284 ENTOMOLOGICAL NEWS. [June, 'l2 



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. 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 

 1/20. 



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 



