in some 
genera is afterwards cast off at the same time as the 
mouth-organs, and the gills. The tarsi are jointless, 
and end in a moveable claw. The food of the insect is 
obtained from the large quantities of mud which it 
swallows. 
The adult nymph sometimes floats on the surface of the 
water, with the dorsum of the thorax exposed to the air, 
buoyed up by gas which at that time accumulates be- 
tween the old and the young integuments, and in the / 
emptied alimentary canal: and sometimes it crawls a 
short distance out of the water. In either case, the, 
thorax opens along the middle dorsal suture. Through 
this opening the subimago extricates its head and fore- 
legs from the old skin: the wings suddenly expand fully ; 
the hinder legs are freed, and then the insect creeps out, 
and flies heavily to some convenient resting place, where 
on alighting it assumes the posture characteristic of its 
genus. In some genera, the subimago is the permanent 
aerial state of the female; in most cases, however, the 
subimaginal pellicle is cast sooner or later, according to 
the temperature of the air and the habit of the genus. 
The dingy appearance of the subimago, the comparative 
shortness of its sete and tarsi, and the ciliate terminal 
border of the wings, nearly always distinguish it from the 
imago. 
8 The term “ pseudimago” used by some authors is spurious, and ‘ sub- 
imago”’ has precedence over “ pro-imago.”’ 
