Anax junius sexual dimorphism -

One of a series of Web pages by Roy J. Beckemeyer devoted to ASPECTS OF INSECT BIOLOGY.

Last updated 9 December 2003

©2003 by Roy J. Beckemeyer.  This information is pending publication.  Please do not copy or use in any published format or venue.  This is the first of a series of pages that will be devoted to flight-related morphology of Anax junius.

Above right: In addition to the obvious color differences in males and females, there are body composition and relative proportion differences that relate to the different physiological and behavioral requirements of the two sexes.  This pair of Common Green Darners is staying in tandem while the female oviposits so that the male can prevent other males from mating with her.  Photo taken by Roy Beckemeyer at Quivira NWR, Stafford Co., Kansas.

Here are some measurements of male and female Anax junius that I made from specimens taken in Sedgwick County, Kansas during August, 1998.

The females are larger and more massive overall, with most of their mass concentrated in the abdomen.  They have more fat, most of it in the abdomen, and associated with ovaries.  They also have larger hind wings in proportion to their fore wings, at least in part to accommodate an aft shift in center of gravity relative to the males.  The next bar chart compares the hind to fore wing ratios:

The next two pie charts compare the mass distributions by body part of the males and females.  Note that the female mass is in her abdomen, the male's in his thorax:

Much of the male's mass is in his flight muscles, as can be seen in the next chart:

There is also a significant difference in tissue type, as shown in the next chart:

Detailed information on center of gravity of male and female Anax junius will eventually be posted here.  For now, I will just note that in both sexes I have found the c.g. to be located aft of the hind wing leading edge at the "wing attach point" (location of the junction of the anterior veins with the axillary plate).

• Males - Center of gravity 15% of mean HW chord aft of HW leading edge
• Females - Center of gravity 38% of mean HW chord aft of HW leading edge

Related Literature

Anholt, B.R., J.H. Marden, and D.M. Jenkins.  1991.  Patterns of mass gain and sexual dimorphism in adult dragonflies (Insecta: Odonata).  Can. J. Zoology.  69:1156-1163.  [Reviews mass data in 54 species in eight families.  Documents that males and females have similar mass at emergence in most species, and that females are significantly heavier at maturity in 37 of those species.  Their data on mature Anax junius were at odds with mine.  They list male dry mass at 368+/-58 mg (n=18), female dry mass at 331+/-28 mg (n=2), a ratio of f:m of 0.9.  (They give for teneral adults: male - 200+/-31 mg (n=4) and female - 208+/- 37 (n=3).)  Note: My data for Anax junius are below after this list of references.]

Ishizawa, N.  1988. Morphological differences in a dragonfly, Sympetrum frequens Selys, with relation to center of gravity.  Yosegaki.  51:856-863.  [This is the first paper I know of in which the sexual dimorphism in center of gravity location in dragonflies is documented and related to the female having  proportionately larger hind wings than the male.  In this species Ishizawa found that the female Sympetrum frequens HW had 31% greater area then the FW, the male 28%, versus my data of 33% for female Anax junius and 29% for males.] 

Marden, J.H.  1989.  Bodybuilding dragonflies: costs and benefits of maximizing flight muscle.  Physiological Zoology.  62(2):505-521. [A "modern classic" in which the rate of growth of adult male and female Plathemis lydia dragonflies are compared.  The female's growth in abdominal mass and the male's growth in thoracic mass are documented and related to reproductive success.] 

My data for male and female fresh, air dry, and lean dry masses, which disagrees with that of Anholt et al:

Here are my data comparing masses for males and females based on fresh, air dry, and lean dry (after chloroform treatment) values in bar chart form:

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