284 ENTOMOLOGICAL NEWS. [June, *12 
The color variations are exhibited by the females which, as in some 
other Odonata (e. g. Ischnura), are often of two types in the same 
species, one (homoeochromatic) resembling, the other (heterochromatic) 
differing from, the colors of the male. “These two color phases are, 
however, not sharply marked off from one another, but are connected 
by intermediate forms. ” 
Geographical variation is best shown in the western half of the con- 
tinent and it is suggested that this is due to the varied topography of 
that region. 
The section on the general life-history contains many notes on the 
general ecology of the imagos and nymphs. Some additional details 
on the copulatory position are furnished and illustrated in plate 2,5 De 
tailed accounts of oviposition in two species are given and figured 
(plate 3). Differences in the eggs of various species are recorded 
(p. 46); the ovaries of a female Ae. umbrosa were found to contain 
839 eggs. The wing-buds appear on the exterior of the nymphs when 
the latter are about I cm. long. How many instars precede this 
appearance was not determined, but “it appears probable that there 
are three or four ecdyses.” Beginning with the stage when the wing- 
buds are barely indicated, “the nymph apparently moults eight times 
before emerging as the adult insect . . . . . making a probable 
total of twelve or thirteen stages.” Characters for distinguishing the last 
eight instars are given. The length of nymphal life in Southern Canada 
and the Northern United States is probably three years. A description 
of the transformation of Ae. canadensis is illustrated by eight figures 
(plate 5). 
The systematic portion of the work opens with separate keys to the 
male and female imagos of the 20 species recognized within the geo- 
graphical limits mentioned. The nymphs of no less than twelve of 
these species are distinguished in a following key. The specific descrip- 
tions are detailed and frequently run to six or more pages. Dr. Walker 
has been careful to give a minute list of the material determined for 
each species, the total number of imagos examined having been about 
1720. 
The plates, reproduced by the Heliotype Co., Boston, from Dr. 
Walker’s own beautiful drawings (some of which we had already seen) 
illustrate the structural and color characters of both sexes of the adults 
and also, as far as possible, of the nymphs. On behalf of odonatologists 
and entomologists generally, we will presume to thank the author’s 
father, Sir Edmund Walker, for the publication of these plates, since 
he has met their cost, as Prof. R. Ramsay Wright states in the prefatory 
note. 
The number of North American species of Aeshna admitted by writers 
at different times affords a curious study of the psychology of “lumping 
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