Sunday, March 28, 2010

More Hilltopping Insects

I finally got the full effect of a hilltopping experience on March 25 when I hiked to the summit of Sentinel Peak (“A” Mountain), just west of downtown Tucson, Arizona. At only 2, 897 ft. (883 meters) above sea level, and rising a mere 522 feet off the desert floor, it is more of a butte than a mountain. Still, its relative isolation makes it an effective magnet for hilltopping insects.

“A” Mountain is named for the immense concrete letter on its eastern slope, painted in the red, white, and blue colors of the University of Arizona. Most folks visit the top for the spectacular panoramic view of Tucson and its surrounding mountain ranges. It can be a romantic setting at dusk, too. Male insects of many species come there for romance as well, but usually have to defend territories or otherwise repel rival males in the process.

Among those insects are bumble bee-sized bot flies, Cuterebra austeni. The big, black and white adults are not often seen during their brief existence. They have no functional mouthparts and are fueled only by burning the fat reserves they accumulated in the larval stage. As larvae they are subdermal parasites of either rodents or rabbits, depending on the species. I’ll spare you the gory details.

The male flies conserve their energy and will allow you to approach extremely closely if you do so carefully. Once startled, or when they detect a rival or a female, they jet off at warp speed. Eventually they return to the same spot, or very close by.

A more typical, larger, and colorful hilltopping insect is the black swallowtail, Papilio polyxenes. These butterflies are in seemingly constant motion, flying swiftly across the summit and only pausing to perch for very brief periods.

They are incredibly aggressive, chasing each other mercilessly. You can actually hear their wings and bodies colliding, so violent are their aerial duels.

Perhaps the most unexpected and unusual insect I discovered was the pollen wasp Pseudomasaris maculifrons. Pollen wasps are solitary insects in the same family as potter wasps, mason wasps, and the social hornets, yellowjackets, and paper wasps. While her cousins feed their larval offspring with other insects, the female pollen wasp stores pollen and nectar in her mud nest.

Males are easily recognized by their peculiar antennae: long and clubbed. Females have much shorter, clubbed antennae. Males of this particular species are known to perch repeatedly in the same small area, day after day, for as long as 29 days (Alcock, John. 1985. “Hilltopping Behavior in the Wasp Pseudomasaris maculifrons(Fox),” Journal of the Kansas Entomological Society, 58(1), pp 162-166). Between February and May, different generations of males occupy the same locations in different years. They are very active insects, but do not appear to be nearly as belligerent as bot flies or black swallowtails.

It turns out the top of the peak also attracts other insect photographers. I crossed paths there with my friend Philip Kline, who I hadn’t seen in probably two years! We had a great time catching up, and helping each other identify the insects we were seeing.

May your own hilltop experiences be just as rewarding and enjoyable. You could very well discover something new (especially regarding bot flies), to yourself or the scientific world.

Sometimes you become the best perch on the peak!


  1. Another great blog post - now if I could just find some hills around here... :-)

  2. I find this a very interesting phenomenon - wish we had something resembling a hilltop here that I could investigate!

  3. Thanks, folks:-) Heidi, you have the ROCKIES for goodness sake! The top of any peak is bound to have something, once the snow melts anyway. Give it a try:-)

  4. Hard to convey sarcasm in a post, I guess. Yup, I've observed hilltopping on the mountaintops.

  5. I have huge sandy nests along my driveway & walk way, with many of these wasps.They are very active in the heat of the day.They are not aggressive at all, as I have tried to aggravate them to make sure they won't sting any visitors.I held one in my hands, it made a racket but never stung me.I am trying to convince my family they are harmless, please reassure me they are!They have been with us for three summers.What do you know of their behaviors? Thanks Karen Mae

  6. Karen: I think you posted your comment to the wrong blog post. None of the insects pictured makes "sandy nests" (meaning burrows I suspect). You may be talking about "cicada killers." Read that post and you'll know they are harmless to people.


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