PRELIMINARY NOTES ON THE ODONATA. 89 
It is to be regretted that the limits of this paper preclude a treat- 
ment of the nymphs. Those desiring descriptions of the nymphs of 
Minnesota species must be referred to the papers of Needham, Walker 
and others. 
The plates accompanying the paper are made from photographs 
taken from life by the writer. 
Classification of the Odonata. 
This group of insects is made up of forms possessing an aquatic 
nymphal or larval stage and a winged imago. These states being 
separated by no marked pupal stage the metamorphosis is Hemi- 
metabolic. Both nymph and adult have large compound eyes, seg- 
mented antennae and mandibulate mouth parts; they are voracious in 
all stages. The four membranous, netted-veined wings of the adult 
usually possess a pterostigma on the costal, apical region of each. The 
antennae and the tarsi are each three segmented. The abdomen is very 
long and slender, and consists of ten distinct segments, exclusive of an 
anal segment ending in the dorsal or superior appendages. The males 
have inferior terminal appendages also, and the opening of the testes 
is on the ninth and the accessory genital organs are on the second seg- 
ment. The females are without terminal inferior appendages and have 
the vulva at the sternal apex of the eighth segment. 
Key to the Sub-families. 
A Slender species with eyes widely separated, thorax weak, wings similar and 
laid together along the back when resting; nymphs with three caudal gills. 
(Sub-order Zygoptera.) Family Agrionidae. 
a More than two antecubitals Sub-family Calopteryginae 
b Two antecubitals only Sub-family Agrioninae 
B Usually larger and heavier bodied, with eyes touching or but slightly separated ; 
thorax large and strong; front and hind wings unlike, the latter widest, and 
both held horizontally in repose; males with single inferior appendage. 
Nymphs without caudal gills. (Sub-order Anisoptera.) 
a Triangle equally distant from arculus in front and hind wings; antecubitals 
of the two series not corresponding throughout. (Family Aeshnidae.) 
a’ Pterostigma with a brace at its inner end in space below it. 
a” Subtriangle of fore wing of single cell. 
a Eyes widely separated by occiput Sub-family Gomphinae 
y Eyes touching for some distance Sub-family Aishninae 
b” Subtriangle of fore wing of three cells Sub-family Petalurinae 
b Pterostigma without a brace vein Sub-family Cordulegastrinae 
b Triangle of hind wing much nearer arculus than that of front wing; ante- 
cubitals of two series practically corresponding. (Family Libellulidae) 
a’ Triangle of hind wing considerably beyond arculus; more than two cubito- 
anal cross veins Sub-family Macromunae 
b’ Triangle of hind wing on a level with arculus or in front of it; one or 
two cubito-anal cross veins 
a” Sectors of arculus distinctly separate at their departure from the ar- 
culus; eyes tuberculed behind Sub-family Cordulinae 
b’ Sectors of the arculus closely approximated or fused for some dis- 
tance beyond arculus; eyes not tuberculed behind 
Sub-family Libellulinae 
