ORDER ODONATA. 
The common dragon-flies of the suborder: Anisoptera are 
familiar to every one, but the damsel-flies, constituting the sub- 
order Zygoptera, might not be recognized as also belonging to 
this order. These damsel-flies are small narrow-winged forms, 
which, like the butterflies, hold their wings back to back while 
at rest. The Odonata have no quiescent pupal state; the 
immature stages after the egg are collectively designated as 
the nymph. The latter is always aquatic. It has highly 
developed thoracic legs but no abdominal ones. Wing-pads 
appear at the third or fourth molt. There are no external gill 
structures except the three terminal appendages of the Zygop- 
tera. The abdomen of the nymph is slender among the 
Zygoptera, but in the ordinary dragon-flies (Anisoptera) it is 
rather short and broad. A very distinctive feature is the large 
and elongate labium, folded beneath the body like an arm, the 
“hand” of which, ending in a pair of claspers, covers the 
mouth or the entire face. The nymphs crawl rather slowly, 
often clumsily, but can dart some distance through the water 
like the crawfish, being propelled by the sudden ejection of the 
water in the rectal respiratory cavity. 
The Odonata and their near relatives the May-flies 
(Ephemerida), isolated remnants of former insect life, in gen- 
eral of primitive character although highly specialized along 
some lines, are probably among the oldest orders of winged 
insects. In younger groups the branching of the “family tree” 
of development may often be traced with some degree of satis- 
faction by a study of the primitive characters retained by still 
existing forms; but in the lapse of ages so many of the earlier 
lines of descent have been obliterated from the earth that in 
the Odonata only the upper parts of a few separated branches 
are traceable, their points of origin being involved in obscurity. 
In Illinois the branch nearest the primitive stock is probably 
that of the “black-wings,” or Caloptery., of our smaller streams, 
representing the Calopterygide. Another branch (Agrionide), 
including the more common damsel-tlies, Agrion, Lestes, ete., 
