Several points underscore the preliminary nature of this survey:

Unsurveyed sites, including Frog Pond on the south side of the mountain and Mitchell Canyon, might yield additional species.

Pine Pond, the largest water feature in the Park and site of the greatest observed diversity, is surrounded by dense stands of emergent vegetation; a more aggressive survey that penetrated this vegetation and reached the main body of the pond might produce additional species there.

During mid-season many dragonflies patrol meadows well away from water; I was unable to identify many of these insects.

Nonetheless, 25 of the 38 species currently known from Contra Costa County were found in the Park, including two observed only by others. I captured and examined in hand at least one individual of each of the 23 species I observed, except California Darner (Rhionaeschna [=Aeshna] californica), Common Whitetail (Plathemis lydia), and Black Saddlebags (Tramea lacerata), which were confidently identified in flight.

Several additional species known from Contra Costa County seem likely to be present in the Park, especially Pacific and Black-fronted Forktails (Ischnura cervula and I. denticollis), Aztec Dancer (Argia nahuana), Exclamation Damsel (Zoniagrion exclamationis), Western Pondhawk (Erythemis collocata), Widow Skimmer (Libellula luctosa), and Blue Dasher (Pachydiplax longipennis). Indeed, I briefly observed a probable male Widow Skimmer in Donner Canyon but was not able to identify it with certainty.

In addition, several species not yet found in Contra Costa County might be expected in the Park. Prominent examples include California and Emma’s Dancers (Argia agrioides and A. emma), Twelve-spotted Skimmer (Libellula pulchella), Wandering Glider (Pantala flavescens), and Western Meadowhawk (Sympetrum occidentale).

The attached tables offer two presentations of the survey results. The annotated checklist indicates the dates on which the four sites were visited and the species found at each. The abundances indicated in the notes for each species should be taken as speculative. Species indicated as “uncommon,” in particular, might well be more common at other sites (or, say, within the perimeter of Pine Pond) or during periods I did not visit the Park. The second table provides a rough indication of seasonal distributions, though no conclusions should be drawn for species observed only once or twice. Note also that this table merges observations from March through mid-May 2004 with those of mid-May through October 2003. The dots in the four upper rows of the table indicate the approximate dates of visits to the four surveyed sites.

In conclusion, Mt. Diablo supports a significant diversity of dragonfly and damselfly species, especially in light of the relatively few ponds and perennial streams within the Park. Particularly notable were the large numbers of patrolling darners (mostly Blue-eyed Darners) over the meadows well away from water during much of the spring and summer, and the striking abundance of Black Spreadwings during the first half of the season.

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