Monday, November 5, 2012

The Inexplicable Mojo of Tiger Beetles.

A Case for
Cicindelidia senilis frosti Varas-Arangua:
Taxonomic Status, Habitat Preferences, Seasonality.

The halophilic tiger beetle, Cicindelidia senilis frosti Varas-Arangua, has been the subject of some debate concerning its taxonomic status as a valid subspecies or merely a color form of the nominate Cicindelidia senilis senilis G.H. Horn.

The two subspecies have been distinguished on the basis of dorsal coloration and distribution: C. senilis senilis, brown to blackish-brown from northern California (San Francisco Bay area to Solano and Sonoma Counties, a disjunct inland population at Carrizo Plain, San Luis Obispo County) and C. senilis frosti, greenish to greenish-brown (rarely bluish), from coastal southern California (San Diego to Ventura and Santa Barbara Counties, a disjunct inland population at Lake Elsinore, Riverside County). Much of the historical distribution of the latter subspecies has been fragmented into isolated populations because of cumulative habitat degradation.

Cicindelidia senilis senilis G.H. Horn
San Franciso Bay, California
Cicindelidia senilis frosti Varas-Arangua
Syntype Male
California Academy of Sciences #8149

Cicindelidia senilis frosti: Labels of Holotype
California Academy of Sciences
When present, the subspecies, C. senilis frosti, occurs in areas of tidal salt marshes and associated salt pans and mudflats. Though more of the exception than the rule, it has also been encountered along the damp/dry sand beach interface. At the turn of the 20th century, when  the type specimen was collected (note type labels, left), Manhattan Beach was an area of wind-swept sand dunes and salt marshes which correspond to the subspecies' ecology. Needless to say, this population has been extirpated. Of the several specimens I have examined, none have approached the distinctive blue-green dorsal coloration of the type. Recently collected specimens are more of a muddy green.

Manhattan Beach, Los Angeles County.
C. 1910

Collection records indicate C. senilis frosti is a spring-fall species but may also be active during summer. Two specimens in my collection from Ventura County were collected in mid-July.

Internet Resources

Cicindelidia senilis senilis image courtesy of Bugguide. C. senilis frosti images courtesy of California Academy of Sciences Research Archive. Period Manhattan Beach photo courtesy of Maureen McGowan.

                                           © Delbert La Rue 2012. All Rights Reserved.

Friday, November 2, 2012

The Inexplicable Mojo of Tiger Beetles.

REPOST: An unusual color variation of
(Cicindela) Cylindera lemniscata lemniscata LeConte
and implications of Circularly Polarized Light (CPL).

The current literature recognizes three subspecies of this small (7-10 mm), ubiquitous tiger beetle:

(CicindelaCylindera lemniscata lemniscata LeConte,
(CicindelaCylindera lemniscata rebaptistata Vaurie, and
(CicindelaCylindera lemniscata bajacalifornica Shook.

Cylindera lemniscata lemniscata LeConte
The nominate subspecies (right) is bright metallic red to reddish-orange with yellowish-green to purple margins and a bold longitudinal macula (stripe) the length of each elytron. It is very common and widespread throughout most of Arizona, adjacent southeastern California (Imperial and Riverside Counties), and New Mexico (Hidalgo County). It is distinguished from the other subspecies by it's yellow unpigmented legs. 

Some publications characterize it as restricted to open grasslands of the Sonoran and Chihuahuan Deserts, but it is frequently encountered in large numbers at lights in urban environments indicating great ecological tolerance and vagility. There is even a small population just outside my front door at home.

Cylindera lemniscata lemniscata LeConte
Greenish-blue coloration
Peppered among the large population of this subspecies in the Sulphur Springs Valley and Willcox Playa, Cochise County, southeast Arizona, is a small distinctive race of deep greenish-blue individuals (left).                                                                                                                                                             
Morphologically, these greenish-blue phenotypes display no appreciable differences from typical Cyl. lemniscata lemniscata - except of course in coloration. Apparently, some Mexican populations of this subspecies are composed entirely of green individuals (R.L. Huber, pers. comm.).

These greenish-blue specimens, along with the typical metallic reddish-orange phenotype, were collected in early July at lights of a shopping center in the town of Willcox, Arizona. This unusual color divergence raises several questions. First being, how does the change in structural coloration effect thermoregulation and heat transfer - if at all?

A recent study of the polymorphic Cicindelidia hornii demonstrated that metallic green morphs attained the same body temperatures as those that were black - under controlled conditions (Schultz & Hadley 1987).

Chrysina gloriosa (LeConte)
It has been shown that some Coleoptera, for example, some scarabs, desert tenebrionids and many non-beetle insects, are sensitive to circularly polarized light (CPL). Consider, the Ruteline scarab genus Chrysina (right), to our eyes, which cannot discern CPL from unpolarized light, appear green. However, when viewed under CPL, green individuals stand out to one another while appearing imperceptible when viewed by potential predators that cannot discriminate CPL (Brady & Cummings 2010). So, can Cicindelids discern CPL, and if so, is the greenish-blue coloration of Cyl. lemniscata lemniscata a form of crypsis?

On the other hand, because of the absence of red light in darkness, red animals, as in typical Cyl. lemniscata lemniscata, are invisible. Red, being a single pigment, is much easier to produce than black pigment while still having the same cryptic effect. But consider, Cyl. lemniscata lemniscata is cathemeral - both nocturnal and diurnal. Interestingly, the nocturnal tiger beetle genus, Amblycheila, are primarily black but some species display a deep reddish dorsal coloration.

Amblycheila cylindriformis (Say)

Literature Cited & Internet Resources

Brady, P., & Cummings, M. 2010. Differential Response to Circularly Polarized Light by the Jewel Scarab Beetle, Chrysina gloriosa. The American Naturalist. 175(5):614-620.

Schultz T.D., & Hadley N.F. 1987. Structural colors of tiger beetles and their role in heat transfer through the integument. Physiological Zoology. 60:737–745.

Cylindera lemniscata lemniscata and Amblycheila cylindriformis image courtesy of Alex Wild,
Chrysina image courtesy of  Sonoran Tree Service.

                                            © Delbert La Rue 2012. All Rights Reserved