4 
has a number of characters in common with the preced- 
ing, and the two constitute the suborder Zygoptera. In 
forms related to Calopteryx the three tracheated caudal sete of 
the May-flies seem to be represented by three shorter and 
thicker respiratory appendages, the laterals especially being 
more or less spike-like and triquetral. In the Agrionide these 
appendages are vertically flattened thin gill-plates. Quite a 
different development of the original type is found in the 
nymphs of the ordinary dragon-flies, the suborder Anisoptera. 
In these respiration is principally performed by internal rectal 
gills, and the three appendages, very short and subtriangular, 
are supplemented by two others, the five converging to protect 
the opening of the respiratory cavity. 
The five families constituting the Anisoptera—Petaluride, 
Aischnide, Gomphide, Cordulegasteride, and Libellulide—are 
variously connected by common characters. It is worth not- 
ing that the obscure color, rough sculpture, and general habits 
of the Calopteryx nymph are also found in the more primitive 
forms of these familhes. The Petaluride is probably the 
oldest of the five. It has not as yet been found in Illinois. 
In this family and the schnide the ovipositor is well devel- 
oped and oviposition is endophytic, as in all the Zygoptera. 
The Petaluride, judging from Mr. Wilhamson’s observations 
(Williamson, ’01), breed in boggy or swampy spots, where 
the imagos may be found ovipositing. The nymph is re- 
markably synthetic in its characters and supplies a hitherto 
missing link in the evolution of the labium, being interme- 
diate in this respect between Aschnide and Cordulegaster- 
ide, thus indicating a very primitive origin. In its form and 
antennal structure, on the other hand, it strongly approaches 
the Gomphide. In general, however, it is clearly most closely 
related to the dvschnide. The nymphs of the A’schnide 
are somewhat elongate, and climb on submerged branches, 
driftwood, roots, and smaller vegetation. The imagos are 
unusually large and high-flying, the eyes large, the wings 
much specialized, The nymphs of the third family, Gom- 
