Sexing: Pea and Freezer Style

In the field, Diptera (fly) scientists were able to identify the gender of  house flies (Muscidae and Fanniidae) as well as blow flies (calliphoridae) by throwing a rock or pea.  (Yes, this is peer reviewed and published literature. No, they did not specify if the pea was fresh, frozen or dried. …Which really should  have been clarified because a dry pea would have a much faster projectile.)  Land and Collet (1975) described that when a pebble or a pea (of unidentified quality) was thrown just above a fly at rest, the fly responded by darting up to intercept, and would then fly with the pebble for several meters before returning to the same perch.  Turns out that a tendency to chase the flying objects is a male trait and that the fly would continue the same pursuit up to 27 times in succession.  (Sheesh, males never learn.) They also stated, “female flies were never seen chasing anything.” Therefore, throwing a pebble or pea was a great indicator of Dipteran sex in the 1970’s.

Fast forward to the present.  What is the modern day technique for determining the gender of flies?   Simple, we stun them in the freezer and examine their eyes.    It takes about 5 minutes of freezer time for the fly to settle down, and once they stop moving, seconds to inspect the eyes.  If the eyes are almost touching, it’s a male, and if there is a gap between them, a female.

For example,


Male Blow Fly, Photo: Mike Hraber


Female Blow Fly, Photo: Mike Hraber

Flies have great vision and males, most likely, have evolved to have larger eyes for a larger field of view.  It serves them well when defending territories and seeking mates.  …Or even when they’re just chasing peas.

It may only take just a few seconds to determine the gender of a single fly but I separate approximately 1000 flies every week!  Because we are working with such large numbers of flies, it is all done in the walk-in freezer.  Of course, the number of flies we are seperating is variable depending on the number of students I have or experiments that we are running… But, on average, with undergraduate slaves students, it takes 1 and a half hours of “freezer time” a week, that’s 234 hours of “freezer time” throughout the last 3 years of my PhD. According to the American Council on Exercise, a person burns 400 calories an hour from shivering. So, if I want to consider work effort in terms of expenditure of calories from shivering and cold, I’ve exerted 93,600 calories in “freezer time” while identifying the sex of flies.

For some, this is a real diet plan! Freezing your way to thin was a phenomenon that was well-studied by the military and space program in the 1950’s and 1960’s (Cronise, 2010).  Although, to be honest, they were more concerned about keeping weight on soldiers in harsh environments.  The good news is, the diet is now available for us civilians, including PhD students.   So, I am now accepting “Feel the Freezer Burn” dieters/ fly sexing volunteers via email!

…Seriously though, it’s too bad I can’t just throw a pea.

Cronise, R. (2010) Ice Age: Mastering Temperature to Manipulate Weight. The 4 Hour Body. (ed. by T. Ferriss) Crown Publishing Group, p. 592.

Land, M. F. and Collett, T.S. (1974) Chasing behavior of houseflies (Fannia canicularis): a description and analysis. Journal of Comparative Physiology. 89:331-357

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